Bernard Lepetit
email: bernard.lepetit@uni-konstanz.de

Douglas A Campbell
email: dcampbel@mta.ca

Johann Lavaud
email: johann.lavaud@univ-brest.fr

Claudia Büchel

Reimund Goss

Benjamin Bailleul
email: bailleul@ibpc.fr

Chapter 2: Photosynthetic light reactions in diatoms. II. The dynamic regulation of the various light reactions

Keywords: alternative electron flow, diatoms, Lhcx, NPQ, photoinhibition, photosynthesis, Photosystem II repair, xanthophyll cycle : 4.3 Polar diatoms C. meneghiniana: Cyclotella meneghiniana CCM: carbon concentrating mechanisms CEF: cyclic electron flow Chl: chlorophyll cyt b6f: cytochrome b6f complex Dd: diadinoxanthin electrochromic shift ETR: electron transport rate FCP: fucoxanthin-chlorophyll-binding protein complex flavodiiron FNR: ferredoxin-NADP + reductase plastoquinone PSI: photosystem I PSII: photosystem II PTOX: plastoquinone terminal oxidase T. pseudonana: Thalassiosira pseudonana

Photosynthesis in diatoms is performed using the same basic modules as cyanobacteria and plants. It can be regulated on multiple levels depending on the environmental cues, allowing diatoms to adjust their photosynthetic light reaction towards optimum whilst at the same time minimizing photodamage induced by light. In recent years, tremendous progress has been gained in understanding these acclimation processes, revealing several diatom-specific features.

In this chapter, we trace several paths through the photosynthetic electron transport chain to optimize photosynthesis. We review how diatoms repair photoinactivated reaction centers and which mechanisms they have to preempt photodamage. Finally, photoprotection is set in an ecophysiological context, highlighting differences in photoprotection of diatoms from different habitats.

Introduction

As outlined in the previous chapter "Photosynthetic light reactions in diatoms. I. The lipids and light-harvesting complexes of the thylakoid membrane", diatoms share the same elementary photosynthetic modules with other organisms performing oxygenic photosynthesis. The linear photosynthetic electron flow involves two photosystems and results in the transfer of electrons from water, the primary electron donor, to the terminal acceptor, nicotinamide adenine dinucleotide phosphate, NADPH (Fig. 1). Photosystem II (PSII) transfers electrons from the Oxygen Evolving Complex, where water oxidation takes place, to the plastoquinones (PQs) in the thylakoid membrane. Photosystem I (PSI) in diatoms catalyzes the electron transfer from the lumen-soluble electron carrier cytochrome c6 [START_REF] Böhme | Photoreactions of cytochromes in algal chloroplasts[END_REF] to ferredoxin (Fd).

In the stroma, the ferredoxin-NADP + reductase (FNR) mediates the electron transfer from Fd to NADP + . The intersystem electron chain, which comprises PQs, the cytochrome b6f complex (cyt b6f) and cytochrome c6, mediates in-series functioning of the two photosystems. In low iron environments, diatoms can replace the iron containing cytochrome c6 and Fd by the coppercontaining plastocyanin and the flavin-containing flavodoxin, respectively (La [START_REF] Roche | Flavodoxin as an in situ marker for iron stress in phytoplankton[END_REF][START_REF] Strzepek | Photosynthetic architecture differs in coastal and oceanic diatoms[END_REF][START_REF] Groussman | Diversity and evolutionary history of iron metabolism genes in diatoms[END_REF]. The coupling of the electron transfer with proton accumulation within the lumen through water splitting and proton pumping across the thylakoid via the cyt b6f generates an electrochemical proton gradient or proton motive force (pmf), which comprises an osmotic component, ΔpH, and an electric one, Δψ. The pmf is used by the chloroplast CF1Fo-ATP synthase to bind a phosphate group to an adenosine diphosphate (ADP) molecule and thus synthesize adenosine triphosphate, ATP. If diatoms share the same elementary bricks with other organisms performing oxygenic photosynthesis, the regulation of their photosynthetic activities and repair processes show several peculiarities, which we will highlight in detail in the following sub chapters.

The other major electron transport route, electron flow in the mitochondrial respiratory chain, is similar in diatoms and the green lineage. It can occur either via the classical electron flow involving complex I, III and IV or via complex I coupled to the Alternative Oxidase (AOX).

Both pathways generate a pmf which fuels ATP synthesis thanks to the mitochondrial ATPase.

The AOX pathway can behave as a valve to dissipate reducing power under stress conditions in the green lineage [START_REF] Selinski | Alternative Oxidase is positive for plant performance[END_REF] which similarly works in diatoms [START_REF] Murik | Downregulation of mitochondrial alternative oxidase affects chloroplast function, redox status and stress response in a marine diatom[END_REF].

Full-chain respiration and AOX pathways are in competition and can compensate each other in diatoms (Bailleul et al. 2015;[START_REF] Schober | Organelle studies and proteome analyses of mitochondria and plastids fractions from the diatom Thalassiosira pseudonana[END_REF]. Mitochondrial electron transport will not be covered in this chapter, except for its involvement in optimizing photosynthesis (see " 1.4 Water-to-water cycles" and"1.5 Chlororespiration, redox state of the plastoquinone pool, and pmf in the dark").

The regulation of photosynthetic electron flow

The role of pmf as a master regulator of photosynthesis

The main constraint for the photosynthetic electron transfer is to adjust its activity to the amount of light received and to the metabolic needs in terms of NADPH and ATP, despite a variable environment [START_REF] Kramer | Balancing the central roles of the thylakoid proton gradient[END_REF][START_REF] Peltier | Auxiliary electron transport pathways in chloroplasts of microalgae[END_REF]. When light absorption exceeds the capacity of the electron transfer chain, the over-reduction of the photosynthetic chain can lead to the photodestruction of both PSII [START_REF] Nixon | Recent advances in understanding the assembly and repair of photosystem II[END_REF] and PSI [START_REF] Sonoike | Photoinhibition of photosystem I[END_REF]) (see also "2.

Photosystem II photoinactivation and repair"). Indeed, when an over-reduction of the acceptor side of PSII and/or PSI occurs, the photochemical yield decreases and the yields of other relaxation pathways increase in a complementary way. The probability of charge recombination between the oxidized special chlorophyll a pair of PSII (P680 + ) and reduced phaeophytin in PSII, forming the triplet state of chlorophyll a ( 3 Chl), increases when electrons cannot be moved fast enough [START_REF] Krieger-Liszkay | Singlet oxygen production in photosynthesis[END_REF][START_REF] Vass | Role of charge recombination processes in photodamage and photoprotection of the photosystem II complex[END_REF]). There is also the possibility of direct population of 3 Chl in the PSII antennae by electron spin flip [START_REF] Santabarbara | Involvement of uncoupled antenna chlorophylls in photoinhibition in thylakoids[END_REF][START_REF] Triantaphylidès | Singlet oxygen in plants: production, detoxification and signaling[END_REF]. When 3 Chl interacts with O2, singlet oxygen ( 1 O2) is produced, a very toxic molecule which can degrade lipids, pigments or proteins, like the D1 protein [START_REF] Krieger-Liszkay | Singlet oxygen production in photosynthesis[END_REF][START_REF] Halliwell | Reactive species and antioxidants. Redox biology is a fundamental theme of aerobic life[END_REF][START_REF] Nixon | Recent advances in understanding the assembly and repair of photosystem II[END_REF][START_REF] Triantaphylidès | Singlet oxygen in plants: production, detoxification and signaling[END_REF][START_REF] Vass | Role of charge recombination processes in photodamage and photoprotection of the photosystem II complex[END_REF]. PSI acceptor side limitation (and possibly PSII acceptor side limitation as well [START_REF] Pospíšil | Production of reactive oxygen species by photosystem II[END_REF])) also favors reduction of O2 into superoxide anion radicals and hydrogen peroxide (H2O2). In the presence of free transition metals such as iron, copper or manganese, H2O2 can be converted via the Fenton reaction into the very reactive hydroxyl radical, which e.g. eventually destroys the iron-sulfur centers involved in the electron transfer [START_REF] Halliwell | Reactive species and antioxidants. Redox biology is a fundamental theme of aerobic life[END_REF][START_REF] Pospíšil | Production of reactive oxygen species by photosystem II[END_REF][START_REF] Dietz | Redox-and reactive oxygen species-dependent signaling into and out of the photosynthesizing chloroplast[END_REF][START_REF] Sonoike | Photoinhibition of photosystem I[END_REF]). The process is almost irreversible because there is no known repair mechanism for PSI [START_REF] Sonoike | Photoinhibition of photosystem I[END_REF]. Collectively, 1 O2, superoxide, H2O2 and the hydroxyl radical are called reactive oxygen species (ROS).

The lumenal pH regulates photoprotection of the two photosystems through the de-excitation of PSII under excess light conditions [START_REF] Kramer | Balancing the central roles of the thylakoid proton gradient[END_REF]Holt et al. 2004, and see "3.4 The molecular regulation and characteristics of qE") and the 'photosynthetic control' which is the modulation of the activity of the cyt b6f by the lumenal pH. The photosynthetic control sets the kinetically limiting step of the linear electron flow at cyt b6f level and by doing so protects PSI by avoiding its acceptor side limitation [START_REF] Rumberg | pH changes in the inner phase of the thylakoids during photosynthesis[END_REF][START_REF] Laisk | Control of cytochrome b6f at low and high light intensity and cyclic electron transport in leaves[END_REF][START_REF] Joliot | Regulation of cyclic and linear electron flow in higher plants[END_REF]. Moreover, the ATP/NADPH ratio must be adjusted to the metabolic needs of the carbon fixation and other metabolic pathways fueled by the light phase of photosynthesis [START_REF] Allen | Photosynthesis of ATP-electrons, proton pumps, rotors, and poise[END_REF]. The ratio of ATP to NADPH produced by the LEF is lower than that required for C fixation in the Calvin-Benson-Bassham cycle (CBB cycle). Based on the stoichiometry of 14 c subunits per ATP synthase from spinach [START_REF] Hahn | Structure, mechanism, and regulation of the chloroplast ATP synthase[END_REF], 14 protons are needed to produce 3 ATP, which are, together with 2 NADPH+H + , required to reduce one molecule of carbon dioxide in the CBB cycle. With the constitutive Q cycle taking place at the cyt b6f [START_REF] Mitchell | The protonmotive Q cycle: a general formulation[END_REF][START_REF] Sacksteder | The proton to electron stoichiometry of steady-state photosynthesis in living plants: a proton-pumping Q cycle is continuously engaged[END_REF], a total of 12 H + are translocated to the lumen for the movement of 4 electrons needed to reduce 2 NADP molecules in LEF. Therefore, the ATP-to-NADPH ratio produced by the LEF is 1.28. However, the ATP-to-NADPH requirement for the CBB cycle is 1.5 and can increase due to carbon concentrating mechanisms (CCM), resulting in an ATP shortfall [START_REF] Eberhard | The dynamics of photosynthesis[END_REF][START_REF] Peltier | Auxiliary electron transport pathways in chloroplasts of microalgae[END_REF][START_REF] Curien | The water to water cycles in microalgae[END_REF]. Here again, the flexibility of the ATP/NADPH stoichiometry produced by the photosynthetic machinery is modulated by the pmf, which is increased by alternative electrons flows (AEFs) fueling the ATP synthase (see "2.2 Alternative electron flows provide photoprotection and flexibility in response to metabolic demand"). In plants and green algae, the flexibility of photosynthesis is therefore largely dependent on the pmf, which plays the dual role of driving ATP and regulating photoprotection mechanisms through its osmotic component, the ΔpH [START_REF] Kramer | Balancing the central roles of the thylakoid proton gradient[END_REF].

Whether the pmf is a master regulator of the photosynthetic electron transfer in diatoms like in plants is still an open question. In diatoms, the ATP/NADPH produced by the LEF could be different depending on the efficiency of the Q cycle and on the (yet unknown) stoichiometry of c subunits per ATP synthase (Fig. 1). The occurrence of a Q cycle in diatoms seems to be at play based on electrochromic shift (ECS) measurements, i.e. changes in absorption of photosynthetic pigments due to Δψ [START_REF] Witt | Energy conversion in the functional membrane of photosynthesis. Analysis by light pulse and electric pulse methods: The central role of the electric field[END_REF]Bailleul et al. 2010a). Like in plants, the increase of ECS concomitant with the cyt b6f turnover indicates the move of two positive charges (protons) for one electron transferred to cytochrome c6 (Bailleul et al. 2015). Also, the high ATP demand of the CCM in diatoms [START_REF] Hopkinson | Efficiency of the CO2concentrating mechanism of diatoms[END_REF]) strongly suggests that mechanisms increasing the pmf are needed (see next section). Clearly, the rapid and reversible, energy dependent regulation of the light harvesting system of PSII (=qE, a major photoprotection mechanism) is regulated by the ΔpH in diatoms (see "3.4 The molecular regulation and characteristics of qE"). So far, there is no clear evidence for a 'photosynthetic control' in diatoms but the observation that P700, the special pair of PSI, is oxidized after a pulse of high light could reflect such a control [START_REF] Shimakawa | Oxidation of P700 ensures robust photosynthesis[END_REF].

Alternative electron flows provide photoprotection and flexibility in response to metabolic demand

In the green lineage, the ATP shortfall is managed by various AEFs, which are auxiliary routes of electron transfer operating along the photosynthetic chain and sharing part of the components of the LEF [START_REF] Eberhard | The dynamics of photosynthesis[END_REF][START_REF] Peltier | Auxiliary electron transport pathways in chloroplasts of microalgae[END_REF][START_REF] Curien | The water to water cycles in microalgae[END_REF]. Here, we name AEFs all electron transfer pathways involving PSII and/or cyt b6f (both generating a pmf), which do not end up in the generation of NADPH. In the green lineage, the AEFs comprise the cyclic electron flow around PSI [START_REF] Shikanai | Cyclic electron transport around photosystem I: genetic approaches[END_REF] and various water-to-water cycles (WWCs), which are electron transfers from water at PSII via O2 back into water by the means of various oxidases [START_REF] Eberhard | The dynamics of photosynthesis[END_REF][START_REF] Curien | The water to water cycles in microalgae[END_REF]. These oxidases include the plastoquinone terminal oxidase (PTOX) which reroutes electrons from the plastoquinols, the Mehler reaction and flavodiiron proteins (Flvs) on the acceptor side of PSI, or the rerouting of reducing power towards the mitochondrial oxidases of the oxidative phosphorylation pathway through the malate shuttle [START_REF] Eberhard | The dynamics of photosynthesis[END_REF][START_REF] Peltier | Auxiliary electron transport pathways in chloroplasts of microalgae[END_REF][START_REF] Curien | The water to water cycles in microalgae[END_REF].

All those pathways increase the pmf and generate extra ATP, allowing the adjustment of the light phase of photosynthesis with the metabolic requirements [START_REF] Eberhard | The dynamics of photosynthesis[END_REF][START_REF] Peltier | Auxiliary electron transport pathways in chloroplasts of microalgae[END_REF][START_REF] Curien | The water to water cycles in microalgae[END_REF]. AEFs also protect both photosystems from excess light by three means. First, because they offer supplementary electron sinks, they avoid over-reduction of the photosynthetic chain which otherwise would lead to the creation of ROS. Then, the adjustment of the ATP-to-NADPH ratio by AEFs optimizes the capacity of the CBB cycle to consume photosynthetic electrons and to prevent such over-reduction. Last, because they modulate ΔpH, all AEFs also contribute to the protection of PSII and PSI through 'photosynthetic control' and qE [START_REF] Kramer | Balancing the central roles of the thylakoid proton gradient[END_REF][START_REF] Peltier | Auxiliary electron transport pathways in chloroplasts of microalgae[END_REF]. AEFs are especially important for photoprotection when the substrates of photosynthesis (CO2 or light) and/or the metabolic demand vary [START_REF] Kramer | Balancing the central roles of the thylakoid proton gradient[END_REF][START_REF] Peltier | Auxiliary electron transport pathways in chloroplasts of microalgae[END_REF]. We next describe the main AEFs in diatoms.

Cyclic electron flow around PSI

The existence of cyclic phosphorylation in chloroplasts was first proposed by Arnon and collaborators [START_REF] Arnon | Photosynthesis by isolated chloroplasts. II. Photosynthetic phosphorylation, the conversion of light into phosphate bond energy[END_REF][START_REF] Arnon | Regulation of ferredoxin-catalyzed photosynthetic phosphorylations[END_REF]. In plants, genetic analyses have identified two pathways allowing the transfer of electrons from PSI acceptors to the PQ pool, one involving the proteins PGRL1 and PGR5 and the other one passing through the chloroplast NADPH dehydrogenase, NDH [START_REF] Shikanai | Cyclic electron transport around photosystem I: genetic approaches[END_REF]. Based on the structure of the cyt b6f, a third mechanism proposed by [START_REF] Mitchell | The protonmotive Q cycle: a general formulation[END_REF] was reiterated recently which involves the injection of electrons from Fd to the Qi site of the cyt b6f [START_REF] Iwai | Isolation of the elusive supercomplex that drives cyclic electron flow in photosynthesis[END_REF][START_REF] Nawrocki | The mechanism of cyclic electron flow[END_REF]. In plants, rates of the cyclic electron flow (CEF) rate can reach up to ~100 electrons s -1 PSI -1 [START_REF] Joliot | Regulation of cyclic and linear electron flow in higher plants[END_REF], but which of the three proposed mechanisms dominates is still a matter of debate. The NDH is not found in diatoms (nor in other microalgae with the notable exception of prasinophytes, Oudot-Le [START_REF] Secq | Chloroplast genomes of the diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana: comparison with other plastid genomes of the red lineage[END_REF]. PGRL1 and PGR5 are present in diatoms [START_REF] Grouneva | The thylakoid membrane proteome of two marine diatoms outlines both diatom-specific and species-specific features of the photosynthetic machinery[END_REF][START_REF] Grouneva | Changes in relative thylakoid protein abundance induced by fluctuating light in the diatom Thalassiosira pseudonana[END_REF] and recent reports suggest an involvement of PGRL1 in CEF of Phaeodactylum tricornutum [START_REF] Zhou | PGRL1 overexpression in Phaeodactylum tricornutum inhibits growth and reduces apparent PSII activity[END_REF]. The existence of CEF, measured as a PSI activity in conditions of PSII inhibition, was demonstrated in diatoms using both P700 reduction and ECS [START_REF] Maxwell | Role of cyclic electron transport in photosynthesis as measured by the photoinduced turnover of P700 in vivo[END_REF]Bailleul et al. 2015) (Fig. 1). The measured rates, however, correspond to 5 to 10 electrons s -1 PSI -1 in most diatom species [START_REF] Maxwell | Role of cyclic electron transport in photosynthesis as measured by the photoinduced turnover of P700 in vivo[END_REF]Bailleul et al. 2015), even though higher rates were measured in polar diatoms (~25 electrons s -1 PSI -1 , [START_REF] Goldman | Gross and net production during the spring bloom along the Western Antarctic Peninsula[END_REF] or in iron stress conditions (up to 55 electrons s -1 PSI -1 , [START_REF] Thamatrakoln | Death-specific protein in a marine diatom regulates photosynthetic responses to iron and light availability[END_REF]. This suggests that if CEF may not play such a constitutive and ubiquitous role as in plants, it can become a key player of the photosynthetic regulation in some species and conditions.

Water-to-water cycles

The occurrence of WWCs in diatoms were first tested by comparing oxygen and fluorescence measurements derived photosynthetic electron transport rates (ETR). While there is a strong correlation between O2 production and ETR under low light conditions, discrepancies were measured under higher light intensities [START_REF] Wagner | Balancing the energy flow from captured light to biomass under fluctuating light conditions[END_REF]. Later, direct measurements of photosynthesis-driven oxygen consumption, even in conditions minimizing the photorespiration, confirmed the presence of the WWC [START_REF] Eisenstadt | Enrichment of oxygen heavy isotopes during photosynthesis in phytoplankton[END_REF].

The Mehler reaction reroutes electrons from Fd to reduce O2 into superoxide (Fig. 1) [START_REF] Mehler | Studies on reactions of illuminated chloroplasts. 1. Mechanism of the reduction of oxygen and other Hill reagents[END_REF][START_REF] Asada | The water-water cycle as alternative photon and electron sinks[END_REF]. Superoxide is then reduced by the superoxide dismutase (SOD) into H2O2, which is further scavenged by the ascorbate peroxidase to form water and oxidized ascorbate [START_REF] Asada | The water-water cycle as alternative photon and electron sinks[END_REF]. Such WWC has been hypothesized to contribute to the generation of extra ATP and to protect PSI from over-reduction of its electron acceptors [START_REF] Ort | A photoprotective role for O2 as an alternative electron sink in photosynthesis?[END_REF]. The Mehler reaction has been identified in plants [START_REF] Asada | The water-water cycle as alternative photon and electron sinks[END_REF], green algae [START_REF] Chen | The iron-containing superoxide dismutaseencoding gene from Chlamydomonas reinhardtii obtained by direct and inverse PCR[END_REF], and cyanobacteria [START_REF] Asada | Superoxide dismutases from a blue-green alga, Plectonema boryanum[END_REF]). In the dinoflagellate genus Symbiodinium, the Mehler reaction was proposed as the main alternative electron flow [START_REF] Roberty | PSI Mehler reaction is the main alternative photosynthetic electron pathway in Symbiodinium sp., symbiotic dinoflagellates of cnidarians[END_REF]. Oxygen photoreduction measured in the diatoms Nitzschia epithemioides and Thalassiosira pseudonana was ascribed to the Mehler reaction [START_REF] Waring | Light-induced responses of oxygen photoreduction, reactive oxygen species production and scavenging in two diatom species[END_REF]) but its rate in vivo is difficult to estimate because of sources of confusion with other WWCs. Based on the nature of the metal cofactor, Fe-containing SODs and CuZn-containing SODs have been described [START_REF] Wolfe-Simon | The role and evolution of superoxide dismutases in algae[END_REF]. In diatoms, a MnSOD has been found in the plastid of T. pseudonana whereas this enzyme is usually present in mitochondria of other species [START_REF] Wolfe-Simon | Localization and role of manganese superoxide dismutase in a marine diatom[END_REF]. This peculiarity could contribute to the ecological success of diatoms in low iron regions of the ocean. Flv proteins play an important role in photoprotection and ATP/NADPH adjustment in cyanobacteria, green algae, mosses and some plants excluding angiosperms [START_REF] Yamamoto | Artificial remodelling of alternative electron flow by flavodiiron proteins in Arabidopsis[END_REF]) but genes encoding Flv are not found in the genomes of diatoms [START_REF] Zhang | Flavodiiron proteins in oxygenic photosynthetic organisms: photoprotection of photosystem II by Flv2 and Flv4 in Synechocystis sp. PCC 6803[END_REF].

The electrons downhill PSI can be rerouted to the mitochondria, a transfer of reducing power enabled by malate shuttles with low rates though [START_REF] Scheibe | Strategies to maintain redox homeostasis during photosynthesis under changing conditions[END_REF]. Also other observations suggest strong energetic exchanges between organelles in the green lineage [START_REF] Cardol | Impaired respiration discloses the physiological significance of state transitions in Chlamydomonas[END_REF][START_REF] Cardol | The onset of NPQ and ΔμH+ upon illumination of tobacco plants studied through the influence of mitochondrial electron transport[END_REF]. The reducing power is then converted into ATP through the oxidative phosphorylation pathway, which can occur either via the classical mitochondrial electron flow or via complex I coupled to AOX. The AOX pathway behaves as a valve and dissipates reducing power from the chloroplast under stress conditions in the green lineage [START_REF] Yoshida | Physiological impact of mitochondrial alternative oxidase on photosynthesis and growth in Arabidopsis thaliana[END_REF][START_REF] Kaye | The mitochondrial alternative oxidase from Chlamydomonas reinhardtii enables survival in high light[END_REF]) but probably also in diatoms [START_REF] Murik | Downregulation of mitochondrial alternative oxidase affects chloroplast function, redox status and stress response in a marine diatom[END_REF]), e.g. under iron stress [START_REF] Allen | Whole-cell response of the pennate diatom Phaeodactylum tricornutum to iron starvation[END_REF]). In addition, regardless of the light conditions, there is a strong constitutive coupling between diatom plastids and mitochondria, involving about 10% of photosynthetically generated electrons, which is coupled to the plastidic import of the extra ATP generated in the mitochondrion (Bailleul et al. 2015). Exchanges of ATP between organelles also occur in the green lineage, which is responsible for a pmf across the thylakoids in the dark, resulting from the hydrolysis of ATP of mitochondrial origin [START_REF] Tanada | The photosynthetic efficiency of carotenoid pigments in Navicula minima[END_REF][START_REF] Paul | Photorespiration in diatoms[END_REF][START_REF] Finazzi | In vivo characterization of the electrochemical proton gradient generated in darkness in green algae and its kinetic effects on cytochrome b6f turnover[END_REF][START_REF] Peers | Copper-containing plastocyanin used for electron transport by an oceanic diatom[END_REF][START_REF] Takizawa | The thylakoid proton motive force in vivo. Quantitative, non-invasive probes, energetics, and regulatory consequences of light-induced pmf[END_REF]). However, the peculiarity of diatom's photosynthesis is the constitutive nature of this energetic interaction. It is unknown how this connection contends with the four membranes surrounding the plastid. One possibility involves an interplay between the CBB cycle and the mitochondrial glycolysis, an unusual metabolic configuration in stramenopiles [START_REF] Liaud | Compartment-specific isoforms of TPI and GAPDH are imported into diatom mitochondria as a fusion protein: Evidence in favor of a mitochondrial origin of the eukaryotic glycolytic pathway[END_REF]Smith et al. 2016;Río Bártulos et al. 2018).

All AEFs listed above form a network of inter-connected, flexible and largely interchangeable electron flows. If the energetic interactions between plastid and mitochondrion seem to be the major constitutive AEF in diatoms, it is likely that other modes of electron flows play a crucial role in specific conditions (high light, nutrient stress, low temperature, or CO2). One should e.g. mention the nitrogen and sulfur assimilation pathways which consume photosynthetic electrons coming from reduced PSI acceptors [START_REF] Schmidt | The mechanism of photosynthetic sulfate reduction by isolated chloroplasts[END_REF][START_REF] Foyer | Photosynthetic nitrogen assimilation and associated carbon and respiratory metabolism[END_REF].

Because they generate extra pmf, resulting in ATP synthesis without providing NADPH to the CBB cycle, these pathways can also be considered as AEFs. Nitrogen assimilation has been proposed to be an important sink for photosynthetic electrons of diatoms under periods of unbalanced light to dark reactions (Lomas & Glibert 1999). Electron cycling around PSII protects PSII and has been described in diatoms, too (Lavaud et al. 2002c;[START_REF] Falciatore | Diatom molecular research comes of age: Model species for studying phytoplankton biology and diversity[END_REF][START_REF] Wagner | Photosystem II cycle activity and alternative electron transport in the diatom Phaeodactylum tricornutum under dynamic light conditions and nitrogen limitation[END_REF]), but it does not participate to pmf and thus does not meet the definition of AEFs of this chapter.

Chlororespiration, redox state of the plastoquinone pool, and pmf in the dark

Chlororespiration is a respiratory-like electron transport taking place in thylakoids and resulting in the oxidation of stromal reductants (Fd, NADPH). It can occur in the dark and involves two enzymes: NADPH dehydrogenase (type I NDH in plants, type II NDH in green algae, [START_REF] Jans | A type II NAD(P)H dehydrogenase mediates light-independent plastoquinone reduction in the chloroplast of Chlamydomonas[END_REF]) which transfers electrons from NADPH or Fd towards the (oxidized) PQs, and the thylakoid-bound plastid terminal oxidase (PTOX) which reduces O2 into water thanks to (reduced) plastoquinols [START_REF] Bennoun | Evidence for a respiratory chain in the chloroplast[END_REF]) (Fig. 1). Because it combines PQ reducing (NDH) and oxidizing (PTOX) enzymatic activities, chlororespiration influences the redox state of the PQ pool [START_REF] Bennoun | Evidence for a respiratory chain in the chloroplast[END_REF] which in the green lineage, cyanobacteria but also in diatoms is an important signaling trigger [START_REF] Pfannschmidt | Potential regulation of gene expression in photosynthetic cells by redox and energy state: approaches towards better understanding[END_REF][START_REF] Lepetit | Light signaling in photosynthetic eukaryotes with 'green' and 'red' chloroplasts[END_REF]. PTOX is present in two copies in diatom genomes [START_REF] Burki | Re-evaluating the green versus red signal in eukaryotes with secondary plastid of red algal origin[END_REF], it is expressed at least under iron stress conditions [START_REF] Mcdonald | Flexibility in photosynthetic electron transport: the physiological role of plastoquinol terminal oxidase (PTOX)[END_REF]) and targeted to the plastid [START_REF] Burki | Re-evaluating the green versus red signal in eukaryotes with secondary plastid of red algal origin[END_REF], and there is physiological evidence for chlororespiration taking place in diatoms [START_REF] Jakob | Activation of diadinoxanthin de-epoxidase due to a chlororespiratory proton gradient in the dark in the diatom Phaeodactylum tricornutum[END_REF][START_REF] Dijkman | Indications for chlororespiration in relation to light regime in the marine diatom Thalassiosira weissflogii[END_REF]. However, no NDH has been found in diatoms so far (Fig. 1) and the importance of chlororespiration for diatoms might have been overestimated in the past. The existence of a pmf in the dark, visualized through the development of Non-Photochemical fluorescence Quenching (NPQ), has long been seen as evidence for an efficient proton pumping chlororespiratory activity in diatom plastids [START_REF] Jakob | Activation of diadinoxanthin de-epoxidase due to a chlororespiratory proton gradient in the dark in the diatom Phaeodactylum tricornutum[END_REF][START_REF] Jakob | Unusual pH-dependence of diadinoxanthin de-epoxidase activation causes chlororespiratory induced accumulation of diatoxanthin in the diatom Phaeodactylum tricornutum[END_REF][START_REF] Grouneva | The regulation of xanthophyll cycle activity and of non-photochemical fluorescence quenching by two alternative electron flows in the diatoms Phaeodactylum tricornutum and Cyclotella meneghiniana[END_REF][START_REF] Cruz | Impact of chlororespiration on non-photochemical quenching of chlorophyll fluorescence and on the regulation of the diadinoxanthin cycle in the diatom Thalassiosira pseudonana[END_REF][START_REF] Kennedy | Dark metabolism: a molecular insight into how the Antarctic sea-ice diatom Fragilariopsis cylindrus survives long-term darkness[END_REF]. However, the almost complete suppression of the dark pmf in the absence of mitochondrial respiration (via anaerobiosis or pharmacological inhibition) in several diatoms demonstrates that the pmf generated in the dark originates instead rather from the hydrolysis of mitochondrial ATP by the plastidic CF1Fo-ATP synthase (Bailleul et al. 2015).

When PTOX re-oxidizes the plastoquinols produced by PSII, it generates a peculiar WWC which does not involve the cyt b6f [START_REF] Nawrocki | The plastid terminal oxidase: its elusive function points to multiple contributions to plastid physiology[END_REF]. The PSII-PTOX pathway can play an important role in the regulation of the ATP/NADPH ratio and photoprotection in stress conditions, e.g. in cyanobacteria and prasinophytes under iron stress [START_REF] Bailey | Alternative photosynthetic electron flow to oxygen in marine Synechococcus[END_REF][START_REF] Cardol | An original adaptation of photosynthesis in the marine green alga Ostreococcus[END_REF], or in alpine plants [START_REF] Streb | Evidence for alternative electron sinks to photosynthetic carbon assimilation in the high mountain plant species Ranunculus glacialis[END_REF]. These cases seem to be exceptions and this pathway is considered to be usually negligible compared to the LEF in the light but plays a crucial role during dark periods [START_REF] Nawrocki | The plastid terminal oxidase: its elusive function points to multiple contributions to plastid physiology[END_REF].

Photosystem II photoinactivation and repair

PSII is susceptible to photoinactivation through multiple, non-exclusive paths (reviews [START_REF] Oguchi | Operation of dual mechanisms that both lead to photoinactivation of Photosystem II in leaves by visible light[END_REF][START_REF] Vass | Molecular mechanisms of photodamage in the Photosystem II complex[END_REF][START_REF] Zavafer | The action spectrum of Photosystem II photoinactivation in visible light[END_REF][START_REF] Campbell | Photoinhibition of Photosystem II in Phytoplankton: Processes and Patterns[END_REF]. A direct path likely acts through a low-probability absorbance of a photon by the manganese cluster of the Oxygen Evolving Complex, leading to loss of capacity for photooxidation of water and then to photochemical destruction of PSII by subsequent photon excitations [START_REF] Tyystjarvi | Mathematical modelling of the light response curve of photoinhibition of photosystem II[END_REF].

This path is provoked primarily by UV and blue photons and shows limited sensitivity to the instantaneous physiological state of the cell [START_REF] Havurinne | Action spectrum of photoinhibition in the diatom Phaeodactylum tricornutum[END_REF][START_REF] Murphy | Photoinactivation of photosystem II in Prochlorococcus and Synechococcus[END_REF]. This direct path occurs across excitation levels at nearly constant probability per photon.

Alternately, photons captured by the PSII antennae can provoke formation of 3 Chl within the antennae or reaction center, which can in turn generate destructive 1 O2. This path is thus influenced by the antennae absorbance spectra and cross section and hence likely by qE (see

"3.

The molecular regulation and characteristics of qE").

There is, as yet, no direct evidence that induction of qE actually lowers susceptibility to photoinactivation in diatoms, although such an effect is usually assumed based on investigations on qE in plants [START_REF] Li | PsbS-dependent enhancement of feedback de-excitation protects photosystem II from photoinhibition[END_REF]. The antenna-dependent photoinactivation path also depends directly upon the degree of PSII closure, which in turn reflects the balance between incoming excitation and the downstream flow of electrons away from PSII, often characterized by the lifetime (τ ) for PSII re-opening as measured by Fast Repetition and Relaxation Fluorescence (FRRF) [START_REF] Kolber | Measurements of variable chlorophyll fluorescence using fast repetition rate techniques: defining methodology and experimental protocols[END_REF].

Thirdly, charge recombination within PSII may lead to generation of 3 Chl, and then subsequent 1 O2 generation and damage to PSII. This photoinactivation path may dominate under low light when delivery of sequential excitations to PSII are widely spaced, increasing the probability of recombination within PSII [START_REF] Ohad | Photoinactivation of photosystem II: is there more than one way to skin a cat?[END_REF][START_REF] Keren | Mechanism of photosystem II photoinactivation and D1 protein degradation at low light: The role of back electron flow[END_REF].

Diatoms show low primary susceptibilities to PSII photoinactivation when compared to other phytoplankton lineages, and within diatoms large centric diatoms show lower susceptibilities than do smaller centrics, probably related to optical attenuation by the package effect in larger cells (Key et al. 2010;[START_REF] Wu | Distinctive photosystem II photoinactivation and protein dynamics in marine diatoms[END_REF][START_REF] Wu | Photosystem II photoinactivation, repair and protection in marine centric diatoms[END_REF][START_REF] Wu | Physiological response of marine centric diatoms to ultraviolet radiation, with special reference to cell size[END_REF][START_REF] Campbell | Photoinhibition of Photosystem II in Phytoplankton: Processes and Patterns[END_REF]. We speculate that the relatively low sensitivity of diatoms to photoinactivation may relate to the photonic crystal nature of the diatom siliceous frustule [START_REF] Goessling | Structure-based optics of centric diatom frustules: modulation of the in vivo light field for efficient diatom photosynthesis[END_REF] which modulates the internal light field of diatom cells. We also note that beyond the rather unusual model species P. tricornutum [START_REF] Havurinne | Action spectrum of photoinhibition in the diatom Phaeodactylum tricornutum[END_REF] there are fewer mechanistic studies of photoinactivation of PSII from pennate [START_REF] Petrou | Rapid photoprotection in sea-ice diatoms from the East Antarctic pack ice[END_REF][START_REF] Petrou | Photophysiological responses of Fragilariopsis cylindrus (Bacillariophyceae) to nitrogen depletion at two temperatures[END_REF]) than from centric diatoms. It is reasonable to expect different optics in the pennates and therefore the centric bias of mechanistic studies to date should be kept in mind, in contrast to the ecological dominance of pennates in many habitats.

Kinetic overlap of photoinactivation and downregulation of PSII

Loss of [PSII]active through either photoinactivation path leads to lowered variable fluorescence from PSII and thus can overlap both phenomenologically and kinetically with reversible downregulation of [PSII]active through induction of qE [START_REF] Grouneva | A new multicomponent NPQ mechanism in the diatom Cyclotella meneghiniana[END_REF][START_REF] Kuzminov | Energy dissipation pathways in Photosystem 2 of the diatom, Phaeodactylum tricornutum, under high-light conditions[END_REF][START_REF] Buck | Identification of sequence motifs in Lhcx proteins that confer qE-based photoprotection in the diatom Phaeodactylum tricornutum[END_REF]. From an ecophysiological point of view the suppression of productivity through sustained loss or downregulation of [PSII]active, through whatever mechanism, may be more important than the direct metabolic burden of PSII repair [START_REF] Raven | The cost of photoinhibition[END_REF]. Prolonged suppression of PSII activity extending after the end of high light exposure imposes an opportunity cost of loss of potential productivity, but keeps the cell in a state preacclimated for a return to higher light, as for example through mixing. Evolution may favor this conservative strategy of prolonged down-regulation over more rapid relaxation of downregulation to maximize short-term productivity at the risk of oxidative damage to the cell. The kinetic overlap of PSII photoinactivation and repair (also summarized under the term qI, photoinhibitory quenching) with the induction and relaxation of qE can be particularly problematic for mechanistic characterization of the photophysiology of diatoms, where large amplitude inductions of qE-like processes (see "3.3 Non-Photochemical fluorescence Quenching (NPQ)") can persist in darkness, requiring low light for timely relaxation (Goss et al. 2006), thereby becoming difficult to distinguish from the overlapping influence of PSII repair under low light or even in darkness [START_REF] Li | A hard day's night: Diatoms continue recycling photosystem II in the dark[END_REF].

PSII repair

Photoinactivation of PSII is countered by a PSII repair cycle [START_REF] Nixon | Recent advances in understanding the assembly and repair of photosystem II[END_REF][START_REF] Komenda | Assembling and maintaining the Photosystem II complex in chloroplasts and cyanobacteria[END_REF][START_REF] Nishiyama | Revised scheme for the mechanism of photoinhibition and its application to enhance the abiotic stress tolerance of the photosynthetic machinery[END_REF]) that proteolytically removes photoinactivated subunit(s), in concert with subunit replacement through translation mediated by chloroplastic ribosomes. It is critical to note that although photoinactivation of PSII may precede recovery through repair, the two processes show distinct kinetics and light responses. PSII photoinactivation rate shows a nearly linear dependence upon instantaneous excitation levels ( [START_REF] Oguchi | Operation of dual mechanisms that both lead to photoinactivation of Photosystem II in leaves by visible light[END_REF][START_REF] Vass | Molecular mechanisms of photodamage in the Photosystem II complex[END_REF][START_REF] Campbell | Photoinhibition of Photosystem II in Phytoplankton: Processes and Patterns[END_REF]. In contrast, PSII repair in diatoms reaches saturation under fairly low light [START_REF] Wu | Photosystem II photoinactivation, repair and protection in marine centric diatoms[END_REF]. Under some conditions diatoms can therefore accumulate considerable pools of [PSII]inactive [START_REF] Wu | Distinctive photosystem II photoinactivation and protein dynamics in marine diatoms[END_REF][START_REF] Wu | Photosystem II photoinactivation, repair and protection in marine centric diatoms[END_REF][START_REF] Campbell | Photosystem II protein clearance and FtsH function in the diatom Thalassiosira pseudonana[END_REF][START_REF] Lavaud | Photosystem II repair in marine diatoms with contrasting photophysiologies[END_REF] representing intermediate stages in the PSII repair cycle [START_REF] Grouneva | The thylakoid membrane proteome of two marine diatoms outlines both diatom-specific and species-specific features of the photosynthetic machinery[END_REF]. This temporal offset between photoinactivation and repair can result in a tradeoff between strategies. A 'just-in-time' repair strategy requires cellular investment in sufficient instantaneous PSII repair capacity to counter PSII photoinactivation immediately, with sufficient metabolic capacity allocated to run key proteases and chloroplastic ribosome activity.

In contrast, a 'warehouse' strategy involves protein investment into larger pools of [PSII]inactive waiting around for eventual repair. In T. pseudonana, the burden of maintaining PSII repair is low under low light, as slow rates of photoinactivation mean that PSII must only be replaced on generational time scales [START_REF] Li | The nitrogen costs of photosynthesis in a diatom under current and future pCO2[END_REF]. The low light cells thus follow a 'just-in-time' strategy. As growth light increases, the burden of PSII repair grows, until under light above the growth optima maintenance of PSII repair comes to dominate chloroplastic nitrogen metabolism, thereby restricting capacity for growth and forcing the cells towards a 'warehouse' strategy where [PSII]inactive waits for repair. [START_REF] Behrenfeld | Compensatory changes in Photosystem II electron turnover rates protect photosynthesis from photoinhibition[END_REF] This implies a 'warehouse' strategy with an offset between photoinactivation and later repair. Interestingly, in diatoms the PsbA (D1) & PsbD (D2) subunits both turnover at similar rates [START_REF] Wu | Distinctive photosystem II photoinactivation and protein dynamics in marine diatoms[END_REF][START_REF] Wu | Photosystem II photoinactivation, repair and protection in marine centric diatoms[END_REF][START_REF] Campbell | Photosystem II protein clearance and FtsH function in the diatom Thalassiosira pseudonana[END_REF][START_REF] Li | A hard day's night: Diatoms continue recycling photosystem II in the dark[END_REF], contrary to text book descriptions which focus on PsbA turnover. We have not pursued the implications as to whether PsbA or PsbD are both subject to direct photoinactivation in parallel, or whether turnover of PsbD simply closely tracks primary photoinactivation of PsbA.

The FtsH protease complex is a membrane associated structure usually comprised of six FtsH isoform subunits, implicated as the primary protease mediating removal of PsbA from thylakoid membranes [START_REF] Nixon | Recent advances in understanding the assembly and repair of photosystem II[END_REF][START_REF] Komenda | Assembling and maintaining the Photosystem II complex in chloroplasts and cyanobacteria[END_REF], along with roles in turnover of other complexes [START_REF] Malnoë | Thylakoid Ftsh protease contributes to photosystem II and cytochrome b6f remodeling in Chlamydomonas reinhardtii under stress conditions[END_REF]. Diatom capacity for PsbA turnover is predictable from the FtsH:PsbA ratio which in turn varies depending upon the predicted light dynamics of the original source habitats of strains [START_REF] Campbell | Photosystem II protein clearance and FtsH function in the diatom Thalassiosira pseudonana[END_REF]). The 'just-in-time' strategy requires a higher FtsH:PsbA ratio than the 'warehouse' strategy. Furthermore, PSII repair continues in the dark in diatoms [START_REF] Li | A hard day's night: Diatoms continue recycling photosystem II in the dark[END_REF]), which we hypothesize is driven by tight integration of chloroplastic and mitochondrial metabolism to fuel the chloroplastic ATP requirements for FtsH, RNA transcription and translation [START_REF] Prihoda | Chloroplastmitochondria cross-talk in diatoms[END_REF]Bailleul et al. 2015). This capacity for dark repair of previously photoinactivated PSII allows for temporal offsets between episodes of net photoinactivation under high light with transient accumulation of [PSII]inactive and subsequent counteracting repair in low light or darkness, and could thereby contribute to the success of diatoms in environments of fluctuating light. Furthermore, dark turnover of PsbA may contribute to a 'warehouse' strategy where [PSII]inactive is recycled in the dark.

Although large centric diatoms show lower susceptibility to photoinactivation than their smaller cousins, they can also show 'luxury' turnover of PSII subunits beyond the rates predicted by the photoinactivation [START_REF] Li | A hard day's night: Diatoms continue recycling photosystem II in the dark[END_REF]. This surprising finding suggests that turnover of subunits can use both [PSII]active and [PSII]inactive as substrates, with a repair system only imperfectly discriminating in favor of [PSII]inactive. The three layer organization of diatom thylakoids may alter or restrict the interaction of the FtsH protease complex with PSII, possibly lowering capacity to discriminate in favor of [PSII]inactive [START_REF] Komenda | The exposed n-terminal tail of the D1 subunit is required for rapid D1 degradation during photosystem II repair in Synechocystis sp PCC 6803[END_REF], and thereby implying a continuous repair process where the lifetime of a PsbA (or PsbD) is determined by lateral delivery to FtsH rather than by the rate of instantaneous photoinactivation. Recently it was shown that photodamaged PSII reaction centers are physically separated from intact PSII complexes and form repair center clusters [START_REF] Levitan | Structural and functional analyses of photosystem II in the marine diatom Phaeodactylum tricornutum[END_REF]. A particular steric puzzle is how any PSII housed in the central of the three thylakoid layers can interact with ribosomes and membrane associated FtsH complexes, but these repair centers may be located rather at the outer layers or the thylakoid margins. As a final idiosyncrasy [START_REF] Grouneva | The thylakoid membrane proteome of two marine diatoms outlines both diatom-specific and species-specific features of the photosynthetic machinery[END_REF] found only a single, plastid encoded FtsH protein isoform in diatom thylakoids (Maxwell, Grouneva, Aro & Campbell; unpublished). In contrast, in other eukaryotic photoautotrophs multiple FtsH protein isoforms are encoded by genes in the nucleus and imported into the chloroplast to form a hetero-hexameric FtsH structure. Thus, either diatoms carry an unusual hexamer of a single isoform of FtsH; or additional divergent subunits have not as yet been annotated as contributing to the chloroplastic FtsH complex of diatoms; or PSII repair in diatoms involves alternate paths.

Photoprotection in diatoms

When the amount of light absorbed by the light harvesting system exceeds the capacity of the photosynthetic electron transport chain to transfer electrons from the sites of photochemistry to electron acceptors, which also depends on energy consuming pathways such as e.g. the CBB cycle, photorespiration or nitrite reduction, high concentrations of ROS both at PSII and PSI can be produced. On one hand, ROS have various signaling functions well documented in plants and green algae [START_REF] Dietz | Redox-and reactive oxygen species-dependent signaling into and out of the photosynthesizing chloroplast[END_REF]) and there is some evidence that ROS may be signaling substances also in diatoms [START_REF] Lepetit | Light signaling in photosynthetic eukaryotes with 'green' and 'red' chloroplasts[END_REF]. On the other hand, ROS are strong oxidants and if produced in large amounts and not detoxified, they will eventually kill the cells [START_REF] Havaux | Vitamin E protects against photoinhibition and photooxidative stress in Arabidopsis thaliana[END_REF][START_REF] Petroutsos | A blue-light photoreceptor mediates the feedback regulation of photosynthesis[END_REF]) (see also "1.1 The role of pmf as a master regulator of photosynthesis"). Hence, all photosynthetic organisms have various strategies to cope with excess light. These strategies include mechanisms to preempt the formation of ROS as well as strategies to deal with the consequences of high levels of ROS. The latter strategy involves both antioxidative agents to directly quench ROS and the capacity to remove and replace photodamaged proteins (see "2.2 PSII Repair").

Detoxification of reactive oxygen species

Carotenoids, besides their role in light harvesting, are important antioxidants. The classical example is β-carotene in the reaction center of PSII, which quenches 1 O2 generated via energy transfer from 3 Chl [START_REF] Latowski | Carotenoids involved in antioxidant system of chloroplasts[END_REF]). Interestingly, diatoms can accumulate huge amounts of xanthophyll cycle (XC) carotenoids (namely diadinoxanthin (Dd) and its de-epoxidized form diatoxanthin (Dt)) in the thylakoid membrane upon prolonged high light acclimation [START_REF] Lepetit | Evidence for the existence of one antenna-associated, lipid-dissolved, and two protein-bound pools of diadinoxanthin cycle pigments in diatoms[END_REF], which is triggered by a reduced PQ pool [START_REF] Lepetit | High light acclimation in the secondary plastids containing diatom Phaeodactylum tricornutum is triggered by the redox state of the plastoquinone pool[END_REF]. For dynamic photoprotection, however, the effective molecule is Dt, whose concentration is depending on XC activity, triggered by the ΔpH (see "3.4 The molecular regulation and characteristics of qE"). The larger the ΔpH, the more Dt is synthesized in the XC. Dt fulfills a dual role: When bound to specific light harvesting proteins, Dt mediates thermal dissipation of absorbed energy via qE, i.e. this absorbed energy cannot be used to drive photosynthesis. When dissolved in the thylakoid lipid membrane (see "Photosynthetic light reactions in diatoms. I. The lipids and light-harvesting complexes of the thylakoid membrane"), Dt acts as a strong antioxidant [START_REF] Lepetit | Evidence for the existence of one antenna-associated, lipid-dissolved, and two protein-bound pools of diadinoxanthin cycle pigments in diatoms[END_REF]. As constitutive thermal dissipation is not desirable for diatom cells especially under fluctuating light conditions and the pH regulated diadinoxanthin de-epoxidase de-epoxidizes both protein bound and membrane dissolved Dd, there is no constitutive antioxidative Dt pool in the thylakoid membrane under low and moderate light conditions. Tocopherols are potent antioxidants (Demmig-Adams and Adams 2002; [START_REF] Havaux | Vitamin E protects against photoinhibition and photooxidative stress in Arabidopsis thaliana[END_REF].

Diatoms possess a full plastid targeted tocopherol biosynthesis pathway based on KEGG predictions, which is different from other heterokonts/stramenopiles [START_REF] Nonoyama | Metabolic innovations underpinning the origin and diversification of the diatom chloroplast[END_REF]. Indeed, tocopherol biosynthesis proteins are present in diatom plastid fractions [START_REF] Schober | Organelle studies and proteome analyses of mitochondria and plastids fractions from the diatom Thalassiosira pseudonana[END_REF]) and pronounced amounts of tocopherol are found in diatom thylakoid membranes, which increase, similarly to the XC pool, upon acclimation to prolonged high light (Fig. 2). As the antioxidative potential of tocopherols even increases in the presence of carotenoids [START_REF] Böhm | Carotenoids enhance vitamin E antioxidant efficiency[END_REF], tocopherols together with the high amounts of membrane dissolved carotenoids provide diatom thylakoids with a strong antioxidative capacity. ] Figure 2 Amount of XC pigments (sum of Dd and Dt) and α-tocopherol, normalized on chlorophyll a, in thylakoids of P. tricornutum and Cyclotella meneghiniana. Diatoms were grown under low light (LL, 40 µmol photons m -2 s -1 ) or high light (HL, 180 µmol photons m -2 s -1 ) for several generations before thylakoid isolation and pigment quantification was performed as described in [START_REF] Lepetit | Evidence for the existence of one antenna-associated, lipid-dissolved, and two protein-bound pools of diadinoxanthin cycle pigments in diatoms[END_REF]. α-tocopherol separation and quantification was performed by Kerstin Flieger (University of Leipzig, Germany) using a reversed phase column in an HPLC with a coupled fluorescence detector.

Furthermore, the plastidic levels of superoxide and H2O2 can be lowered via superoxide dismutase and ascorbate peroxidase, respectively. While this route exists in diatoms, its respective importance is not clarified yet and species-specific differences do exist [START_REF] Rijstenbil | Effects of periodic, low UVA radiation on cell characteristics and oxidative stress in the marine planktonic diatom Ditylum brightwellii[END_REF][START_REF] Rijstenbil | Effects of UVB radiation and salt stress on growth, pigments and antioxidative defence of the marine diatom Cylindrotheca closterium[END_REF][START_REF] Roncarati | Photosynthetic performance, oxidative damage and antioxidants in Cylindrotheca closterium in response to high irradiance, UVB radiation and salinity[END_REF][START_REF] Waring | Light-induced responses of oxygen photoreduction, reactive oxygen species production and scavenging in two diatom species[END_REF]. Other antioxidants such as ascorbic acid, glutathione and phenolic compounds including flavonoids complement the antioxidative capacity of diatoms, but huge species-specific differences in their concentrations are apparent [START_REF] Foo | Antioxidant capacities of fucoxanthin-producing algae as influenced by their carotenoid and phenolic contents[END_REF][START_REF] Smerilli | Photoprotective and antioxidant responses to light spectrum and intensity variations in the coastal diatom Skeletonema marinoi[END_REF][START_REF] Smerilli | Antioxidant and photoprotection networking in the coastal diatom Skeletonema marinoi[END_REF].

Prevention of reactive oxygen species formation

Besides detoxifying ROS and ROS generated products, carotenoids also protect photosynthetic organisms including diatoms before ROS can actually be created. They are involved in NPQ discussed in chapter 3.3, but carotenoids are also very effective quenchers of 3 Chl if orientated closely enough and thereby preempting the formation of 1 O2 [START_REF] Triantaphylidès | Singlet oxygen in plants: production, detoxification and signaling[END_REF]. Other strategies to avoid the formation of ROS include photorespiration as well as various AEFs (see "1.2 Alternative electron flows provide photoprotection and flexibility in response to metabolic demand"). Interestingly, reversible plastid movement in response to light intensity has been observed in the diatom Pleurosira laevis, thus increasing the absorption cross section under low light conditions to maximize light harvesting, but decreasing it under high light conditions due to plastid crowding in the cell center and resulting self-shading [START_REF] Furukawa | Green-and blue-light-mediated chloroplast migration in the centric diatom Pleurosira laevis[END_REF]. Whether such a phenomenon can contribute to photoprotection in other diatoms remains questionable, as in the diatom Coscinodiscus granii plastid movement is also induced after prolonged high light exposure, which is not reversible and is correlated with cell death though [START_REF] Goessling | Photo-protection in the centric diatom Coscinodiscus granii is not controlled by chloroplast high-light avoidance movement[END_REF].

Non-Photochemical fluorescence Quenching (NPQ)

NPQ is a collective term for different phenomena, which all act to lower photosystem II fluorescence. However, these phenomena are distinct from the suppression of fluorescence caused by the onset of photochemical quenching, i.e. the oxidation of the primary PSII electron acceptor QA by a drain of electrons from the ETR due to consumption of NADPH and ATP in the CBB cycle. Importantly, it is by measuring fluorescence that NPQ can be tracked in the lab, but for cellular physiology the amount of fluorescence emission is less relevant than the two other major processes an excited Chl* a electron can use to revert to the ground state (heat dissipation, photochemistry). A decrease in fluorescence by NPQ can have different origins, and several distinct NPQ components are described in plants, see e.g. [START_REF] Derks | Diverse mechanisms for photoprotection in photosynthesis. Dynamic regulation of photosystem II excitation in response to rapid environmental change[END_REF], [START_REF] Malnoë | Photoinhibition or photoprotection of photosynthesis? Update on the (newly termed) sustained quenching component qH[END_REF], and [START_REF] Pinnola | Molecular mechanisms involved in plant photoprotection[END_REF]. Often, but not always, an increase in NPQ indicates an increase in photoprotection [START_REF] Li | PsbS-dependent enhancement of feedback de-excitation protects photosystem II from photoinhibition[END_REF][START_REF] Takahashi | Photoprotection in plants: a new light on photosystem II damage[END_REF].

There are three classical, primarily kinetically defined NPQ components: qE (energy dependent quenching), qI (photoinhibitory quenching), and qT (quenching based on state transitions). qE comprises photoprotection mechanisms which are rapidly induced upon a generation of a Δph (usually upon exposure to excess light conditions) over time scale of seconds to minutes. qE is also rapidly relaxed upon return to non-saturating light conditions. qI describes processes characterized by a much slower recovery under low light or dark conditions than qE related processes. By virtue of this kinetic definition, qI encompasses drops in fluorescence yield induced by damaged or inactivated PSII reaction centers (see "2.1 Kinetic overlap of photoinactivation and downregulation of PSII"), but also processes induced by a sustained thermal dissipation independent of PSII damage, e.g. in evergreen plants during winter.

Therefore, for plants the term qZ was introduced to distinguish some of these slowly recovering processes by their underlying mechanism. qZ describes thermal dissipation processes, which are formed within minutes and exclusively rely on the de-epoxidation of violaxanthin via antheraxanthin to zeaxanthin, independent of the qE providing protein PsbS [START_REF] Nilkens | Identification of a slowly inducible zeaxanthin-dependent component of nonphotochemical quenching of chlorophyll fluorescence generated under steady-state conditions in Arabidopsis[END_REF]. Its relaxation also follows strictly the epoxidation of zeaxanthin via antheraxanthin to violaxanthin. In diatoms, recent works start to disentangle the various qI components by introducing the sustained NPQ (NPQs) parameter, which describes within qI those slowly recovering NPQ processes that are independent of PSII damage and repair [START_REF] Wu | Photosystem II photoinactivation, repair and protection in marine centric diatoms[END_REF][START_REF] Lacour | The role of sustained photoprotective non-photochemical quenching in low temperature and high light acclimation in the bloom-forming arctic diatom Thalassiosira gravida[END_REF].

Despite some debates, qT is well understood in the green lineage. qT includes a reversible movement of LHCII antennae between PSII and PSI, where these LHCII antennae modulate the PSII and PSI absorption cross section [START_REF] Nawrocki | State transitions redistribute rather than dissipate energy between the two photosystems in Chlamydomonas[END_REF][START_REF] Pan | Structure of the maize photosystem I supercomplex with light-harvesting complexes I and II[END_REF]. Also, state transitions are often paralleled by a change in thylakoid macrostructure, with fewer thylakoid stacked regions when LHCII detaches from PSII and shuttles towards PSI (Chuartzman et al. 2008;[START_REF] Nagy | Chloroplast remodeling during state transitions in Chlamydomonas reinhardtii as revealed by noninvasive techniques in vivo[END_REF]. The trigger for state transitions is the redox state of the PQ pool, which is sensed via a thylakoidal kinase, STN7 in plants, STT7 in Chlamydomonas [START_REF] Depege | Role of chloroplast protein kinase Stt7 in LHCII phosphorylation and state transition in Chlamydomonas[END_REF][START_REF] Bellafiore | State transitions and light adaptation require chloroplast thylakoid protein kinase STN7[END_REF]. A reduced PQ pool activates STN7/STT7, which then phosphorylates LHCII and thus induces its detachment from PSII. Diatoms do not display stacked and non-stacked thylakoidal regions under normal growth conditions, which historically has been used to argue against a possible existence of state transitions. Moreover, in the diatom P. tricornutum previous experiments indicated the absence of state transitions [START_REF] Owens | Light-harvesting function in the diatom Phaeodactylum tricornutum: II. Distribution of excitation energy between the photosystems[END_REF]. In this study, a protocol adapted from plants was applied which uses green light as 'PSII light' and far-red light as 'PSI light'. We meanwhile know that there is no strict and stable discrimination of far-red absorption in PSII in P. tricornutum, as also PSII can absorb in the far-red due to the presence of Lhcf15 [START_REF] Herbstová | Molecular basis of chromatic adaptation in pennate diatom Phaeodactylum tricornutum[END_REF][START_REF] Herbstová | Red-light phenotype in a marine diatom involves a specialized oligomeric red-shifted antenna and altered cell morphology[END_REF]) (see "Photosynthetic light reactions in diatoms. I. The lipids and light-harvesting complexes of the thylakoid membrane"). Interestingly, in P. tricornutum there is a putative kinase with strong homology to STT7/STN7, whose function is as yet unclear. Moreover, time resolved fluorescence analyses revealed a spectral component developing under NPQ conditions, which the authors speculated to result from a transfer of some PSII antennae to PSI [START_REF] Miloslavina | Ultrafast fluorescence study on the location and mechanism of non-photochemical quenching in diatoms[END_REF]. Given the fact that not all aspects of diatom NPQ assigned to qE are fully clarified yet (see next chapter) and that most studies on diatom NPQ characteristics have been performed on only a handful of species while there are ~ 100,000 diatom species to explore [START_REF] Mann | An inordinate fondness? The number, distributions, and origins of diatom species[END_REF], the absence of state transitions in diatoms should not been taken for granted yet.

The molecular regulation and characteristics of qE

In recent years, major understandings of the molecular regulation of qE in diatoms have been acquired. In addition, there are numerous reports on the ecophysiological distribution and importance of qE in diatoms (see "4. Variable adjustment of diatom light-response in different ecosystems"). qE strongly depends on the presence of the xanthophyll pigment Dt [START_REF] Arsalane | Influence of the pool size of the xanthophyll cycle on the effects of light stress in a diatom -competition between photoprotection and photoinhibition[END_REF][START_REF] Olaizola | Non-photochemical fluorescence quenching and the diadinoxanthin cycle in a marine diatom[END_REF]Lavaud et al. 2002b;[START_REF] Goss | The importance of a highly active and delta pH regulated diatoxanthin epoxidase for the regulation of the PSII antenna function in diadinoxanthin cycle containing algae[END_REF][START_REF] Park | Dynamic response of the transcriptome of a psychrophilic diatom, Chaetoceros neogracile, to high irradiance[END_REF][START_REF] Lavaud | An explanation for the inter-species variability of the photoprotective non-photochemical chlorophyll fluorescence quenching in diatoms[END_REF][START_REF] Roháček | Relaxation of the non-photochemical chlorophyll fluorescence quenching in diatoms: kinetics, components and mechanisms[END_REF][START_REF] Blommaert | Contrasting NPQ dynamics and xanthophyll cycling in a motile and a non-motile intertidal benthic diatom[END_REF][START_REF] Lacour | Diversity in xanthophyll cycle pigments content and related non-photochemical quenching (NPQ) among microalgae: implications for growth strategy and ecology[END_REF]). This pigment is produced from Dd via action of a de-epoxidase in the presence of a ΔpH across the thylakoid membrane [START_REF] Hager | Das Carotinoidmuster und die Verbreitung des lichtinduzierten Xanthophyllcyclus in verschiedenen Algenklassen[END_REF]. Usually, this happens in excess light when the photosynthetic light reactions are limited by what the CBB cycle can metabolize (oxidation of NADPH, hydrolysis of ATP). However, the de-epoxidation also occurs to some extent in dark conditions, likely due to a backwards reaction of the thylakoidal ATP synthase pumping protons into the lumen (Bailleul et al. 2015) or possibly some kind of 'chlororespiration' [START_REF] Jakob | Activation of diadinoxanthin de-epoxidase due to a chlororespiratory proton gradient in the dark in the diatom Phaeodactylum tricornutum[END_REF]) (see also "1.5 Chlororespiration, redox state of the plastoquinone pool, and pmf in the dark").

Dt is converted back to Dd in the absence of a pronounced ∆pH, i.e. under low light and to some extent also dark conditions, catalyzed via an epoxidase which then operates faster than the de-epoxidase [START_REF] Blommaert | The fine-tuning of NPQ in diatoms relies on the regulation of both xanthophyll cycle enzymes[END_REF]. Interestingly, after a switch from high light the epoxidation reaction in diatoms is faster under low light conditions than in darkness (Goss et al. 2006;[START_REF] Blommaert | Contrasting NPQ dynamics and xanthophyll cycling in a motile and a non-motile intertidal benthic diatom[END_REF]. As the epoxidase requires NADPH as co-substrate, its synthesis through photochemical reactions under low light conditions (not in darkness) in combination with a putatively small NADPH/NADP pool has been proposed to cause this peculiar feature [START_REF] Goss | The importance of a highly active and delta pH regulated diatoxanthin epoxidase for the regulation of the PSII antenna function in diadinoxanthin cycle containing algae[END_REF]), but also another mode of redox control has been postulated recently [START_REF] Blommaert | The fine-tuning of NPQ in diatoms relies on the regulation of both xanthophyll cycle enzymes[END_REF]. The XC of diatoms shows further differences compared to the green lineage: The epoxidation reaction of diatoms is much faster [START_REF] Goss | The importance of a highly active and delta pH regulated diatoxanthin epoxidase for the regulation of the PSII antenna function in diadinoxanthin cycle containing algae[END_REF]), but gets downregulated under high light, possibly reflecting its regulation by the stromal pH [START_REF] Mewes | Supplementary ultraviolet-b radiation induces a rapid reversal of the diadinoxanthin cycle in the strong light-exposed diatom Phaeodactylum tricornutum[END_REF][START_REF] Goss | The importance of a highly active and delta pH regulated diatoxanthin epoxidase for the regulation of the PSII antenna function in diadinoxanthin cycle containing algae[END_REF] or redox regulation [START_REF] Blommaert | The fine-tuning of NPQ in diatoms relies on the regulation of both xanthophyll cycle enzymes[END_REF]). The de-epoxidase is activated at a much lower ∆pH than in the greens [START_REF] Jakob | Unusual pH-dependence of diadinoxanthin de-epoxidase activation causes chlororespiratory induced accumulation of diatoxanthin in the diatom Phaeodactylum tricornutum[END_REF], especially when high concentrations of the co-substrate ascorbate are available [START_REF] Grouneva | Influence of ascorbate and pH on the activity of the diatom xanthophyll cycle-enzyme diadinoxanthin de-epoxidase[END_REF].

Also, the de-epoxidase of diatoms has a considerable higher affinity for ascorbate [START_REF] Grouneva | Influence of ascorbate and pH on the activity of the diatom xanthophyll cycle-enzyme diadinoxanthin de-epoxidase[END_REF]. Finally, in contrast to the 'greens', the abolishment of the ∆pH does not directly remove qE. As long as a Dt is present, it still provides qE [START_REF] Goss | The importance of a highly active and delta pH regulated diatoxanthin epoxidase for the regulation of the PSII antenna function in diadinoxanthin cycle containing algae[END_REF]. Hence, the amount of short-term high light induced de-epoxidation of Dd to Dt is usually linearly correlated to the establishment of qE in diatoms, while the epoxidation of Dt back to Dd is linearly correlated to the relaxation of qE (Lavaud et al. 2002a;[START_REF] Goss | The importance of a highly active and delta pH regulated diatoxanthin epoxidase for the regulation of the PSII antenna function in diadinoxanthin cycle containing algae[END_REF][START_REF] Lavaud | An explanation for the inter-species variability of the photoprotective non-photochemical chlorophyll fluorescence quenching in diatoms[END_REF][START_REF] Blommaert | Contrasting NPQ dynamics and xanthophyll cycling in a motile and a non-motile intertidal benthic diatom[END_REF]. In this aspect, the diatom qE rather resembles the qZ mechanism described for plants (see "3.3 Non-Photochemical fluorescence Quenching"), except that diatom qE/qZ operates on a faster timescale. Therefore, the tuning of the XC in diatoms can be best explained in the framework of its utmost importance for induction of qE [START_REF] Wilhelm | The regulation of carbon and nutrient assimilation in diatoms is significantly different from green algae[END_REF]): As changes in the pH of the lumen/stroma only indirectly trigger qE via activation/deactivation of the XC enzymes, the enzymatic reactions must proceed very rapidly.

In line with this, the kinetic properties of both de-epoxidase and epoxidase reaction can be altered in P. tricornutum depending on the long term acclimation to different light intensities [START_REF] Kuczynska | The xanthophyll cycle in diatom Phaeodactylum tricornutum in response to light stress[END_REF]. We must emphasize that in the diatoms C. meneghiniana and Navicula sp. there is a component of qE that is independent of the XC, but rather relies on the establishment of the ∆pH alone [START_REF] Grouneva | A new multicomponent NPQ mechanism in the diatom Cyclotella meneghiniana[END_REF][START_REF] Derks | Rapid regulation of excitation energy in two pennate diatoms from contrasting light climates[END_REF]. So far, mechanistically little is known about this NPQ component, but for C. meneghiniana, [START_REF] Grouneva | A new multicomponent NPQ mechanism in the diatom Cyclotella meneghiniana[END_REF] demonstrated that it requires the initial presence of certain amounts of Dt.

In addition to Dt, thylakoid membrane proteins called Lhcx have a key role for qE in diatoms.

These proteins belong to the Lhc superfamily and usually are encoded by multiple slightly different copies in the diatom genome (see "Photosynthetic light reactions in diatoms. I. The lipids and light-harvesting complexes of the thylakoid membrane"). When individual Lhcx proteins are lowered or increased by reverse genetics, the capacity for qE is simultaneously either lowered or increased (Bailleul et al. 2010b;[START_REF] Ghazaryan | Involvement of the Lhcx protein Fcp6 of the diatom Cyclotella meneghiniana in the macro-organisation and structural flexibility of thylakoid membranes[END_REF]Taddei et al. 2016;[START_REF] Lepetit | The diatom Phaeodactylum tricornutum adjusts nonphotochemical fluorescence quenching capacity in response to dynamic light via fine-tuned Lhcx and xanthophyll cycle pigment synthesis[END_REF][START_REF] Buck | Identification of sequence motifs in Lhcx proteins that confer qE-based photoprotection in the diatom Phaeodactylum tricornutum[END_REF]. Moreover, in various diatom species and communities, Lhcx proteins increase in response to light stress conditions (Bailleul et al. 2010b;[START_REF] Park | Dynamic response of the transcriptome of a psychrophilic diatom, Chaetoceros neogracile, to high irradiance[END_REF]Zhu and Green 2010;[START_REF] Lepetit | High light acclimation in the secondary plastids containing diatom Phaeodactylum tricornutum is triggered by the redox state of the plastoquinone pool[END_REF][START_REF] Laviale | Response of intertidal benthic microalgal biofilms to a coupled light-temperature stress: evidence for latitudinal adaptation along the Atlantic coast of Southern Europe[END_REF][START_REF] Blommaert | Contrasting NPQ dynamics and xanthophyll cycling in a motile and a non-motile intertidal benthic diatom[END_REF][START_REF] Lepetit | The diatom Phaeodactylum tricornutum adjusts nonphotochemical fluorescence quenching capacity in response to dynamic light via fine-tuned Lhcx and xanthophyll cycle pigment synthesis[END_REF]Taddei et al. 2018). Other stressors such as iron, copper, phosphorus, or nitrogen limitation increase the amount of some Lhcx, too, also followed by an increase of qE capacity (Zhu et al. 2010;Taddei et al. 2016;[START_REF] Helliwell | A novel Ca 2+ signalling pathway co-ordinates environmental phosphorus sensing and nitrogen metabolism in marine diatoms[END_REF]. Three (Lhcx1,2,3) of the four Lhcx proteins present in P. tricornutum can confer qE independently of each other, while also synergistically adding additional qE capacity [START_REF] Buck | Identification of sequence motifs in Lhcx proteins that confer qE-based photoprotection in the diatom Phaeodactylum tricornutum[END_REF]. By using multiple functionally complementary Lhcx genes whose expression is dependent on different environmental triggers, diatoms tune their qE capacity depending on the environmental constraints. Interestingly, the Lhcx4 protein in P. tricornutum cannot induce qE by itself [START_REF] Buck | Identification of sequence motifs in Lhcx proteins that confer qE-based photoprotection in the diatom Phaeodactylum tricornutum[END_REF], although it might do so on top of Lhcx1 mediated NPQ (Taddei et al. 2016). Lhcx4 is also regulated differently, being strongly upregulated under constant darkness (Nymark et al. 2009;Taddei et al. 2016).

Lhcx proteins intimately interact with Dt to provide qE. In P. tricornutum, neither Dt nor Lhcx are able to induce any qE individually [START_REF] Buck | Identification of sequence motifs in Lhcx proteins that confer qE-based photoprotection in the diatom Phaeodactylum tricornutum[END_REF], indicating that both components are involved in the same mechanism. Lhcx and Dt induced qE reduces the functional absorption cross section of PSII (σPSII) [START_REF] Kuzminov | Energy dissipation pathways in Photosystem 2 of the diatom, Phaeodactylum tricornutum, under high-light conditions[END_REF][START_REF] Giovagnetti | Detachment of the fucoxanthin chlorophyll a/c binding protein (FCP) antenna is not involved in the acclimative regulation of photoprotection in the pennate diatom Phaeodactylum tricornutum[END_REF][START_REF] Xu | Phytoplankton σPSII and Excitation Dissipation; Implications for estimates of primary productivity[END_REF][START_REF] Buck | Identification of sequence motifs in Lhcx proteins that confer qE-based photoprotection in the diatom Phaeodactylum tricornutum[END_REF]. Strikingly, when extrapolating the linear relationship between decrease of σPSII and increase of qE to a thermal dissipation rate of 100%, a σPSII value remains which is the size of a dimeric PSII core without any peripheral antenna both in P. tricornutum [START_REF] Buck | Identification of sequence motifs in Lhcx proteins that confer qE-based photoprotection in the diatom Phaeodactylum tricornutum[END_REF]) and in T. pseudonana [START_REF] Xu | Phytoplankton σPSII and Excitation Dissipation; Implications for estimates of primary productivity[END_REF]). Thus, most likely Lhcx and Dt induce a disconnection of the outer antennae from the PSII core, resulting in the strong drop in σPSII [START_REF] Buck | Identification of sequence motifs in Lhcx proteins that confer qE-based photoprotection in the diatom Phaeodactylum tricornutum[END_REF]. Such an antennae detachment is corroborating previous results obtained by time resolved fluorescence spectroscopy [START_REF] Miloslavina | Ultrafast fluorescence study on the location and mechanism of non-photochemical quenching in diatoms[END_REF]). In this model, the antenna detachment thereby preempts flow of excitation energy from the antenna to the PSII core, lowering excitation pressure on the photosynthetic system.

How the fluorescence in the detached antennae complexes is actually quenched is still a matter of debate. Lhcx genes (also termed LhcSR/Li818) are spread throughout the eukaryotic photosynthetic kingdom, being present in most taxa which rely on light harvesting proteins as main antennae, with the major exception of land plants [START_REF] Dittami | Chlorophyll-binding proteins revisited-a multigenic family of light-harvesting and stress proteins from a brown algal perspective[END_REF][START_REF] Blommaert | Light regulation of Lhcx genes in the benthic diatom Seminavis robusta[END_REF]. So far, an involvement of Lhcx/LhcSR proteins in qE, besides in diatoms, has been demonstrated in the green alga C. reinhardtii [START_REF] Peers | An ancient light-harvesting protein is critical for the regulation of algal photosynthesis[END_REF]), the eustigmatophyte Nannochloropsis oceanica [START_REF] Park | Chlorophyll-carotenoid excitation energy transfer and charge transfer in Nannochloropsis oceanica for the regulation of photosynthesis[END_REF], and the moss Physcomitrella patens [START_REF] Alboresi | Physcomitrella patens mutants affected on heat dissipation clarify the evolution of photoprotection mechanisms upon land colonization[END_REF]. Insights from these different organisms may be extrapolated to get clues about the nature of diatom quenching. Based on ultrafast transient absorption and fluorescence spectroscopy measurements, multiple different quenching mechanisms including chlorophyllcarotenoid excitation energy transfer as well as chlorophyll-chlorophyll and chlorophyllcarotenoid charge transfer states have been suggested to be responsible for qE in plants and green algae (summarized in de [START_REF] De La | Molecular mechanisms of nonphotochemical quenching in the LhcsSR3 protein of Chlamydomonas reinhardtii[END_REF][START_REF] Bennett | Models and mechanisms of the rapidly reversible regulation of photosynthetic light harvesting[END_REF]. This ambiguity may partly be due to two major facts: i) In order to avoid scattering artifacts, ultrafast spectroscopy experiments are often recorded on isolated thylakoids or pigment protein complexes which need to be specifically treated to induce NPQ (such as by lowering pH or removing detergent). Whether the so induced quenching complexes actually represent the in vivo situation remains questionable (see also [START_REF] Elnour | Energy dissipation mechanisms in the FCPb light-harvesting complex of the diatom Cyclotella meneghiniana[END_REF]. Also, for interpretation of the different spectroscopic features, one needs to impose several assumptions on them by referring to spectra obtained from other individually measured pigment protein complexes, the latter also not necessarily reflecting the true in vivo situation.

ii) These ultrafast spectroscopy experiments are experimentally challenging, with difficulties to find the 'right' experimental laser power and exposure time to avoid artefacts due to bleaching and annihilation processes or due to generation of qI phenomena (see [START_REF] Van Oort | Revisiting the role of xanthophylls in nonphotochemical quenching[END_REF][START_REF] Buck | Identification of sequence motifs in Lhcx proteins that confer qE-based photoprotection in the diatom Phaeodactylum tricornutum[END_REF]).

Because of these technical challenges, [START_REF] Van Oort | Revisiting the role of xanthophylls in nonphotochemical quenching[END_REF] argued that many conclusions based on ultrafast spectroscopic experiment are actually experimental artifacts. As even in the green line the nature of the quenching pigments is unclear, in diatoms there is currently no solid idea of the pigments involved in the actual heat dissipation process.

However, several reports indicate the presence of two major qE sites in diatoms. One site is assigned directly dependent on Dt and resides at the PSII core, while the other one is rather induced by ∆pH and located in detached antennae complexes [START_REF] Miloslavina | Ultrafast fluorescence study on the location and mechanism of non-photochemical quenching in diatoms[END_REF][START_REF] Chukhutsina | Disentangling two non-photochemical quenching processes in Cyclotella meneghiniana by spectrally-resolved picosecond fluorescence at 77K[END_REF][START_REF] Giovagnetti | Detachment of the fucoxanthin chlorophyll a/c binding protein (FCP) antenna is not involved in the acclimative regulation of photoprotection in the pennate diatom Phaeodactylum tricornutum[END_REF]Taddei et al. 2018). Such an interpretation is, however, difficult to reconcile with the prime dependency of diatom qE on Dt, with the ∆pH rather having an indirect effect on qE by regulating the XC. We assume that these results may, at least partially, be cofounded by the problems described above and may instead rather represent a mixture of qE and qI processes [START_REF] Kuzminov | Energy dissipation pathways in Photosystem 2 of the diatom, Phaeodactylum tricornutum, under high-light conditions[END_REF][START_REF] Buck | Identification of sequence motifs in Lhcx proteins that confer qE-based photoprotection in the diatom Phaeodactylum tricornutum[END_REF]). Also, there may exist diatom species specific differences, with Cyclotella possessing a more complex qE behavior than P. tricornutum [START_REF] Grouneva | A new multicomponent NPQ mechanism in the diatom Cyclotella meneghiniana[END_REF]) which could explain the observation of even a third qE site in Cyclotella [START_REF] Chukhutsina | Disentangling two non-photochemical quenching processes in Cyclotella meneghiniana by spectrally-resolved picosecond fluorescence at 77K[END_REF]). For P. tricornutum there is strong evidence that the major qE mechanism is dependent on the presence of Lhcx proteins and the conversion of Dd into Dt, which detaches the peripheral antennae and thus establishes the major site of qE (Fig. 3) [START_REF] Kuzminov | Energy dissipation pathways in Photosystem 2 of the diatom, Phaeodactylum tricornutum, under high-light conditions[END_REF][START_REF] Buck | Identification of sequence motifs in Lhcx proteins that confer qE-based photoprotection in the diatom Phaeodactylum tricornutum[END_REF].

It remains to be elucidated whether Lhcx proteins form actual part of these detached quenching complexes or whether, in concert with Dt, they only induce the uncoupling of peripheral antennae and their transformation into the dissipative state. Different biochemical studies do not provide a conclusive picture about localization of Lhcx proteins yet: Lhcx have been found in preparations of the peripheral antennae [START_REF] Beer | Subunit composition and pigmentation of fucoxanthin-chlorophyll proteins in diatoms: Evidence for a subunit involved in diadinoxanthin and diatoxanthin binding[END_REF][START_REF] Lepetit | Evidence for the existence of one antenna-associated, lipid-dissolved, and two protein-bound pools of diadinoxanthin cycle pigments in diatoms[END_REF]Taddei et al. 2018), PSI [START_REF] Grouneva | The thylakoid membrane proteome of two marine diatoms outlines both diatom-specific and species-specific features of the photosynthetic machinery[END_REF]Taddei et al. 2018) but also in PSII (Taddei et al. 2018[START_REF] Kansy | Pre-purification of diatom pigment protein complexes provides insight into the heterogeneity of FCP complexes[END_REF]. It is clear that LhcSR/Lhcx proteins can bind pigments including substantial amounts of carotenoids [START_REF] Bonente | Analysis of Lhcsr3, a protein essential for feedback de-excitation in the green alga Chlamydomonas reinhardtii[END_REF], strikingly different to PsbS, the protein mediating qE in plants. Hence, in diatoms Lhcx proteins may bind XC pigments, which would be the most parsimonious explanation for the intimate connection of the XC and Lhcx in conferring qE.

Unfortunately, recent x-ray structures of diatom antennae do not include Lhcx proteins [START_REF] Nagao | ) Structural basis for energy harvesting and dissipation in a diatom PSII-FCPII supercomplex[END_REF][START_REF] Pi | The pigment-protein network of a diatom photosystem II-light-harvesting antenna supercomplex[END_REF] and no in vitro Lhcx protein reconstitution including pigments is available yet. However, it was recently shown that some specific part of the Lhcx proteins is essential for providing qE [START_REF] Buck | Identification of sequence motifs in Lhcx proteins that confer qE-based photoprotection in the diatom Phaeodactylum tricornutum[END_REF]. By modeling this modified Lhcx sequence on the structure of Lhcf4, it became clear that especially one prominent tryptophan in this region may most likely coordinate the binding of a Dd/Dt molecule in the second membrane spanning helix, which therefore would provide an explanation for the intimate link of Lhcx and Dt in conferring qE [START_REF] Buck | Identification of sequence motifs in Lhcx proteins that confer qE-based photoprotection in the diatom Phaeodactylum tricornutum[END_REF].

Given the difficulty of identifying Lhcx proteins within isolated pigment protein complexes in diatoms, a rather loose association amongst them is likely. We assume that Lhcx proteins may primarily be located within the PSII supercomplexes, such as in C. reinhardtii [START_REF] Tokutsu | Energy-dissipative supercomplex of photosystem II associated with LHCSR3 in Chlamydomonas reinhardtii[END_REF][START_REF] Scholz | Light-dependent N-terminal phosphorylation of LHCSR3 and LHCB4 are interlinked in Chlamydomonas reinhardtii[END_REF]. As experimental evidence suggests that qE in diatoms involves a disconnection of the peripheral antennae from PSII, we assume a localization of Lhcx rather in between the peripheral antennae and the PSII core complex. Here, the conversion of Dd into Dt bound to Lhcx could induce a conformational shift or rearrangement of Lhcx which leads to a disconnection of the peripheral antennae including a switch into the thermal dissipative state (Fig. 3). The Lhcx proteins could move together with the peripheral antennae or stay associated with PSII. This model is similar to our earlier model [START_REF] Lavaud | The peculiar features of non-photochemical fluorescence quenching in diatoms and brown algae[END_REF][START_REF] Goss | Biodiversity of NPQ[END_REF], but including only one qE quenching site.

Variable adjustment of diatom light-response in different ecosystems

In the recent years, there has been a significant effort in exploring the light-response diversity among diatoms in relation to their growth form and their specific marine ecosystem and habitat.

Diatoms are unique in that they conquered virtually every aquatic habitat, whose characteristics diverge enormously. In that regard, one of the processes that has received the most interest is NPQ and associated XC (see [START_REF] Bertrand | Carotenoid biosynthesis in diatoms[END_REF][START_REF] Brunet | Can the xanthophyll cycle help extract the essence of the microalgal functional response to a variable light environment ?[END_REF][START_REF] Lavaud | The peculiar features of non-photochemical fluorescence quenching in diatoms and brown algae[END_REF][START_REF] Lacour | Diversity in xanthophyll cycle pigments content and related non-photochemical quenching (NPQ) among microalgae: implications for growth strategy and ecology[END_REF]. Different diatom strains and ecotypes grown under similar conditions in stable culture collection conditions for years and even decades retain differences in effective and potential NPQ and related XC and Lhcx features [START_REF] Dimier | Photoprotective capacity as functional trait in planktonic algae: relationship between xanthophyll cycle and ecological characteristics in three diatoms[END_REF][START_REF] Lavaud | Photoprotection capacity differs among diatoms: Possible consequences on the spatial distribution of diatoms related to fluctuations in the underwater light climate[END_REF]Bailleul et al. 2010b;[START_REF] Petrou | Heterogeneity in the photoprotective capacity of three Antarctic diatoms during short-term changes in salinity and temperature[END_REF][START_REF] Barnett | Growth form defines physiological photoprotective capacity in intertidal benthic diatoms[END_REF][START_REF] Blommaert | Contrasting NPQ dynamics and xanthophyll cycling in a motile and a non-motile intertidal benthic diatom[END_REF][START_REF] Croteau | Contrasting nonphotochemical quenching patterns under high light and darkness aligns with light niche occupancy in Arctic diatoms[END_REF]. Different ecotypes and species can show similar Dd+Dt pool sizes and de-epoxidation The quenching efficiency of Dt has been considered as a solid index of the adaptation of a strain to a given habitat with a specific light climate [START_REF] Lacour | Diversity in xanthophyll cycle pigments content and related non-photochemical quenching (NPQ) among microalgae: implications for growth strategy and ecology[END_REF]. Diatoms adapt their fast light-response to the light climate of their habitat, in terms of average light dose and light fluctuations, through the molecular control of NPQ by Dt and Lhcx synthesis. Some authors even proposed a classification of Dd-containing microalgae in three distinct ecological groups based on their XC properties vs. irradiance and light fluctuations [START_REF] Dimier | Photoprotective capacity as functional trait in planktonic algae: relationship between xanthophyll cycle and ecological characteristics in three diatoms[END_REF][START_REF] Dimier | Comparative ecophysiology of the xanthophyll cycle in six marine phytoplanktonic species[END_REF]. The quenching efficiency and other NPQ features are indeed influenced by ecological niche adaptation with no apparent phylogenetic connection, the most obvious example being benthic forms that inhabit estuarine intertidal flats [START_REF] Barnett | Growth form defines physiological photoprotective capacity in intertidal benthic diatoms[END_REF][START_REF] Blommaert | Contrasting NPQ dynamics and xanthophyll cycling in a motile and a non-motile intertidal benthic diatom[END_REF][START_REF] Blommaert | Behavioural versus physiological photoprotection in epipelic and epipsammic benthic diatoms[END_REF]. In turn, NPQ is regarded as a functional trait likely participating in the cost-benefit photoadaptive strategy, the geographical distribution and seasonal dynamics of a species [START_REF] Irwin | Phytoplankton functional types: a trait perspective[END_REF]. NPQ characteristics have been indeed related to i) latitude (Bailleul et al. 2010b;[START_REF] Laviale | Response of intertidal benthic microalgal biofilms to a coupled light-temperature stress: evidence for latitudinal adaptation along the Atlantic coast of Southern Europe[END_REF], ii) different marine [START_REF] Strzepek | Photosynthetic architecture differs in coastal and oceanic diatoms[END_REF][START_REF] Lavaud | Fast regulation of photosynthesis in diatoms: mechanisms, evolution and ecopysiology[END_REF]) and freshwater [START_REF] Shi | Habitat-specific differences in adaptation to light in freshwater diatoms[END_REF]) ecosystems, iii) different habitats of the same ecosystem, i.e. seasonal habitat succession in polar systems [START_REF] Petrou | Heterogeneity in the photoprotective capacity of three Antarctic diatoms during short-term changes in salinity and temperature[END_REF][START_REF] Croteau | Contrasting nonphotochemical quenching patterns under high light and darkness aligns with light niche occupancy in Arctic diatoms[END_REF] or different intertidal sediment types [START_REF] Barnett | Growth form defines physiological photoprotective capacity in intertidal benthic diatoms[END_REF][START_REF] Blommaert | Contrasting NPQ dynamics and xanthophyll cycling in a motile and a non-motile intertidal benthic diatom[END_REF], and iv) seasonal dynamics of phytoplankton in a given ecosystem or habitat [START_REF] Dimier | Photoprotective capacity as functional trait in planktonic algae: relationship between xanthophyll cycle and ecological characteristics in three diatoms[END_REF][START_REF] Polimene | Modelling a light-driven phytoplankton succession[END_REF]). In the following, we will focus on three ecosystems where diatoms are found abundantly: the water column, the coastal intertidal flats and the polar regions.

Diatoms of the phytoplankton

The light climate of planktonic diatoms shows a depth-dependent attenuation of irradiance and modification of the light spectrum [START_REF] Stomp | Colorful niches of phototrophic microorganisms shaped by vibrations of the water molecule[END_REF][START_REF] Jaubert | Light sensing and responses in marine microalgae[END_REF]. The dynamics of the XC and NPQ, together with other fast-responding photophysiological processes such as the PSII cyclic electron cycle (Lavaud et al. 2002c;[START_REF] Lavaud | Photoprotection capacity differs among diatoms: Possible consequences on the spatial distribution of diatoms related to fluctuations in the underwater light climate[END_REF][START_REF] Wagner | Photosystem II cycle activity and alternative electron transport in the diatom Phaeodactylum tricornutum under dynamic light conditions and nitrogen limitation[END_REF], are relevant for the regulation of the photosynthetic productivity of plankton diatoms in the mixed upper layer of the water column [START_REF] Brunet | Can the xanthophyll cycle help extract the essence of the microalgal functional response to a variable light environment ?[END_REF][START_REF] Polimene | Modelling a light-driven phytoplankton succession[END_REF]) and during blooming [START_REF] Fujiki | Photoprotective response of xanthophyll pigments during phytoplankton blooms in Sagami Bay, Japan[END_REF]. Diatoms cope especially well with a fluctuating light climate where they lose less of the absorbed light via alternative electron pathways compared to green algae and thus reach higher growth rates [START_REF] Wagner | Balancing the energy flow from captured light to biomass under fluctuating light conditions[END_REF]. This is achieved by a more balanced and cost-effective qE via a concerted regulation of XC pigments and Lhcx synthesis in a narrow range of irradiance [START_REF] Wagner | Balancing the energy flow from captured light to biomass under fluctuating light conditions[END_REF][START_REF] Lavaud | Photoprotection capacity differs among diatoms: Possible consequences on the spatial distribution of diatoms related to fluctuations in the underwater light climate[END_REF][START_REF] Giovagnetti | The velocity of light intensity increase modulates the photoprotective response in coastal diatoms[END_REF][START_REF] Lepetit | The diatom Phaeodactylum tricornutum adjusts nonphotochemical fluorescence quenching capacity in response to dynamic light via fine-tuned Lhcx and xanthophyll cycle pigment synthesis[END_REF]. Such response is an adaptive advantage in well-mixed habitats where competition for light strongly influences phytoplankton groups and species dynamics [START_REF] Widdicombe | Long-term phytoplankton community dynamics in the Western English Channel[END_REF][START_REF] Polimene | Modelling xanthophyll photoprotective activity in phytoplankton[END_REF][START_REF] Schofield | Decadal variability in coastal phytoplankton community composition in a changing West Antarctic Peninsula[END_REF] and their functional and evolutionary ecology [START_REF] Tozzi | Historical climate change and ocean turbulence as selective agents for two key phytoplankton functional groups[END_REF][START_REF] Benoiston | The evolution of diatoms and their biogeochemical functions[END_REF].

The ecophysiological connection between NPQ and XC features and the in situ light climate was investigated by comparing strains from different habitats [START_REF] Lavaud | Fast regulation of photosynthesis in diatoms: mechanisms, evolution and ecopysiology[END_REF][START_REF] Lacour | Diversity in xanthophyll cycle pigments content and related non-photochemical quenching (NPQ) among microalgae: implications for growth strategy and ecology[END_REF]. In general, species adapted to higher average light doses or stronger light fluctuations (estuaries and coasts) showed higher Dt quenching efficiencies than counterparts adapted to more stable light environments (semi-enclosed bays and open ocean). Contrastingly, the latter rely more on regulatory processes better suited to cope with more gradual light changes such as a fast turnover of the D1 protein to balance photodamage [START_REF] Wu | Photosystem II photoinactivation, repair and protection in marine centric diatoms[END_REF][START_REF] Lavaud | Photosystem II repair in marine diatoms with contrasting photophysiologies[END_REF]) (see also "2.2 PSII repair"). Diatoms that can have both a planktonic and a benthic life (tychoplankton) perform diverse responses, with some representatives showing NPQ features similar to coastal centric diatoms [START_REF] Perkins | Time-dependent upregulation of electron transport with concomitant induction of regulated excitation dissipation in Haslea diatoms[END_REF][START_REF] Lacour | Diversity in xanthophyll cycle pigments content and related non-photochemical quenching (NPQ) among microalgae: implications for growth strategy and ecology[END_REF], while those that inhabit intertidal flats resemble open ocean species [START_REF] Barnett | Growth form defines physiological photoprotective capacity in intertidal benthic diatoms[END_REF][START_REF] Lacour | Diversity in xanthophyll cycle pigments content and related non-photochemical quenching (NPQ) among microalgae: implications for growth strategy and ecology[END_REF].

Diatoms of the microphytobenthos

Benthic diatoms that inhabit the sediment of intertidal flats experience an extremely complex light environment depending on a multifaceted combination of tidal and diurnal cycles that control the alternation between sharp very low light/darkness periods (during immersion and at night) and open-field like light fluctuations during daytime emersions. They also have to deal with a photic zone condensed to the first hundreds of µm in the sediment [START_REF] Kühl | The light field of microbenthic communities: Radiance distribution and microscale optics of sandy coastal sediments[END_REF]. Additionally, other environmental cues such as temperature and salinity can vary rapidly between extremes [START_REF] Paterson | Microphytobenthos in contrasting coastal ecosystems: Biology and dynamics[END_REF]. Similarly to plankton counterparts, NPQ and related XC pigment dynamics in benthic diatom strongly depend on light as a function of latitude, season, tidal cycle, penetration in the sediment, daily light dose and light fluctuations during emersion [START_REF] Lavaud | The peculiar features of non-photochemical fluorescence quenching in diatoms and brown algae[END_REF]. These temporal NPQ dynamics, together with other photoprotective processes such as the PSII repair cycle [START_REF] Frankenbach | Photoinactivation, repair and the motility-physiology trade-off in microphytobenthos[END_REF], help benthic diatoms in maintaining their photosynthetic efficiency during emersion [START_REF] Laviale | Response of intertidal benthic microalgal biofilms to a coupled light-temperature stress: evidence for latitudinal adaptation along the Atlantic coast of Southern Europe[END_REF][START_REF] Cartaxana | Regulation of intertidal microphytobenthos photosynthesis over a diel emersion period is strongly affected by diatom migration patterns[END_REF]. Differences in photoadaptive traits between benthic diatoms are believed to partially explain the differential distribution of growth forms between contrasting sedimentary habitats (i.e. muddy vs. sandy), and they are also claimed to be involved in the hourly species succession within the biofilm during emersion [START_REF] Underwood | Patterns in microphytobenthic primary productivity: Speciesspecific variation in migratory rythms and photosynthetic efficiency in mixed-species biofilms[END_REF].

The two main growth forms of benthic diatoms are the non-motile epipsammon and the motile epipelon. Epipsammon usually dominates in coarser sandy sediment where light penetrates deeper, and it lives in close association with sediment particles. This growth form -similar to plankton counterparts adapted to high average light doses or strong light fluctuations -first relies on XC based NPQ for its photoprotection [START_REF] Cartaxana | Physiological versus behavioral photoprotection in intertidal epipelic and epipsammic benthic diatom communities[END_REF][START_REF] Barnett | Growth form defines physiological photoprotective capacity in intertidal benthic diatoms[END_REF], and some of them reach some of the highest quenching efficiencies [START_REF] Lacour | Diversity in xanthophyll cycle pigments content and related non-photochemical quenching (NPQ) among microalgae: implications for growth strategy and ecology[END_REF]. Their high NPQ capacity is based on a large Dd+Dt pool size, high kinetic constants of the XC enzymes, a strong DES and possibly the use of the violaxanthin-antheraxanthin-zeaxanthin cycle under harsh light exposure [START_REF] Barnett | Growth form defines physiological photoprotective capacity in intertidal benthic diatoms[END_REF][START_REF] Blommaert | Contrasting NPQ dynamics and xanthophyll cycling in a motile and a non-motile intertidal benthic diatom[END_REF][START_REF] Blommaert | Behavioural versus physiological photoprotection in epipelic and epipsammic benthic diatoms[END_REF].

Contrastingly, epipelon dominates in fine cohesive muddy sediment where it undergoes active 'vertical migration' modelled onto the tidal and diurnal cycles, i.e. it migrates upward the sediment at daytime emersion to form a biofilm at the surface and perform photosynthesis.

Epipelic diatoms can also use motility to position themselves along the sediment light gradient and escape from excess light [START_REF] Consalvey | The ups and downs of life in a benthic biofilm: migration of benthic diatoms[END_REF]), which generates a strong interplay between behavioral (phototactic motility) and physiological regulation of photosynthesis [START_REF] Laviale | The importance of being fast: comparative kinetics of vertical migration and non-photochemical quenching of benthic diatoms under light stress[END_REF][START_REF] Blommaert | Behavioural versus physiological photoprotection in epipelic and epipsammic benthic diatoms[END_REF][START_REF] Frankenbach | Photoinactivation, repair and the motility-physiology trade-off in microphytobenthos[END_REF]. Therefore, epipelic diatoms rely less on NPQ and generally show a weaker quenching efficiency [START_REF] Lacour | Diversity in xanthophyll cycle pigments content and related non-photochemical quenching (NPQ) among microalgae: implications for growth strategy and ecology[END_REF]. Besides, epipelic diatoms rely on a more efficient repair capacity of PSII photodamage than epipsammon [START_REF] Frankenbach | Photoinactivation, repair and the motility-physiology trade-off in microphytobenthos[END_REF], an ability strikingly similar to their plankton counterparts adapted to a relatively stable light environment [START_REF] Lavaud | Photosystem II repair in marine diatoms with contrasting photophysiologies[END_REF]. Also, benthic diatoms may contain a higher number of Lhcx proteins (17 in Seminavis robusta, [START_REF] Blommaert | Light regulation of Lhcx genes in the benthic diatom Seminavis robusta[END_REF] to adjust their NPQ capacity to cope with combined environmental stresses [START_REF] Laviale | Response of intertidal benthic microalgal biofilms to a coupled light-temperature stress: evidence for latitudinal adaptation along the Atlantic coast of Southern Europe[END_REF][START_REF] Blommaert | Contrasting NPQ dynamics and xanthophyll cycling in a motile and a non-motile intertidal benthic diatom[END_REF], but this awaits confirmation by more sequenced benthic diatoms. Critically, studies regarding the full budget of absorbed light energy and its fate including diverse alternative electron paths, as done in planktonic species [START_REF] Wagner | Balancing the energy flow from captured light to biomass under fluctuating light conditions[END_REF][START_REF] Jakob | A complete energy balance from photons to new biomass reveals a light-and nutrient-dependent variability in the metabolic costs of carbon assimilation[END_REF]Bailleul et al. 2015), are not yet available for benthic species.

Polar diatoms

Diatoms also thrive in polar ecosystems, characterized by very different light climates related to specific, often seasonal, habitats: in sea-ice, underneath the ice, at the marginal ice zone, and in summer free-ice open waters. Furthermore, high latitude habitats are characterized by extreme changes in photoperiods over the year, from complete darkness to 24 h light per day.

The combined effects of light with temperature, nutrient availability, and ocean acidification on the regulation of photosynthesis in polar diatoms have been recently extensively considered (Petrou et al. 2016;[START_REF] Trimborn | Two Southern Ocean diatoms are more sensitive to ocean acidification and changes in irradiance than the prymnesiophyte Phaeocystis antarctica[END_REF][START_REF] Kvernvik | Fast reactivation of photosynthesis in arctic phytoplankton during the polar night[END_REF][START_REF] Lewis | Photoacclimation of Arctic Ocean phytoplankton to shifting light and nutrient limitation[END_REF][START_REF] Strzepek | Photosynthetic adaptation to low iron, light, and temperature in Southern Ocean phytoplankton[END_REF][START_REF] Yan | Increased temperature benefits growth and photosynthetic performance of the sea ice diatom Nitzschia cf. neglecta (Bacillariophyceae) isolated from saroma lagoon, Hokkaido, Japan[END_REF]. In general, polar diatoms show photoadaptive features of very low light and temperature acclimation, for instance: i) specific features of Rubisco activity [START_REF] Valegård | Structural and functional analyses of Rubisco from arctic diatom species reveal unusual posttranslational modifications[END_REF]) including a high cellular Rubisco content [START_REF] Young | Slow carboxylation of Rubisco constrains the rate of carbon fixation during Antarctic phytoplankton blooms[END_REF][START_REF] Gerecht | High ribulose-1,5bisphosphate carboxylase/oxygenase content in northern diatom species[END_REF] to compensate for the slow enzymatic rate, ii) very active AEF processes [START_REF] Alderkamp | The effect of iron limitation on the photophysiology of Phaeocystis antarctica (Prymnesiophyceae) and Fragilariopsis cylindrus (Bacillariophyceae) under dynamic irradiance[END_REF][START_REF] Goldman | Gross and net production during the spring bloom along the Western Antarctic Peninsula[END_REF][START_REF] Schuback | Interacting effects of light and iron availability on the coupling of photosynthetic electron transport and CO2-assimilation in marine phytoplankton[END_REF][START_REF] Trimborn | Susceptibility of two Southern Ocean phytoplankton key species to iron limitation and high light[END_REF], iii) the ability to repair damaged PSII even below 0°C [START_REF] Petrou | Rapid photoprotection in sea-ice diatoms from the East Antarctic pack ice[END_REF], and iv) a strong antioxidant activity (mycosporine-like amino acids and ROS scavengers [START_REF] Hernando | Effects of solar radiation on growth and mycosporine-like amino acids content in Thalassiosira sp, an Antarctic diatom[END_REF][START_REF] Janknegt | Oxidative stress responses in the marine antarctic diatom Chaetoceros brevis (Bacillariophyceae) during photoacclimation[END_REF][START_REF] Ha | Strategy of photo-protection in phytoplankton assemblages in the Kongsfjorden, Svalbard, Arctic[END_REF]).

The pattern of NPQ-related light-response and Dd+Dt pool size is similar to the one of temperate plankton diatoms. It correlates with diurnal and seasonal light dynamics [START_REF] Schallenberg | Diel quenching of Southern Ocean phytoplankton fluorescence is related to iron limitation[END_REF][START_REF] Schuback | Diurnal regulation of photosynthetic light absorption, electron transport and carbon fixation in two contrasting oceanic environments[END_REF], depth [START_REF] Kashino | Strategies of phytoplankton to perform effective photosynthesis in the North Water[END_REF][START_REF] Hashihama | Light acclimation states of phytoplankton in the Southern Ocean, determined using photosynthetic pigment distribution[END_REF][START_REF] Alderkamp | Photoacclimation and nonphotochemical quenching under in situ irradiance in natural phytoplankton assemblages from the Amundsen Sea, Antarctica[END_REF]) and horizontal light climate heterogeneity, i.e. Dd+Dt pool size is larger in free-ice open waters and at the marginal ice zone than in waters covered with ice. It also depends on the depth of snow covering the floe [START_REF] Aikawa | Diel tuning of photosynthetic systems in ice algae at Saroma-ko Lagoon, Hokkaido, Japan[END_REF][START_REF] Alou-Font | Snow cover affects ice algal pigment composition in the coastal Arctic Ocean during spring[END_REF]Galindo et al. 2017). Also noteworthy is the high number of FCP and Lhcx proteins (11 isoforms) found in the bipolar strain Fragilariopsis cylindrus (Mock et al. 2017). Such as in temperate counterparts, most Lhcx isoforms are downregulated in darkness [START_REF] Kennedy | Dark metabolism: a molecular insight into how the Antarctic sea-ice diatom Fragilariopsis cylindrus survives long-term darkness[END_REF]) and some are upregulated under high light exposure [START_REF] Park | Dynamic response of the transcriptome of a psychrophilic diatom, Chaetoceros neogracile, to high irradiance[END_REF][START_REF] Kvernvik | Higher sensitivity towards light stress and ocean acidification in an Arctic sea-iceassociated diatom compared to a pelagic diatom[END_REF].

Although they are adapted to very low light, bottom sea-ice diatom communities possess the physiological panoply for responding to sudden and relatively prolonged irradiance increase, e.g. in spring, due to snow flash melt [START_REF] Griffith | Diadinoxanthin cycle of the bottom ice algal community during spring in McMurdo Sound, Antarctica[END_REF][START_REF] Katayama | Photoprotective responses of an ice algal community in Saroma-Ko Lagoon, Hokkaido, Japan[END_REF][START_REF] Arrigo | Sea ice algal biomass and physiology in the Amundsen Sea, Antarctica[END_REF]. Alderkamp and colleagues [START_REF] Alderkamp | Can photoinhibition control phytoplankton abundance in deeply mixed water columns of the Southern Ocean?[END_REF][START_REF] Alderkamp | Short-term photoacclimation effects on photoinhibition of phytoplankton in the Drake Passage (Southern Ocean)[END_REF][START_REF] Alderkamp | Photoacclimation and nonphotochemical quenching under in situ irradiance in natural phytoplankton assemblages from the Amundsen Sea, Antarctica[END_REF][START_REF] Joy-Warren | Light is the primary driver of early season phytoplankton production along the western Antarctic Peninsula[END_REF]) demonstrated how NPQ, XC pigments and PSII repair mitigate photoinhibition in polar diatom communities, and how this controls their productivity by strongly modulating the coupling between ETR and carbon fixation [START_REF] Schuback | Primary productivity and the coupling of photosynthetic electron transport and carbon fixation in the Arctic Ocean[END_REF][START_REF] Thomalla | Seasonal regulation of the coupling between photosynthetic electron transport and carbon fixation in the Southern Ocean[END_REF].

Polar diatoms are able to sustain a functional photosynthetic machinery during winter polar night at least for 1.5 months and to reactivate photosynthesis upon the spring onset of light within less than 24 h [START_REF] Kvernvik | Fast reactivation of photosynthesis in arctic phytoplankton during the polar night[END_REF]. While this involves many concerted physiological and metabolic adjustments [START_REF] Kennedy | Dark metabolism: a molecular insight into how the Antarctic sea-ice diatom Fragilariopsis cylindrus survives long-term darkness[END_REF][START_REF] Morin | Response of the sea-ice diatom Fragilariopsis cylindrus to simulated polar night darkness and return to light[END_REF], NPQ and XC pigments (and likely AEFs) are of primary importance especially during re-illumination to reactivate photosynthesis [START_REF] Lacour | Decoupling light harvesting, electron transport and carbon fixation during prolonged darkness supports rapid recovery upon re-illumination in the Arctic diatom Chaetoceros neogracilis[END_REF][START_REF] Morin | Response of the sea-ice diatom Fragilariopsis cylindrus to simulated polar night darkness and return to light[END_REF]. In these conditions, even a low photon flux is dramatically in excess of photosynthetic capacity, and light onset is anticipated in some species via Dt synthesis in the dark, switching PSII in the dissipative state (='dark NPQ') [START_REF] Kennedy | Dark metabolism: a molecular insight into how the Antarctic sea-ice diatom Fragilariopsis cylindrus survives long-term darkness[END_REF][START_REF] Lacour | Decoupling light harvesting, electron transport and carbon fixation during prolonged darkness supports rapid recovery upon re-illumination in the Arctic diatom Chaetoceros neogracilis[END_REF]. Dt synthesis in darkness occurs possibly through the build-up of a transthylakoidal ΔpH via chlororespiration and/or plastid-mitochondria coupling (see "1.5 Chlororespiration, redox state of the plastoquinone pool, and pmf in the dark"). Upon re-illumination, dark NPQ provides immediate protection [START_REF] Lacour | Decoupling light harvesting, electron transport and carbon fixation during prolonged darkness supports rapid recovery upon re-illumination in the Arctic diatom Chaetoceros neogracilis[END_REF], onto which light-driven Dt accumulation based NPQ is added [START_REF] Morin | Response of the sea-ice diatom Fragilariopsis cylindrus to simulated polar night darkness and return to light[END_REF], which supports the rapid transition towards fully operational photosynthesis [START_REF] Kvernvik | Fast reactivation of photosynthesis in arctic phytoplankton during the polar night[END_REF][START_REF] Morin | Response of the sea-ice diatom Fragilariopsis cylindrus to simulated polar night darkness and return to light[END_REF].

Outlook

A wealth of knowledge about photosynthesis in diatoms has been gathered in the last decades, but most of these data have been acquired using only a handful of model species. Considering the ecophysiological diversity of diatoms, these data can only represent a small part of the puzzle. The remarkable flexibility of photosynthetic electron flows and especially the tight interplay with mitochondrial respiration needs to be addressed under various stress conditions and in different diatom species to understand how diatoms optimize electron flows. In this regard, a closer look into mitochondrial respiration may help dissecting further peculiarities of diatoms compared to the green lineage. We also need a first highly resolved image of the diatom ATP synthase to get an idea of the proton requirement per ATP synthesis. In addition, the identity of the actual fluorescence quenching pigments involved in thermal dissipation is unclear so far, and the molecular details of the photosynthetic repair machinery are not known yet. For all these questions, the application of modern genome editing tools such as CRISPR-Cas will be essential. Moreover, the development of heterotrophic diatom model systems, as proposed for Cyclotella cryptica and Cylindrotheca fusiformis [START_REF] Falciatore | Diatom molecular research comes of age: Model species for studying phytoplankton biology and diversity[END_REF], could enable for the first time studying processes in heterotrophic mutants (lethal in autotrophic diatoms) which include plastid division and development, but also thylakoid (protein complex) and pigment synthesis. Finally, the further exploration of the functional and ecological diversity of different growth forms and species isolated from different marine habitats will help better understanding how the specificities of diatom photosynthesis support their worldwide ecological success.

Figure 1

 1 Figure 1 Schematic representation of the photosynthetic electron transfer chains. The linear electron flow is represented as a bold blue arrow. Proton movements are represented by red arrows. The CBB cycle is represented in grey. The unknowns are represented by blue dashed lines (for alternative electron flows), a green dashed line (PSII cyclic electron flow), a red dashed line and red question marks (Q cycle in the cyt b6f, ATP demand for CCM and number of c subunits). PSI cyclic electron flow could be mediated via PGRL1/PGR5 or via another yet unknown plastoquinone reducing pathway.

  noted that phytoplankton can tolerate a degree of photoinhibitory burn down of the standing pool of [PSII]active without suffering an immediate loss of net photosynthesis, showing that the remaining pool of [PSII]active can accelerate their photochemical turnover to compensate for loss of [PSII]active.

  [START_REF] Lavaud | Photosystem II repair in marine diatoms with contrasting photophysiologies[END_REF] found that in P. tricornutum a large capacity for NPQ accompanied a 'justin-time' strategy of repairing [PSII]inactive rapidly. In contrast, Skeletonema costatum compensated for limited capacity to induce sustained NPQ with a 'warehouse' strategy by maintaining a large pool of [PSII]inactive awaiting repair. We thus find a compensatory balance between capacity to downregulate PSII through NPQ with capacity to accumulate large pools of [PSII]inactive or excess [PSII]active.

Figure 3

 3 Figure 3 Model of how Lhcx and Dt generate qE in P. tricornutum. Depending on their environmental conditions, cells are in a low or high, but yet inactive qE state, which potential qE capacity is determined by the amount of XC pigments and Lhcx proteins present. Upon excess light exposure, the established ∆pH leads to the conversion of Dd into Dt, which induces a reduction in σPSII by thermal dissipation in the outer, uncoupled FCP antennae (Fucoxanthin chlorophyll binding proteins), shifting the cells into the active qE state.

  states (DES) with completely different XC-mediated NPQ kinetics and capacities. This results possibly from Dd+Dt pools with different functionality and localization and different expression of Lhcx proteins (see "3.4 The molecular regulation and characteristics of qE"), generating a different overall quenching efficiency of Dt.

Acknowledgements

BB acknowledges financial support from the European Research Council (ERC) under the European Union Horizon 2020 research and innovation program (grant agreement no. 715579). BL thanks the Deutsche Forschungsgemeinschaft (LE3358/3-2) and the Baden-Württemberg Stiftung (Elite program) for financial support, and Kerstin Flieger (University of Leipzig) for analyzing α-tocopherol. CB acknowledges support by the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 675006 and from the Deutsche Forschungsgemeinschaft, grant Bu 812 10-1. DAC thanks the Canada Research Chairs and Natural Science and Engineering Research Council of Canada for support. JL thanks the Centre National de la Recherche Scientifique-CNRS, the Natural

Sciences and Engineering Research Council of Canada-NSERC (Discovery and Northern Supplement grants), the Canada First Research Excellence Fund-Sentinelle Nord, and the strategic research cluster Québec-Océan for their financial support. All authors thank the reviewer for valuable suggestions.