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

Reimund Goss

The Peculiar Features of Non-Photochemical Fluorescence Quenching in Diatoms and Brown Algae

Diatoms and brown algae are major contributors to marine primary production. They are biologically diverse, with thousands of different species, and are extremely successful, occupying almost every marine ecosystem ranging from the coastal-estuarine to deep-sea regions. Their ecological success is based in part on their ability to rapidly regulate photosynthesis in response to pronounced fluctuations in their natural light environment. Regulation of light harvesting, and the use of excitation energy, is largely based on effective dissipation of excessive energy as heat. Thermal dissipation of excitation energy is assessed as non-photochemical quenching of chlorophyll a fluorescence (NPQ). NPQ depends strongly on the conversion of xanthophylls: diadinoxanthin (Dd) to diatoxanthin (Dt) in the Dd-Dt cycle of diatoms and violaxanthin (V) to zeaxanthin (Z), via the intermediate antheraxanthin (A), in the VAZ cycle present in brown algae. Xanthophyll cycle (XC)-dependent thermal energy dissipation underlying NPQ represents one of the most important photoprotection mechanisms of diatoms and brown algae. In the present chapter, we review the biochemistry of XC enzymes with a special focus on co-substrate requirements and regulation of enzyme activity. In addition, we present a new model for the structural basis of XC-dependent NPQ in diatoms based on the latest experimental findings. In the last section, we highlight the importance of XC-dependent photoprotection for the ecological success of diatoms and brown algae in their natural environments.

H II MGDG inverted hexagonal phase

I. Introduction

In marine ecosystems, the main in situ abiotic driving forces for autotrophic growth are temperature, nutrients and light, the latter showing the highest variations in amplitude and frequency [START_REF] Macintyre | The effect of water motion on short-term rates of photosynthesis by marine phytoplankton[END_REF][START_REF] Kirk | Light and Photosynthesis in Aquatic Ecosystems[END_REF]). Consequently, the response of algae to their natural light environment is influenced by at least one irradiance-dependent process [START_REF] Li | Sensing and responding to excess light[END_REF]). Because rapid light fluctuations are unpredictable, this process must be fast and flexible. The xanthophyll (XC)-dependent non-photochemical quenching of chlorophyll (Chl)

a fluorescence (NPQ) turns out to provide such features. Photoprotective thermal energy dissipation that is reflected in NPQ is one of the main physiological processes used by algae to respond to the fluctuations of their natural light environment [START_REF] Lavaud | Fast regulation of photosynthesis in diatoms: mechanisms, evolution and ecophysiology[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] Goss | Regulation and function of xanthophyll cycle-dependent photoprotection in algae[END_REF][START_REF] Depauw | Exploring the molecular basis of responses to light in marine diatoms[END_REF][START_REF] Niyogi | Evolution of flexible non-photochemical quenching mechanisms that regulate light harvesting in oxygenic photosynthesis[END_REF]. Diatoms and brown macroalgae can exert a strong NPQ (Lavaud et al. 2002a;[START_REF] Ruban | The superexcess energy dissipation in diatom algae: comparative analysis with higher plants[END_REF][START_REF] Lavaud | Fast regulation of photosynthesis in diatoms: mechanisms, evolution and ecophysiology[END_REF]Garcia-Mendoza and Colombo-Pallotta 2007).

Diatoms, as well as macrophytic brown algae (Phaeophyceae), belong to the stramenopiles that are classified as Chromista [START_REF] Kooistra | The origin and the evolution of the diatoms: Their adaptation to a planktonic existence[END_REF]). The chloroplasts of the Chromista are most likely the result of different endosymbiotic events, including a significant gene transfer from red algal and from prasinophyte cells [START_REF] Frommolt | Ancient recruitment by chromists of green algal genes encoding enzymes for carotenoid biosynthesis[END_REF][START_REF] Moustafa | Genomic footprints of a cryptic plastid endosymbiosis in diatoms[END_REF]). Among the genes that were retained are the ones encoding for the XC enzymes violaxanthin (V) de-epoxidase (VDE) and zeaxanthin (Z) epoxidase (ZEP). It was suggested that the latter genes were retained because, through the control of thermal energy dissipation underlying NPQ, the XC provides an efficient photoprotection mechanism, thereby improving photosynthetic performance [START_REF] Frommolt | Ancient recruitment by chromists of green algal genes encoding enzymes for carotenoid biosynthesis[END_REF]. The plastids of the stramenopiles are further characterized by the existence of four envelope membranes and by a different pigment assemblage [START_REF] Wilhelm | The regulation of photosynthetic carbon and nutrient assimilation in diatoms is significantly different from green algae, a putative consequence of secondary endosymbiosis[END_REF][START_REF] Lavaud | Fast regulation of photosynthesis in diatoms: mechanisms, evolution and ecophysiology[END_REF][START_REF] Lepetit | Molecular dynamics of the diatom thylakoid membrane under different light conditions[END_REF]. The thylakoid membranes are arranged in regular stacks of three with no grana systems as in vascular plants [START_REF] Pyszniak | Immunocytochemical localization of photosystem I and the fucoxanthin-chlorophyll a/c light-harvesting complex in the diatom Phaeodactylum tricornutum[END_REF].

Diatoms show a fascinating biodiversity with up to 100,000 different species divided into three clades and two morphological groups: the pennates and the centrics (longitudinal versus radial symmetry, respectively) [START_REF] Kooistra | The origin and the evolution of the diatoms: Their adaptation to a planktonic existence[END_REF]. Planktonic diatoms evolved from benthic forms [START_REF] Kooistra | The origin and the evolution of the diatoms: Their adaptation to a planktonic existence[END_REF]. Diatoms are almost equally distributed in marine and limnic habitats [START_REF] Falkowski | The evolution of modern eukaryotic phytoplankton[END_REF]) and they are major contributors to marine primary production [START_REF] Armbrust | The life of diatoms in the world's oceans[END_REF]). The light-harvesting system of diatoms consists of the fucoxanthin-chlorophyll protein (FCP) complexes that form oligomers whose organization is groups-/species-dependent (see Büchel,Chap. 11). In addition to the peripheral, main lightharvesting complex (LHC), which serves as antenna for both photosystem I (PS I) and II (PS II), diatoms contain a specific PS I FCP complex [START_REF] Veith | Identification of a specific fucoxanthin-chlorophyll protein in the light-harvesting complex of photosystem I in the diatom Cyclotella meneghiniana[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]. FCP complexes are apoproteins of up to 18-19 kDa [START_REF] Wilhelm | The regulation of photosynthetic carbon and nutrient assimilation in diatoms is significantly different from green algae, a putative consequence of secondary endosymbiosis[END_REF][START_REF] Lavaud | Fast regulation of photosynthesis in diatoms: mechanisms, evolution and ecophysiology[END_REF][START_REF] Lepetit | order to amplify Dt-dependent energy dissipation and thus NPQ (Lavaud and Lepetit 2013)[END_REF]. The pigments of the XC cycle involving diadinoxanthin (=Dd) and diatoxanthin (=Dt) are bound to FCPs or located in a monogalactosyldiacylgycerol (MGDG) shield around the peripheral FCP complexes [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]). These complexes are the site, where the majority of energy dissipation that results in NPQ takes place (see Büchel,Chap. 11).

Macrophytic brown algae are important coastal primary producers (Enriquez and Borowitzka 2010). There are about 2000 known species (de Reviers and Rousseau 1999)

whose phylogeny is still under debate [START_REF] Philips | Resolving evolutionary relationships among the brown algae using chloroplast and nuclear genes[END_REF]). Their morphology is extremely diverse, with forms ranging from microscopic filaments to the well-known macrophytes like the Fucales and the Laminariales. Most of them are sessile organisms with few pelagic (open water) species. The LHC system of brown algae is also composed of FCP complexes [START_REF] Dittami | Chlorophyll-binding proteins revisited -a multigenic family of light-harvesting and stress proteins from a brown algal perspective[END_REF]. The latter are isolated as trimeric units of 20-kDa apoproteins [START_REF] Martino | Characterization of two light-harvesting subunits isolated from the brown alga Pelvetia canaliculata: heterogeneity of xanthophyll distribution[END_REF][START_REF] Fujii | Isolation and purification of the major photosynthetic antenna, fucoxanthin-Chl a/c protein, from cultured discoid germlings of the brown alga, Cladosiphon okamuranus TOKIDA (Okinawa Mozuku)[END_REF]) that can possibly form higher dodecameric structures [START_REF] Katoh | Light-harvesting particles isolated from a brown alga, Dictyota dichtoma. A supramolecular assembly of fucoxanthin-chlorophyll-protein complexes[END_REF][START_REF] Passaquet | Light-harvesting complexes of brown algae. Biochemical characterization and immunological relationships[END_REF]. These complexes exhibit pigmentation similar to that of the corresponding complexes in diatoms, with the exception of the XC pigments that are the same as in green algae and vascular plants, i.e. V, antheraxanthin (A), and Z [START_REF] Passaquet | Light-harvesting complexes of brown algae. Biochemical characterization and immunological relationships[END_REF].

The variability of the XC-NPQ responses among lineages/genera/species has been a matter of growing debate. It was suggested that XC-NPQ diversity, and to a larger extent the diversity of photoadaptative strategy and capacity, would contribute to promoting competitive exclusion and spatial co-existence and/or temporal succession of algae in both pelagic and benthic (region along the bottom) systems (see [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] Goss | Regulation and function of xanthophyll cycle-dependent photoprotection in algae[END_REF]. The high efficiency of photoprotective XC-dependent energy dissipation underlying NPQ could also explain the amazing capacity of diatoms and brown algae to exploit differences in underwater light climate and to occupy a wide range of ecological niches.

II. Xanthophyll Cycle-Dependent NPQ

A. Xanthophyll Cycles of Diatoms and Brown Algae

Diatoms

Diatoms possess both the VAZ and the Dd-Dt cycle [START_REF] Lohr | Carotenoid metabolism in phytoplankton[END_REF]. While the Dd-Dt cycle is the main XC and is essential for NPQ, the VAZ-cycle pigment V serves as precursor pigment in the biosynthesis of Dd and the main light-harvesting xanthophyll fucoxanthin [START_REF] Lohr | Carotenoid metabolism in phytoplankton[END_REF][START_REF] Dambek | Biosynthesis of fucoxanthin and diadinoxanthin and function of initial pathway genes in Phaeodactylum tricornutum[END_REF]. In line with their function as xanthophyll synthesis intermediates, the VAZ-cycle pigments are only present in significant amounts after exposure to prolonged excess light, which triggers profound de-novo carotenoid synthesis [START_REF] Lohr | Algae displaying the diadinoxanthin cycle also possess the violaxanthin cycle[END_REF].

The Dd-Dt cycle (Fig. 1) consists of a de-epoxidation step from Dd, with one epoxy group, to the epoxy-free Dt [START_REF] Lohr | Carotenoid metabolism in phytoplankton[END_REF]. The back epoxidation reaction re-introduces the epoxy group into the Dt molecule. De-epoxidation from Dd to Dt takes place under excess light intensities but may also be triggered during long periods of darkness [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] 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]) (see Section II.C). Epoxidation is usually observed during periods of low light or darkness that follow excess light exposure (Goss et al. 2006a).

Typesetter to place Figure 1 here

De-epoxidation is catalyzed by the enzyme Dd de-epoxidase (DDE) while Dt epoxidase (DEP) triggers the epoxidation. Important differences exist between the latter two enzymes and the respective enzymes of the VAZ cycle of vascular plants, VDE and ZEP.

DDE exhibits a different pH-dependence [START_REF] Jakob | Unusual pH-dependence of diadinoxanthin de-epoxidase activation causes chlororespiratory induced accumulation of diatoxanthin in the diatom Phaeodactylum tricornutum[END_REF]; while VDE activity occurs at pH values of 6.5 or lower, DDE activity can already be observed at neutral pH values. The pH optimum of DDE is also shifted to higher pH values (5.5) in isolated thylakoids of diatoms as compared to VDE [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 | Influence of ascorbate and pH on the activity of the diatom xanthophyll cycle-enzyme diadinoxanthin de-epoxidase[END_REF]). These differences imply that, in diatoms, Dd de-epoxidation can be triggered by a lower light-driven proton gradient across the thylakoid membrane ( pH) (see Section II.C). It is not clear if DDE activation at less acidic pH values results in a conformational change of the enzyme as has been proposed for VDE of vascular plants. Inactive VDE is monomeric, and forms a dimer after exposure to low/acid pH values [START_REF] Arnoux | A structural basis for the pHdependent xanthophyll cycle in Arabidopsis thaliana[END_REF][START_REF] Saga | Mutation analysis of violaxanthin de-epoxidase indentifies substrate-binding sites and residues involved in catalysis[END_REF]. It has been proposed that the dimeric form of VDE is able to simultaneously de-epoxidize both epoxy groups of V [START_REF] Arnoux | A structural basis for the pHdependent xanthophyll cycle in Arabidopsis thaliana[END_REF]. Such a direct de-epoxidation of V to Z is, however, in contrast to the various studies that describe V de-epoxidation under natural conditions in intact plant leaves. These studies have shown that a significant accumulation of A can occur during the morning with increasing light intensities and during the afternoon when the irradiance decreases [START_REF] Adams | Operation of the xanthophyll cycle in higher plants in response to diurnal changes in incident sunlight[END_REF]).

Since Dd de-epoxidation only comprises one de-epoxidation step, the greatest advantage of dimeric VDE, i.e. simultaneous de-epoxidation of both epoxy-groups of V, does not apply to the Dd-Dt cycle. Additionally, DDE exhibits a significantly lower K M value (than VDE) for the co-substrate of de-epoxidation, ascorbate [START_REF] Grouneva | Influence of ascorbate and pH on the activity of the diatom xanthophyll cycle-enzyme diadinoxanthin de-epoxidase[END_REF]. The K M value for isolated DDE of Cyclotella meneghiniana is 0.7 mM versus 2.3 mM for VDE of spinach.

DDE is also able to maintain efficient de-epoxidation in the presence of very low concentrations of ascorbate, although this leads to a shift of the pH-optimum towards lower pH values [START_REF] Grouneva | Influence of ascorbate and pH on the activity of the diatom xanthophyll cycle-enzyme diadinoxanthin de-epoxidase[END_REF]). The optimized co-substrate usage of DDE indicates that the diatom cell is not providing a high concentration of ascorbate for Dd de-epoxidation.

A further, albeit more indirect, optimization is seen in the lipid requirement of the deepoxidation. Efficient Dd de-epoxidation depends on presence of the main thylakoid lipid MGDG [START_REF] Goss | Role of hexagonal structure-forming lipids in diadinoxanthin and violaxanthin solubilization and de-epoxidation[END_REF][START_REF] Goss | Lipid dependence of diadinoxanthin solubilization and de-epoxidation in artificial membrane systems resembling the lipid composition of the natural thylakoid membrane[END_REF]. MGDG serves to solubilize the hydrophobic pigment Dd, making it accessible for DDE. For the complete solubilization of a certain amount of Dd, a lower concentration of MGDG is sufficient than for the same number of V molecules [START_REF] Goss | Role of hexagonal structure-forming lipids in diadinoxanthin and violaxanthin solubilization and de-epoxidation[END_REF](Goss et al. , 2007)). This increased solubilization efficiency of Dd is in line with the often large XC pigment pool of diatoms with values of up to 800 mM Dd+Dt per M Chl a [START_REF] Lavaud | Enrichment of the light-harvesting complex in diadinoxanthin and implications for the nonphotochemical quenching fluorescence quenching in diatoms[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]. A higher number of Dd molecules can presumably be incorporated into the MGDG phase, resulting in increased de-epoxidation [START_REF] Lavaud | Enrichment of the light-harvesting complex in diadinoxanthin and implications for the nonphotochemical quenching fluorescence quenching in diatoms[END_REF]. A high concentration of Dt in the thylakoid lipid phase is in line with increased photoprotection by Dt [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]. Additionally, MGDG forms what are termed inverted hexagonal (H II )

phases essential for efficient de-epoxidation [START_REF] Latowski | Kinetics of violaxanthin de-epoxidation by violaxanthin de-epoxidase, a xanthophyll cycle enzyme, is regulated by membrane fluidity in model lipid bilayers[END_REF][START_REF] Goss | Role of hexagonal structure-forming lipids in diadinoxanthin and violaxanthin solubilization and de-epoxidation[END_REF][START_REF] Goss | Lipid dependence of diadinoxanthin solubilization and de-epoxidation in artificial membrane systems resembling the lipid composition of the natural thylakoid membrane[END_REF].

H II phases are likely associated with the FCP complexes and represent the docking sites for DDE once the enzyme has been activated by pH [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][START_REF] Lepetit | Molecular dynamics of the diatom thylakoid membrane under different light conditions[END_REF], i.e. the MGDG shield around the LHC targets DDE to the site where the majority of Dd is located.

The DEP also shows significant differences compared to ZEP, especially with regard to the epoxidation kinetics that can be almost 20 fold higher (Goss et al. 2006a).

Consequently, while Z epoxidation in vascular plants and green algae is slow compared to V de-epoxidation (Goss et al. 2006b;[START_REF] Schaller | Investigating the interaction between the violaxanthin cycle enzyme zeaxanthin epoxidase and the thylakoid membrane[END_REF] 2002;Goss et al. 2006a). The mechanism of this inactivation is, however, still unknown.

Nevertheless, upon exposure to gradually increasing light intensities, a complete inactivation of the DEP can be circumvented [START_REF] Dimier | Comparative ecophysiology of the xanthophyll cycle in six marine phytoplanktonic species[END_REF].

With respect to the localization of the XC enzymes, the pH-dependent activation of DDE and the NADPH requirement of DEP indicate that the enzymes are situated in the thylakoid lumen and on the stromal side of the thylakoid membrane, respectively, as it has been described for VDE and ZEP of vascular plants (Jahns et al. 2009;[START_REF] Latowski | Xanthophyll cycle -a mechanism protecting plants against oxidative stress[END_REF].

The MGDG-requirement of Dd de-epoxidation indicates that DDE, like VDE [START_REF] Schaller | The main thylakoid membrane lipid monogalactosyldiacylglycerol (MGDG) promotes the deepoxidation of violaxanthin associated with the light-harvesting complex of photosystem II (LHCII)[END_REF], binds to MGDG-enriched regions of the thylakoid membrane that are located in the vicinity of the FCP complexes [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]. The binding of DDE to MGDG-enriched regions occurs after the DDE pH-activation. Whether DEP is a peripheral, loosely-bound membrane protein, as it has been recently suggested for ZEP [START_REF] Schaller | Investigating the interaction between the violaxanthin cycle enzyme zeaxanthin epoxidase and the thylakoid membrane[END_REF]), remains to be assessed.

Genetic studies of the Dd-Dt cycle demonstrated that the genes encoding for DDE and DEP are closely related to the respective genes of the Prasinophytes [START_REF] Frommolt | Ancient recruitment by chromists of green algal genes encoding enzymes for carotenoid biosynthesis[END_REF].

The comparable structure of the gene arrangement in diatoms and Prasinophytes, i.e. a cluster of the DDE-VDE gene with one of the multiple copies of the DEP-ZEP gene, further strengthens the notion that the Chromista have acquired the XC genes through an early endosymbiosis involving a primitive green alga, in addition to the later endosymbiosis events with red algae. A recent study reported the genetic manipulation of the DDE gene in Phaeodactylum tricornutum [START_REF] Lavaud | Silencing of the violaxanthin de-epoxidase gene in the diatom Phaeodactylum tricornutum reduces diatoxanthin synthesis and non-photochemical quenching[END_REF]. By suppressing the transcript level of the DDE in vivo, it was possible to inhibit up to 50% of Dt synthesis and to strongly reduce the extent of NPQ, as well as to disturb the pH-Dt-NPQ relationship. Such approaches will help to decipher the mechanism underlying NPQ in diatoms in the future (see Section II.B.1).

Brown Algae

Unlike diatoms, brown algae only possess the VAZ cycle [START_REF] Rodrigues | A smaller and impaired xanthophyll cycle makes the deep sea algae Laminaria abyssalis (Phaeophyceae) highly sensitive to daylight when compared with shallow water Laminaria digitata[END_REF][START_REF] Gévaert | Laminaria saccharina photosynthesis measured in situ: photoinhibition and xanthophyll cycle during a tidal cycle[END_REF]Garcia-Mendoza and Colombo-Pallotta 2007). The VAZ cycle (Fig. 1) consists of a two-step de-epoxidation from the di-epoxy xanthophyll V to Z via the intermediate A [START_REF] Yamamoto | Studies on the light and dark interconversions of leaf xanthophylls[END_REF]Hager 1967a,b,). This reaction sequence, taking place under excess light (as is the case for the VAZ cycle in plants and the Dd-Dt cyle in diatoms), is reversed in low light or in darkness. The enzymes catalyzing the forward and backward reaction are VDE and ZEP, respectively. Almost nothing is known about the biochemistry of these enzymes in brown algae. Since the overall characteristics of the XC, i.e. kinetic, rates of the first and second de-epoxidation and epoxidation steps, and ratio of the first to the second deepoxidation and epoxidation step, are generally comparable to vascular plants [START_REF] Rodrigues | A smaller and impaired xanthophyll cycle makes the deep sea algae Laminaria abyssalis (Phaeophyceae) highly sensitive to daylight when compared with shallow water Laminaria digitata[END_REF], the properties of the VAZ-cycle enzymes in brown algae are most likely also comparable. However, V de-epoxidation may be slightly slower and Z epoxidation slightly [START_REF] Miloslavina | Ultrafast fluorescence study on the location and mechanism of non-photochemical quenching in diatoms[END_REF]Jahns and Holzwarth 2012). The model includes differences observed between diatom cells exhibiting a low versus high NPQ amplitude (see also Section III.A).

B. Mechanism of

Typesetter to place Figure 2 here

The quenching site Q2 is proposed to be located in a part of the LHC antenna that remains attached to the PS II core; its quenching properties presumably rely on an aggregation state induced by lowering of the luminal pH and further depends on the thylakoid membrane lipid environment as shown in vitro (Gundermann and Büchel 2012) and on the binding of Dd-Dt [START_REF] Miloslavina | Ultrafast fluorescence study on the location and mechanism of non-photochemical quenching in diatoms[END_REF]). The quenching Q1 is proposed to be located in a part of the LHC antenna that disconnects from PS II upon illumination and forms oligomeric FCP complexes (potentially including both Lhcf and Lhcx proteins, see below) [START_REF] Miloslavina | Ultrafast fluorescence study on the location and mechanism of non-photochemical quenching in diatoms[END_REF]). The quenching property of Q1 appears to be relatively independent of the presence of Dd-Dt in vitro (Gundermann and Büchel 2012). Chl a 710-712 oligomeric FCP complexes are already present in dark-and low light-adapted cells [START_REF] Miloslavina | Ultrafast fluorescence study on the location and mechanism of non-photochemical quenching in diatoms[END_REF] thereby dissipating excessive excitation energy as heat at the Q1 site in addition to the Q2 site.

Recent data obtained in P. tricornutum support such a mechanism; they revealed that FCP macrodomains undergo a fast and reversible light-induced reorganization that correlates with the kinetics of NPQ [START_REF] Szabo | Structurally flexible macro-organization of the pigment-protein complexes of the diatom Phaeodactylum tricornutum[END_REF].

The exact regulatory role of Dt remains under debate; two alternatives have been proposed that are not mutually exclusive (Fig. 2 The persistence of Dt in the dark was proposed to be responsible for keeping the FCP oligomers in an aggregated state (Goss et al. 2006a). The latter feature would be in line with the persistence of part of the quenching at the Q2 (and/or Q1) site(s) in the dark, i.e. as long as the majority of the Dt molecules are not converted back to Dd, the FCP oligomers cannot reconnect to PS II (Fig. 2). Retention of Dt and its engagement in thermal energy dissipation resulting in sustained NPQ in the dark were reported for several species, including pennate and centric representatives (Goss et al. 2006a;[START_REF] Lavaud | An explanation for the inter-species variability of the photoprotective non-photochemical chlorophyll fluorescence quenching in diatoms[END_REF]. Because its dark relaxation is slow, this sustained component of NPQ, also called "dark NPQ" (see Section III.B), probably contributes in part to qI, together with photo-damage/-inactivation of PS II [START_REF] Wu | Photosystem II photoinactivation, repair, and protection in marine centric diatoms[END_REF]). Sustained "dark NPQ" is often observed after a prolonged exposure to excess light [START_REF] Lavaud | An explanation for the inter-species variability of the photoprotective non-photochemical chlorophyll fluorescence quenching in diatoms[END_REF], which exhausts the pool of NADPH+H + and leads to a slower epoxidation of Dt back to Dd in the dark (Goss et al. 2006a). Hence, at least part of the "dark NPQ" could be interpreted as a slow recovery from the light-dependent reversible reorganization of the LHC system. "Dark NPQ" is also observed after a prolonged exposure to darkness (several hours), where it is induced by Dt molecules generated by a chlororespiratory ∆pH (see Section II.C).

The identity of the proteins involved in formation of the Q1 and Q2 sites remains unclear. These proteins are obviously different among diatom species (see Büchel,Chap. 11 for further details). In the centric diatom C. meneghiniana, the basic organizational units of It has recently been demonstrated that the transcription of some Lhcx genes is coregulated with the Dd+Dt content via the redox state of the 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]). In P.

tricornutum, an increase, via genetic engineering, of the in-vivo Lhcx1 level generates a higher NPQ [START_REF] Bailleul | An atypical member of the light-harvesting complex stress-related protein family modulates diatom responses to light[END_REF][START_REF] Depauw | Exploring the molecular basis of responses to light in marine diatoms[END_REF]. [START_REF] Zhu | Photoprotection in the diatom Thalassiosira pseudonana: Role of LI818-like proteins in response to high light stress[END_REF] proposed a structural role for Lhcx1 that fits well with its presence in significant amounts in low lightacclimated cells [START_REF] Bailleul | An atypical member of the light-harvesting complex stress-related protein family modulates diatom responses to light[END_REF]Schellenberger et al. 2013;[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]).

Nevertheless, this proposal is not in contradiction with Lhcx1 providing Dd-and Dt-binding sites or the amount of Dd and -Dt bound to the LHC system being modulated by the amount of Lhcx1 as a function of strains/species and of the organism's acclimation to light [START_REF] Lavaud | An explanation for the inter-species variability of the photoprotective non-photochemical chlorophyll fluorescence quenching in diatoms[END_REF]. There is currently no information on the Lhcf/Lhcx composition of the FCP oligomers responsible for Chl a 710-712 .

In addition to Dt-dependent NPQ, part of the thermal dissipation of the excessive excitation energy can be independent of Dt synthesis. The latter NPQ is a PS II reaction center-type quenching, may include recombination of Q A -and the S2/S3 state of the oxygenevolving complex (OEC) [START_REF] Eisenstadt | Changes in the photosynthetic reaction centre II in the diatom Phaeodactylum tricornutum result in non-photochemical fluorescence quenching[END_REF], and is most likely caused by an excess light-induced structural change of the PS II core complex itself. Earlier measurements (Lavaud et al. 2002c;[START_REF] Feikema | Cyclic electron transfer in photosystem II in the marine diatom Phaeodactylum tricornutum[END_REF]) revealed the existence of cyclic electron flow within PS II that includes an electron transfer pathway from the acceptor to the donor side [START_REF] Lavaud | Fast regulation of photosynthesis in diatoms: mechanisms, evolution and ecophysiology[END_REF]. This process has been interpreted as a protection mechanism against acceptor side photoinhibition in the OEC and donor side photoinhibition in PS II where OEC is temporarily disabled.

Brown Algae

Little is known about the mechanism of xanthophyll-dependent NPQ in brown algae [START_REF] Enriquez | The use of the fluorescence signal in studies of seagrasses and macroalgae[END_REF]. Although the latter algae display a different XC (see Section II.A.2), it seems that the NPQ of brown algae shows similar features to that of diatoms. As observed for diatoms, NPQ is mainly composed of the qE component, as a wavelength-dependent qT seems to be absent [START_REF] Fork | Light energy distribution in the brown alga Macrocystis pyrifera (giant kelp)[END_REF]. However, the possibility of an excess light-dependent qT in brown algae has yet to be studied systematically. Also similar to the situation in diatoms, pH alone cannot generate NPQ (Lavaud et al. 2002b;[START_REF] Lavaud | In diatoms, the transthylakoid proton gradient regulates the photoprotective non-photochemical fluorescence quenching beyond its control on the xanthophyll cycle[END_REF]; NPQ requires the concomitant binding of A and Z to the LHC system (Garcia- Alternatively, NPQ in brown algae was proposed to depend only on the synthesis of Z and A, and thus would be controlled by the activity of the XC enzymes and by the number of quenching sites in the light-harvesting complex [START_REF] Ocampo-Alvarez | Antagonistic effect between violaxanthin and de-epoxidated pigments in nonphotochemical quenching induction in the qE deficient brown alga Macrocystis pyrifera[END_REF]. Such a ∆pHindependent but Z-and A-dependent NPQ would be comparable to the slowly reversible, pHindependent form of NPQ in evergreen vascular plants persisting throughout harsh seasons (Demmig- Adams et al. 2006; see also Chap. 24). In general, it can be concluded that the considerable NPQ observed in some diatoms (Lavaud et al. 2002a;[START_REF] Ruban | The superexcess energy dissipation in diatom algae: comparative analysis with higher plants[END_REF]) and brown algae (Garcia-Mendoza and Colombo-Pallotta 2007) is independent of the specific nature of the XC pigments, i.e. Dt or Z and A. Instead, NPQ most likely depends on the ability to synthesize a large amount of either Dt or Z and A [START_REF] Lavaud | Enrichment of the light-harvesting complex in diadinoxanthin and implications for the nonphotochemical quenching fluorescence quenching in diatoms[END_REF]Garcia-Mendoza and Colombo-Pallotta 2007) and on the spatial organization and the protein composition of the FCP complexes [START_REF] Lavaud | Fast regulation of photosynthesis in diatoms: mechanisms, evolution and ecophysiology[END_REF]; see Section II.A.1).

The Lhcf/Lhcx proteins involved in thermal energy dissipation and thus NPQ in brown algae have not been identified yet. However, the recent sequencing of the Ectocarpus silicosus genome highlighted their large diversity with about 40 Lhcf genes and 13 Lhcx genes [START_REF] Dittami | Chlorophyll-binding proteins revisited -a multigenic family of light-harvesting and stress proteins from a brown algal perspective[END_REF]. Interestingly, and comparable to the situation in diatoms, an FCP complex with a high content of XC pigments exists [START_REF] Martino | Characterization of two light-harvesting subunits isolated from the brown alga Pelvetia canaliculata: heterogeneity of xanthophyll distribution[END_REF][START_REF] Tuji | The effect of irradiance on the growth of different forms of freshwater diatoms: implications for succession in attached diatom communities[END_REF]; it most likely is one of the NPQ sites (Q2). In M. pyrifera, a large VAZ pool is found in blades growing at the surface and exposed to high and changing light conditions [START_REF] Ocampo-Alvarez | Antagonistic effect between violaxanthin and de-epoxidated pigments in nonphotochemical quenching induction in the qE deficient brown alga Macrocystis pyrifera[END_REF]. This is concomitant with a higher expression of two LI818 isoforms ("LHC7" and "LHC8") [START_REF] Konotchick | Transcriptomic analysis of metabolic function in th egiant kelp, Macrocystis pyrifera, across depth and season[END_REF], as the functional orthologs of the diatom Lhcx and the green algal LHCSR [START_REF] Niyogi | Evolution of flexible non-photochemical quenching mechanisms that regulate light harvesting in oxygenic photosynthesis[END_REF] and see also Morosinotto and Bassi, Chap 14;

Finazzi and Minagawa, Chap. 21). It is thus very likely that LI818 proteins are involved in the NPQ process in brown algae, although a thorough biochemical investigation is needed to decipher their exact role.

C. Regulation of Xanthophyll Cycle-Dependent NPQ

Because Dt and Z are mandatory for NPQ in diatoms and brown algae, respectively, the most important factors for regulation of the majority of energy dissipation underlying NPQ are the kinetics of the build-up and relaxation of the pH and the availability of the co-substrates of the de-epoxidation and epoxidation reactions (summarized in Fig. 1). In general, Dd deepoxidation is faster than the respective conversion of V to A and Z in vascular plants and brown algae (Lavaud et al. 2002b;Goss et al. 2006a). The faster kinetics are caused by the almost immediate activation of DDE due to the shift of its pH-optimum to higher pH-values [START_REF] Jakob | Unusual pH-dependence of diadinoxanthin de-epoxidase activation causes chlororespiratory induced accumulation of diatoxanthin in the diatom Phaeodactylum tricornutum[END_REF]). This shift is accompanied by complete inhibition of Dt epoxidation by establishment of the 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]Goss et al. 2006a; see also Section II.A.1). Rapid Dd de-epoxidation correlates with a very fast induction of NPQ (Lavaud et al. 2002;[START_REF] Lavaud | In diatoms, the transthylakoid proton gradient regulates the photoprotective non-photochemical fluorescence quenching beyond its control on the xanthophyll cycle[END_REF]. The almost immediate generation of NPQ is further aided by the existence of an NPQ component that shows extremely fast kinetics after the onset of illumination [START_REF] Grouneva | A new multicomponent NPQ mechanism in the diatom Cyclotella meneghiniana[END_REF]). This component relies on establishment of the pH and depends on the concentration of Dt already present at the onset of illumination [START_REF] Lavaud | In diatoms, the transthylakoid proton gradient regulates the photoprotective non-photochemical fluorescence quenching beyond its control on the xanthophyll cycle[END_REF][START_REF] Grouneva | A new multicomponent NPQ mechanism in the diatom Cyclotella meneghiniana[END_REF]. A comparably fast NPQ component that depends on the presence of Z at the onset of actinic illumination has also been observed in vascular plants [START_REF] Demmig-Adams | Zeaxanthin and the induction and relaxation kinetics of the dissipation of excess excitation energy in leaves in 2% O 2 , 0% CO 2[END_REF][START_REF] Kalituho | The transiently generated non-photochemical quenching of excitation energy in Arabidopsis leaves is modulated by zeaxanthin[END_REF]; see also Chap 24).

Due to the prominent differences in the pH-dependent activation of DDE of diatoms and VDE of vascular plants, low pH-gradients are already sufficient to activate DDE of diatoms (see Section II.A.1). This means that, in addition to light-dependent DDE activation, Dd de-epoxidation can also be triggered by a chlororespiratory ∆pH in the dark [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] 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]. Although establishment of a ∆pH by chlororespiration is rather slow, significant synthesis of Dt occurs during prolonged periods of darkness. In P.

tricornutum, accumulation of Dt is further supported by inhibition of DEP by the chlororespiratory ∆pH [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]. Dt epoxidation is also limited when the co-substrate of DEP, NADPH, is not available in sufficient concentration. Such a partial inhibition of Dt epoxidation occurs during dark periods that follow excess light illumination (Goss et al. 2006a). It was proposed that both the ongoing Calvin-Benson Cycle and the chlororespiratory electron flow act as sinks for NADPH, thereby depriving DEP of its cosubstrate (Grouneva et al. 2009). In contrast, the conversion of Dt to Dd is very fast and efficient after a transition from high to low light (Goss et al. 2006a;[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], probably because electron transport is sufficient to generate the NADPH needed for Dt epoxidation. Both the fast deprivation of NADPH during dark periods following excess light illumination and the maintenance of NADPH synthesis during low light illumination is in line with the assumption that diatoms contain a significantly smaller pool of NADP + /NADPH compared to vascular plants (Goss et al. 2006;[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].

The synthesis of Dt in darkness results in the establishment of NPQ [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] 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]). However, the quenching efficiency of Dt generated by a chlororespiratory ∆pH may be lower than that of Dt formed by a light-driven ∆pH [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]. In Thalasiosira pseudonana, a high concentration of Dt can be induced in the dark by anaerobic conditions without development of a correspondingly high NPQ [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].

The return to aerobic conditions strongly increases the quenching efficiency of Dt, most likely by a significant increase of lumen acidification via a combination of electron transport from the plastoquinone (PQ) pool to oxygen by PQ oxidase and proton translocation by a type-1 NAD(P)H dehydrogenase (Ndh) [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].

Chlororespiration also contributes to Dt synthesis and NPQ during excess light illumination [START_REF] Eisenstadt | Changes in the photosynthetic reaction centre II in the diatom Phaeodactylum tricornutum result in non-photochemical fluorescence quenching[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]). This, however, depends on the diatom species [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]). The complete inhibition of light-driven linear electron transport by DCMU in P. tricornutum does not lead to a breakdown of ∆pH. In contrast, electron flow from the stroma to the PQ pool and the PQ oxidase, in combination with a type-1 Ndh, maintains a chlororespiratory ∆pH that leads to further Dd de-epoxidation and NPQ (see above). Since this latter electron flow is not coupled to NADP + reduction, the substrate of DEP is not synthesized, thus restricting a competing epoxidation of Dt to Dd. In the diatom C. While changes in the pool size of Dd+Dt provide information on the average irradiance to which the diatom cells have been exposed during the past hours/days, the dynamics of Dt synthesis can be used as a tracer for cell movements within the euphotic zone [START_REF] Brunet | Can the xanthophyll cycle help extract the essence of the microalgal functional response to a variable light environment?[END_REF]Hashihama et al. 2010). The dynamics of the XC and NPQ, together with other physiological processes [START_REF] Macintyre | The effect of water motion on short-term rates of photosynthesis by marine phytoplankton[END_REF], thus appear relevant for the regulation of the photosynthetic productivity of planktonic diatoms in the mixed upper layer of the water column, especially in the context of a regular exposure to excessive light intensities [START_REF] Alderkamp | Can photoinhibition control phytoplankton abundance in deeply mixed water columns of the Southern Ocean?[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] coupled with other environmental stresses [START_REF] Petrou | Heterogeneity in the photoprotective capacity of three Antarctic diatoms during short-term changes in salinity and temperature[END_REF]) and during bloom conditions [START_REF] Fujiki | Photoprotective response of xanthophyll pigments during phytoplankton blooms in Sagami Bay, Japan[END_REF].

Planktonic diatoms, and to a larger extent planktonic microalgae, show a large intergroup/species XC and NPQ diversity [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] Brunet | Can the xanthophyll cycle help extract the essence of the microalgal functional response to a variable light environment?[END_REF][START_REF] Goss | Regulation and function of xanthophyll cycle-dependent photoprotection in algae[END_REF]. It was proposed that the XC-NPQ activity and efficiency may be influenced by ecological niche adaptation, i.e. the light environment of the habitat from which the strains/ecotypes originate [START_REF] Lavaud | Fast regulation of photosynthesis in diatoms: mechanisms, evolution and ecophysiology[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]. In other words, the XC and NPQ efficiency would be a functionally adaptive trait that would participate in the cost/benefit balance of photoadaptative strategies [START_REF] Mckew | The trade-off between the light-harvesting and photoprotective functions of fucoxanthin-chlorophyll proteins dominates light acclimation in Emiliana huxleyi (clone CCMP 1516)[END_REF]) and that could, in part, explain the geographical/spatial species distribution and their temporal succession [START_REF] Lavaud | Fast regulation of photosynthesis in diatoms: mechanisms, evolution and ecophysiology[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] Dimier | Comparative ecophysiology of the xanthophyll cycle in six marine phytoplanktonic species[END_REF] even proposed a classification of ecological groups of phytoplankton based on their XC dynamics.

The hypothesis of XC and NPQ as functional traits was investigated by a comparison of representatives of the main phytoplankton groups (Casper-Lindley and Bjorkman 1998; [START_REF] Meyer | Xanthophyll cycling in Phaeocystis globosa and Thalassiosira sp.: a possible mechanism for species succession[END_REF][START_REF] Van Leeuwe | Taxon-specific differences in photoacclimation to fluctuating irradiance in an Antarctic diatom and a green flagellate[END_REF][START_REF] Wagner | Balancing the energy flow from captured light to biomass under fluctuating light conditions[END_REF][START_REF] Kropuenske | Photophysiology in two major Southern Ocean phytoplankton taxa: Photoprotection in Phaeocystis antartica and Fragilaria cylindricus[END_REF][START_REF] Van De Poll | Non-photochemical quenching of chlorophyll fluorescence and xanthophyll cycle responses after excess PAR and UVR in Chaetoceros brevis, Phaeocystis antarctica and coastal Antarctic phytoplankton[END_REF]. In general, diatoms seem to cope well with a fluctuating light climate, i.e.

they can exert strong NPQ (Lavaud et al. 2002a;[START_REF] Ruban | The superexcess energy dissipation in diatom algae: comparative analysis with higher plants[END_REF][START_REF] Lavaud | Fast regulation of photosynthesis in diatoms: mechanisms, evolution and ecophysiology[END_REF]). This feature is an adaptive advantage in deeply-mixed aquatic habitats: it drives, in part, the competition for light among phytoplankton groups that ultimately participate in species succession independent of other environmental cues (Huisman et al. 2004;[START_REF] Edwards | Functional traits explain phytoplankton community structure and seasonal dynamics in a marine ecosystem[END_REF]). Interestingly, it has been debated how modifications of upper-ocean turbulence may have shaped the functional and evolutionary ecology of planktonic diatoms and may currently influence their geographical distribution [START_REF] Falkowski | The evolution of modern eukaryotic phytoplankton[END_REF][START_REF] Tozzi | Historical climate change and ocean turbulence as selective agents for two key phytoplankton functional groups[END_REF]). The ecophysiological connection to XC-NPQ diversity of planktonic diatoms was investigated by comparing strains/ecotypes from different pelagic habitats (estuary, coast, semi-enclosed bay, open ocean) [START_REF] Lavaud | General features of photoprotection by energy dissipation in planktonic diatoms (Bacillariophyceae)[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] Strzepek | Photosynthetic architecture differs in coastal and oceanic diatoms[END_REF][START_REF] Van De Poll | Photoacclimation modulates excessive photosynthetically active and ultraviolet radiation effects in a temperate and an Antarctic marine diatom[END_REF][START_REF] Petrou | Heterogeneity in the photoprotective capacity of three Antarctic diatoms during short-term changes in salinity and temperature[END_REF], including different seasonal distributions [START_REF] Dimier | Photoprotective capacity as functional trait in planktonic algae: relationship between xanthophyll cycle and ecological characteristics in three diatoms[END_REF]) and pico-phytoplankton representatives [START_REF] Giovagnetti | Growth and photophysiological responses of two picoplanktonic Minutocellus species, strains RCC967 and RCC703 (Bacillariophyceae)[END_REF]. A special focus was recently given to diatom representatives of polar ecosystems [START_REF] Van Leeuwe | Taxon-specific differences in photoacclimation to fluctuating irradiance in an Antarctic diatom and a green flagellate[END_REF][START_REF] Van De Poll | Photoacclimation modulates excessive photosynthetically active and ultraviolet radiation effects in a temperate and an Antarctic marine diatom[END_REF][START_REF] Van De Poll | Non-photochemical quenching of chlorophyll fluorescence and xanthophyll cycle responses after excess PAR and UVR in Chaetoceros brevis, Phaeocystis antarctica and coastal Antarctic phytoplankton[END_REF][START_REF] Kropuenske | Photophysiology in two major Southern Ocean phytoplankton taxa: Photoprotection in Phaeocystis antartica and Fragilaria cylindricus[END_REF][START_REF] Mills | Photophysiology in two Southern ocean phytoplankton taxa: Photosynthesis of Phaeocystis antarctica (Prymnesiophyceae) and Fragilaria cylindrus (Bacillariophyceae) under simulated mixed-layer irradiance[END_REF][START_REF] Petrou | Heterogeneity in the photoprotective capacity of three Antarctic diatoms during short-term changes in salinity and temperature[END_REF]. Species/ecotypes adapted to a more stable light environment (semi-enclosed bays/open ocean versus estuaries/coasts) seem to rely more on regulatory processes other than NPQ and the PS II electron cycle (see Section II.B.1), which are likely better suited to cope with more gradual light changes: (1) a fast turnover of the PS II D1 protein [START_REF] Wu | Photosystem II photoinactivation, repair, and protection in marine centric diatoms[END_REF]Lavaud J. and Campbell D. unpublished results 2013), and (2) employment of Dt in the prevention of lipid peroxidation [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][START_REF] Lavaud | An explanation for the inter-species variability of the photoprotective non-photochemical chlorophyll fluorescence quenching in diatoms[END_REF]. Finally, the adjustment of light harvesting pigment content [START_REF] Brunet | Pigments and photoacclimation processes[END_REF]) and of the metabolic capacity, and especially the balance of energy between lipids and carbohydrates, is of importance for the response to dynamic light regimes of diatoms with different NPQ capacity [START_REF] Su | The impact of nonphotochemical quanching of fluorescence on the photon balance in diatoms under dynamic light conditions[END_REF].

A recent report suggested a molecular basis for the diversity in NPQ capacity observed among planktonic diatoms based on the mechanistic model described in Section II.B.1 [START_REF] Lavaud | An explanation for the inter-species variability of the photoprotective non-photochemical chlorophyll fluorescence quenching in diatoms[END_REF]. A high capacity for NPQ seems to depend on a large amount of Dt present at the Q2 quenching site (see Figure 2) and the capacity to form many/large Q1 quenching sites [START_REF] Lavaud | An explanation for the inter-species variability of the photoprotective non-photochemical chlorophyll fluorescence quenching in diatoms[END_REF]. In addition, the number of Lhcx proteins present in the light-harvesting system seems to be crucial [START_REF] Bailleul | An atypical member of the light-harvesting complex stress-related protein family modulates diatom responses to light[END_REF][START_REF] Zhu | Photoprotection in the diatom Thalassiosira pseudonana: Role of LI818-like proteins in response to high light stress[END_REF][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]). NPQ capacity is based more on the plastidic distribution of Dt and its effective involvement in NPQ than on the total amount of Dd+Dt and the de-epoxidation state of the XC (DES), i.e. species/ecotypes can show the same Dd+Dt content and DES with a drastically different NPQ amplitude and kinetics [START_REF] Lavaud | An explanation for the inter-species variability of the photoprotective non-photochemical chlorophyll fluorescence quenching in diatoms[END_REF]. For that reason, the slope of the relationship between Dt and NPQ appears to be a good indicator of the adaptation of diatom species/ecotypes to a specific light regime and, possibly, an aquatic niche [START_REF] Lavaud | General features of photoprotection by energy dissipation in planktonic diatoms (Bacillariophyceae)[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] Lavaud | An explanation for the inter-species variability of the photoprotective non-photochemical chlorophyll fluorescence quenching in diatoms[END_REF].

were also reported for isolated benthic diatoms [START_REF] Perkins | Light response curve methodology and possible implications in the application of chlorophyll fluorescence to benthic diatoms[END_REF][START_REF] Salleh | The effects of temperature on the photosynthetic parameters and recovery of two temperate benthic microalgae, Amphora cf.coffeaeformis and Cocconeis cf. sublittoralis (Bacillariophyceae)[END_REF][START_REF] Serôdio | A model for describing the light response of the nonphotochemical quenching of chlorophyll fluorescence[END_REF].

Of considerable interest are differences in the ability of species/groups to cope with excessive light/UV exposure [START_REF] Waring | Impact of elevated UV-B radiation on photosynthetic electron transport, primary productivity and carbon allocation in estuarine epipelic diatoms[END_REF][START_REF] Jesus | Adaptations of microphytobenthos assemblages to sediment type and tidal position[END_REF][START_REF] Cartaxana | Physiological versus behavioral photoprotection in intertidal epipelic and epipsammic benthic diatom communities[END_REF], which is similar to the situation in planktonic diatoms (see Section III.A). Differences in adaptation are believed to potentially impact spatial distribution (muddy versus sandy habitats) as well as species succession within the MPB biofilm during emersion and different seasons [START_REF] Tuji | The effect of irradiance on the growth of different forms of freshwater diatoms: implications for succession in attached diatom communities[END_REF][START_REF] Serôdio | Non-photochemical quenching of chlorophyll fluorescence and operation of the xanthophyll cycle in estuarine microphytobenthos[END_REF][START_REF] Underwood | Patterns in microphytobenthic primary productivity: Species-specific variation in migratory rhythms and photosynthetic efficiency in mixed-species biofilms[END_REF]. Epipsammic species (firmly attached to sediment particles) use physiological processes such as the XC and energy dissipation underlying NPQ for their photoprotection [START_REF] Jesus | Adaptations of microphytobenthos assemblages to sediment type and tidal position[END_REF][START_REF] Cartaxana | Physiological versus behavioral photoprotection in intertidal epipelic and epipsammic benthic diatom communities[END_REF]. Epipelic (freely living in sediment particles) motile species rely on both physiology and "behavioral photoprotection", i.e. the cells use their vertical mobility to escape from excess light [START_REF] Consalvey | The ups and downs of life in a benthic biofilm: migration of benthic diatoms[END_REF][START_REF] Perkins | Vertical cell movement is the primary response of intertidal benthic biofilms to increasing light dose[END_REF].

The respective importance of physiological and behavioral photoprotection has long been a topic of interest and has shown major recent advances [START_REF] Jesus | Effects of vertical migration by benthic microalgae on fluorescence measurements of photophysiology[END_REF][START_REF] Mouget | Migration or photoacclimation to prevent high irradiance and UV-B damage in marine microphytobenthic communities[END_REF][START_REF] Perkins | Vertical cell movement is the primary response of intertidal benthic biofilms to increasing light dose[END_REF][START_REF] Cartaxana | Physiological versus behavioral photoprotection in intertidal epipelic and epipsammic benthic diatom communities[END_REF][START_REF] Serôdio | Efficiency of photoprotection in microphytobenthos: role of vertical migration and the xanthophyll cycle against photoinhbition[END_REF]. In general, both XCdependent photoprotection and behavioral photoprotection provide the MPB diatoms with a strong capacity to resist periods of prolonged excessive light [START_REF] Blanchard | Characterizing and quantifying photoinhibition in intertidal microphytobenthos[END_REF][START_REF] Serôdio | Photosynthetic activity, photoprotection and photoinhibition in intertidal microphytobenthos as studied in situ using variable chlorophyll fluorescence[END_REF][START_REF] Serôdio | Efficiency of photoprotection in microphytobenthos: role of vertical migration and the xanthophyll cycle against photoinhbition[END_REF][START_REF] Perkins | Vertical cell movement is the primary response of intertidal benthic biofilms to increasing light dose[END_REF], including strong resistance to UV radiation [START_REF] Waring | Responses of estuarine intertidal microphytobenthic algal assemblages to enhanced ultraviolet B radiation[END_REF][START_REF] Mouget | Migration or photoacclimation to prevent high irradiance and UV-B damage in marine microphytobenthic communities[END_REF]. Additionally, MPB diatoms are usually characterized by a strong "dark NPQ" [START_REF] Perkins | The application of variable chlorophyll fluorescence to microphytobenthic biofilms[END_REF]) (see Section II.B.1), which presumably allows the cells to maintain their antenna system in a basal dissipative state and fully activate their NPQ machinery more rapidly upon sudden light exposure. Such a system would be especially relevant for the MPB diatoms that can spend more than 18 h per day in the dark (immersion and night emersion) before being exposed to sunlight again.

C. Brown Algae

Because most of the main macrophytic brown algae are sessile organisms, their light environment strongly depends on the water depth where they are located. The light environment of immerged brown algae is similar to that of planktonic diatoms at a given depth, while the light environment of emerged thalli bears greater semblance to that of the benthic diatoms. Brown algae are able to perform XC-dependent energy dissipation reflected in NPQ, with high NPQ reported for some species adapted to harsh environments, i.e. species living at the air-water interface and in the upper zone of the intertidal rocky shore (Harker et al. 1999;[START_REF] Colombo-Pallotta | Photosynthetic performance, light absorption, and pigment composition of Macrocystis pyrifera (Laminariales, Phaeophyceae) blades from different depths[END_REF]. Increases in XC carotenoid levels, conversion of those carotenoids to the forms responsible for energy dissipation, and their engagement in energy dissipation underlying NPQ are involved in the response to natural irradiance changes, including excessive light exposure and UV radiation [START_REF] Uhrmacher | Zeaxanthin content and the degree of photoinhibition are linearly correlated in the brown alga Dictyota dichotoma[END_REF]Hanelt et al. 1997;[START_REF] Schofield | Photosystem II quantum yields and xanthophyllcycle pigments of the macroalga Sargassum natans (Phaeophyceae): responses under natural sunlight[END_REF][START_REF] Apprill | Effects of ultraviolet radiation on Laminaria saccharina in relation to depth and tidal height in the Gulf of Maine[END_REF], sometimes coupled with other environmental stresses, especially desiccation (Harker et al. 1999) and high temperature [START_REF] Poulson | Photosynthetic response of Nereocystis luetkeana (Phaeophyta) to high light[END_REF]. For the same individual, the level of XC-dependent NPQ can be diverse among different parts of the thallus, dependent on the respective age of the thallus parts [START_REF] Colombo-Pallotta | Photosynthetic performance, light absorption, and pigment composition of Macrocystis pyrifera (Laminariales, Phaeophyceae) blades from different depths[END_REF][START_REF] Poulson | Photosynthetic response of Nereocystis luetkeana (Phaeophyta) to high light[END_REF] as well as life-cycle stage (sporophytes versus gametophytes) (Hanelt et al. 1997). For instance, in Laminaria saccharina, gametophytes are more resistant to excessive light stress than sporophytes, partially due to a stronger XC, which may influence their respective distribution on the sea shore (Hanelt et al. 1997).

The zonation of brown macroalgal species strongly depends on their depth distribution in and out of the water column (depending on the tide) as a function of irradiance and the light climate, with the exception of pelagic species like Sargassum spp. [START_REF] Schofield | Photosystem II quantum yields and xanthophyllcycle pigments of the macroalga Sargassum natans (Phaeophyceae): responses under natural sunlight[END_REF].

ongoing debate [START_REF] Goss | Regulation and function of xanthophyll cycle-dependent photoprotection in algae[END_REF], red algae are characterized by a high, stable concentration of Z, thus demonstrating the importance of de-epoxidized xanthophylls for efficient photoprotection.

IV. Conclusion

Investigations of the XC and photoprotective thermal energy dissipation (as reflected in NPQ) in diatoms and brown algae are of high importance for two obvious reasons: (1) the algae are essential primary producers in marine ecosystems, and, (2) their productivity is based mainly on light availability that can fluctuate drastically in aquatic habitats. XC-dependent energy dissipation (NPQ) appears to be the central mechanism for the rapid adjustment of light harvesting and photochemistry of the cell, thereby enabling diatoms and brown algae to maintain an optimal productivity in a permanently changing light environment. The most recent advances provide new insights into the regulation of the XC and the underlying mechanism of NPQ [START_REF] Goss | Regulation and function of xanthophyll cycle-dependent photoprotection in algae[END_REF][START_REF] Depauw | Exploring the molecular basis of responses to light in marine diatoms[END_REF][START_REF] Lepetit | Molecular dynamics of the diatom thylakoid membrane under different light conditions[END_REF][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] and the species/strain diversity of the XC and NPQ, including the latter features' potential ecophysiological importance [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] Petrou | Heterogeneity in the photoprotective capacity of three Antarctic diatoms during short-term changes in salinity and temperature[END_REF][START_REF] Lavaud | An explanation for the inter-species variability of the photoprotective non-photochemical chlorophyll fluorescence quenching in diatoms[END_REF]. There are numerous areas that deserve further attention in the near future, starting with the investigation of the protein partners involved in the regulation of NPQ (especially the Lhcx proteins and the Chl a 710-712 FCP complexes) and experiments to confirm the importance of the XC-dependent process(es) underlying NPQ as a functional trait essential to the ecology of diatoms and brown algae. To further investigate the latter, more thorough in-situ studies are needed that take advantage of recent improvements in fluorescence methodology and analysis of NPQ [START_REF] Perkins | The application of variable chlorophyll fluorescence to microphytobenthic biofilms[END_REF][START_REF] Enriquez | The use of the fluorescence signal in studies of seagrasses and macroalgae[END_REF][START_REF] Lefebvre | Duration of rapid light curves for determining in situ the photosynthetis of microphytobenthos biofilms[END_REF][START_REF] Serôdio | A model for describing the light response of the nonphotochemical quenching of chlorophyll fluorescence[END_REF][START_REF] Nitschke | Chlorophyll a fluorescence responses of temperate Phaeophyceae under submersion and emersion regimes: a comparison of rapid and steadystate light curves[END_REF]. To gain further knowledge about the mechanistic basis of NPQ in diatoms and brown algae, genomic/genetic tools in conjunction with sophisticated spectroscopic and physiological measurements will have to be performed. While such approaches have already shown their utility in diatoms [START_REF] Materna | First induced plastid genome mutations in an alga with secondary plastids: psbA mutations in the diatom Phaeodactylum tricornutum (Bacillariophyceae) reveal consequences on the regulation of photosynthesis[END_REF][START_REF] Bailleul | An atypical member of the light-harvesting complex stress-related protein family modulates diatom responses to light[END_REF][START_REF] Depauw | Exploring the molecular basis of responses to light in marine diatoms[END_REF][START_REF] Lavaud | Silencing of the violaxanthin de-epoxidase gene in the diatom Phaeodactylum tricornutum reduces diatoxanthin synthesis and non-photochemical quenching[END_REF][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], this field has just opened up for brown algae [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] Konotchick | Transcriptomic analysis of metabolic function in th egiant kelp, Macrocystis pyrifera, across depth and season[END_REF].

  complex 'a', fucoxanthin chlorophyll protein complex 'b', fucoxanthin chlorophyll protein complex 'o', fucoxanthin chlorophyll protein '6' (= Lhcx1)H + protons LHC light-harvesting complexLhcf, Lhcx light-harvesting complex protein binding fucoxanthin, light-harvesting complex protein 'x' LHCSR stress-related light-harvesting complex protein (= LI818) LHC7, LHC8 light-harvesting complex protein '7', light-harvesting complex protein '8'LI818 light-induced light-harvesting complex protein '818'

  energy dissipation and relaxation of NPQ is presumably possible only by efficient removal of Dt from its FCP-binding sites. However, the possibility of an extremely fast Dt epoxidation poses a problem for the Dd-Dt cycle when, during excess light illumination, a fast net accumulation of Dt is needed to trigger thermal energy dissipation resulting in NPQ(Lavaud et al. 2002b; Goss et al. 2006a;[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]) (see Section II.B.1). Without an efficient control of Dt epoxidation, excess light could not result in a fast and strong increase of Dt concentration. Therefore, it is of great importance that DEP be under the strict control of light-driven pH, i.e. ∆pH almost completely inhibits epoxidation of Dt(Mewes and Richter

  faster in brown algae(Garcia-Mendoza et al. 2011). Brown algae thus exhibit slow Z epoxidation kinetics similar to those in vascular plants(Garcia-Mendoza and Colombo- Pallotta 2007). This means that, during a low-light or dark period following excess light illumination, a significant amount of Z remains bound to the FCP complexes. As described above for Dt in diatoms (see also Section II.B.1), Z in brown algae also seems to be able to contribute to a sustained NPQ even after pH relaxation (Garcia-Mendoza and Colombo-Pallotta 2007). Hence the structure/organization of the FCP complexes appears to be responsible for this stable Dt-or Z-dependent NPQ. In the LHC II complexes of vascular plants such a pronounced stable Z-dependent NPQ is normally not seen after ∆pH relaxation(Goss et al. 2006a), but can be found in plants exposed to excess light for extended time periods with or without additional stresses (Demmig-Adams and Adams 2006; Demmig-Adams et al. 2006; see alsoAdams et al., Chap. 23, Chap. 24). It remains an open question as to why brown algae are unable to epoxidize Z to V with comparably fast kinetics as observed for the conversion of Dt to Dd in diatoms. Based on the observation that the conversion rate of A to V in darkness decreases in M. pyrifera exposed to longer illumination periods[START_REF] Ocampo-Alvarez | Antagonistic effect between violaxanthin and de-epoxidated pigments in nonphotochemical quenching induction in the qE deficient brown alga Macrocystis pyrifera[END_REF], it has been proposed that the slow epoxidation reaction represents an adaptive strategy of brown algae to cope with light stress conditions. Maintaining A in the antenna system may allow for a faster response to saturating light by accelerating Z accumulation and NPQ induction(Garcia-Mendoza et al. 2011).

  ;[START_REF] Enriquez | Direct determination of the excited state energies of the xanthophylls diadinoxanthin and diatoxanthin from Phaeodactylum tricornutum[END_REF]; see also Holzwarth and Jahns Chap. 5): (1) direct quenching of excited Chl a at the Q2 site by Dt via a transfer of energy from the respective S 1 states of Chl a and Dt as measured in vitro, or (2), together with proton binding, de-epoxidation of Dd to Dt could support a conformational change of the Dd-Dt-binding Lhcf/Lhcx proteins, thereby promoting disconnection of the FCP oligomers and formation of the Q1 site. The latter possibility (2) is supported by (i) the sensitivity of the 522-nm fingerprint to the pH-uncoupler NH 4 Cl and to the Dd de-epoxidation-inhibitor dithiothreitol (DTT) and (ii) numerous studies that succeeded in uncoupling pH, Dt and NPQ in several pennate and centric species(Lavaud et al. 2002b; Goss et al. 2006a;[START_REF] Lavaud | In diatoms, the transthylakoid proton gradient regulates the photoprotective non-photochemical fluorescence quenching beyond its control on the xanthophyll cycle[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] Gundermann and Büchel 2012;[START_REF] Lavaud | Silencing of the violaxanthin de-epoxidase gene in the diatom Phaeodactylum tricornutum reduces diatoxanthin synthesis and non-photochemical quenching[END_REF]. The latter studies showed three essential features of NPQ in diatoms: (1) a certain magnitude of the pH is needed for complete induction of NPQ, (2) NPQ can be modulated to a certain extent by Dt in the absence of a pH, once Dt had been synthesized in the presence of a pH, and (3) Dt is mandatory for NPQ. The possibility of a coupling between both a direct and an indirect effect of Dt in NPQ is consistent with the synergistic effect of pH and Z in the NPQ/qE allosteric model of vascular plants (Jahns and Holzwarth 2012;[START_REF] Ruban | The photoprotective molecular switch in the photosystem II antenna[END_REF]). Nevertheless, a significant part of NPQ can be maintained in the dark without any pH as long as Dt is present(Goss et al. 2006a;[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], see also Section II.A.1).

FCPs

  are trimers (FCPa) and hexamers (FCPb) composed of different Lhcf/Lhcx polypeptides. Their respective features (binding of Dd and Dt, ability to aggregate, dependence of the quenching property on pH and Dt) support the possible identification of FCPa as Q2 and FCPb as Q1[START_REF] Miloslavina | Ultrafast fluorescence study on the location and mechanism of non-photochemical quenching in diatoms[END_REF] Gundermann and Büchel 2012;[START_REF] Lepetit | Molecular dynamics of the diatom thylakoid membrane under different light conditions[END_REF]), although such a proposal remains to be further evaluated. In the pennate diatom P. tricornutum, a clear differentiation between FCP fractions that form trimers or hexa-to nonameric complexes (termed "FCPo") does not exist[START_REF] Lepetit | Spectroscopic and molecular characterization of the oligomeric antenna of the diatom Phaeodactylum tricornutum[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][START_REF] Joshi-Deo | Characterization of a trieric light-harvesting complex in the diatom Phaeodactylum tricornutum built of FcpA and FcpE proteins[END_REF]. FCPa contains the Fcp6 polypeptide, a member of the Lhcx family (Gundermann and[START_REF] Dambek | Biosynthesis of fucoxanthin and diadinoxanthin and function of initial pathway genes in Phaeodactylum tricornutum[END_REF], the functional ortholog of LHCSR in green algae[START_REF] Niyogi | Evolution of flexible non-photochemical quenching mechanisms that regulate light harvesting in oxygenic photosynthesis[END_REF] and see also Morosinotto and Bassi, Chap 14; Finazzi and Minagawa, Chap. 21), termed Lhcx1 in P. tricornutum. It was proposed that Fcp6 may interact with Fcp2, a member of the Lhcf family, to form a quenching center (Gundermann and Büchel 2012) that we hypothesize to be Q2. While Fcp2 might be responsible for the pH sensitivity of FCPa (Gundermann and Büchel 2012), and could therefore drive the above-mentioned aggregation of FCPs at the Q2 site, the role of Fcp6/Lhcx1 remains unclear. While the latter complexes' ability to bind protons has not been completely confirmed, they appear to bind Dt[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][START_REF] Zhu | Photoprotection in the diatom Thalassiosira pseudonana: Role of LI818-like proteins in response to high light stress[END_REF] Gundermann and Büchel 2012).

  [START_REF] García-Mendoza | The giant kelp Macrocystis pyrifera presents a different nonphotochemical quenching control than higher plants[END_REF] Garcia-Mendoza et al. 2011). The relationship between NPQ and Z (and A) is similar to that of diatoms, demonstrating an obligatory role of Z and A (Garcia-Mendoza and Colombo-Pallotta 2007; Ocampo-Alvarez et al. 2013). Strikingly, this relationship depends on light acclimation and the size of the VAZ pool (Ocampo-Alvarez et al. 2013), as is also the case in diatoms (Schumann et al. 2007). Because NPQ in brown algae reacts as in diatoms to DTT, pH uncouplers (NH 4 Cl, nigericin), and shows the same differences in pre-illuminated versus dark-adapted samples (Garcia-Mendoza and Colombo-Pallotta 2007; Garcia-Mendoza et al. 2011), it is likely to display similar mechanistic features as well (see Section 2.B.1).

  meneghiniana, the first steps of the alternative electron flow are comparable and also result in donation of electrons from stromal sources to the PQ pool. Further electron transfer reactions are, however, different and include electron transport from the PQ pool to PS I, which results in the production of NADPH. Chlororespiratory electron transport in C. meneghiniana does not lead to the generation of a ∆pH, most likely due to the absence of electron donation to PQ oxidase and due to involvement of a type-2 Ndh that does not translocate protons.Consequently, chlororespiratory-dependent Dd de-epoxidation and NPQ is not observed as long as the generation of NADPH simultaneously enables epoxidation of preformed Dt to Dd.II. Importance of the Xanthophyll Cycle and NPQ in the FieldA. Diatoms of the PhytoplanktonPlanktonic diatoms live in a three-dimensional environment characterized by strong physical driving forces coupled with a depth-dependent attenuation of the irradiance. They usually undergo passive movements determined by their sinking rate and the water turbulence. Many studies have explored the in-situ functioning of the XC and NPQ in planktonic diatoms in different ecosystems and in relation to changes of the diurnal, seasonal, and latitudinal underwater light climate (see[START_REF] Brunet | Can the xanthophyll cycle help extract the essence of the microalgal functional response to a variable light environment?[END_REF]. On a yearly scale, the pool size of Dd+Dt, the extent of Dd de-epoxidation, and NPQ correlate well with day length and seasonal changes in irradiance. On a daily scale, Dt and NPQ track the course of the sun and the water depth, showing an increase at midday and in the upper layer of the water column. In that framework, the dynamics of the Dd+Dt pool size and Dt synthesis were often used to characterize the coupling of phytoplankton photosynthesis with light and hydrodynamics.

Fig. 1 .

 1 Fig. 1. Reaction sequence and enzymes of the VAZ and the Dd-Dt cycles. The VAZ cycle is

  Xanthophyll-Dependent NPQ1. DiatomsIn diatoms, NPQ is mainly composed of qE, i.e. pH-or energy-dependent quenching (seePapageorgiou and Govindjee, Chap. 1, and Logan et al., Chap. 7 for a definition of the different quenching components). Indeed, qT, the NPQ component based on state-transitions, i.e. the reversible, wavelength-dependent translocation of part of the PS II antenna to PS I, seems to be absent in diatoms[START_REF] Owens | Light-harvesting function in the diatom Phaeodactylum tricornutum II. Distribution of excitation energy between the photosystems[END_REF]. It is, however, still unclear if a light intensitydependent structural reorganization of PS II and PS I, which might lead to an increased spillover of excitation energy, takes place. The origin and mechanism of qI, the slowly

reversible so-called "photoinhibitory" quenching, remain unclear, although it is likely that XC pigments are involved (Demmig-Adams and Adams 2006; see also

Adams et al., Chap. 23, Chap. 24)

. Mechanistic models of xanthophyll-dependent NPQ in diatoms have been proposed before

[START_REF] Lavaud | Fast regulation of photosynthesis in diatoms: mechanisms, evolution and ecophysiology[END_REF][START_REF] Goss | Regulation and function of xanthophyll cycle-dependent photoprotection in algae[END_REF][START_REF] Lepetit | Molecular dynamics of the diatom thylakoid membrane under different light conditions[END_REF]

and significant recent advances have been made since

[START_REF] Miloslavina | Ultrafast fluorescence study on the location and mechanism of non-photochemical quenching in diatoms[END_REF] Gundermann and Büchel 2012;[START_REF] Lavaud | An explanation for the inter-species variability of the photoprotective non-photochemical chlorophyll fluorescence quenching in diatoms[END_REF]

. The following tentative NPQ model for pennate diatoms (Fig.

2

, see

Büchel, Chap. 11

for a model for centric diatoms) is based on the recent observation of two quenching sites (Q1 and Q2) in diatoms and vascular plants

  ;[START_REF] Chukhutsina | Variations in the first steps of photosynthesis for the diatom Cyclotella meneghiniana grown under different light conditions[END_REF][START_REF] Lavaud | An explanation for the inter-species variability of the photoprotective non-photochemical chlorophyll fluorescence quenching in diatoms[END_REF]. However, until the latter disconnect from PS II, their dissipative property remains negligible. Presence of large and stable Chl a 710-712 FCP oligomers in high NPQ, dark-acclimated cells, might be induced by the decrease of the thylakoid lipid ratio MGDG/SQDG[START_REF] Lepetit | Molecular dynamics of the diatom thylakoid membrane under different light conditions[END_REF]. We propose that disconnection of Chl a 710-712 FCP oligomers upon illumination is generated by both protonation of the

Lhcf/Lhcx proteins involved in NPQ

(Goss et al. 2006;[START_REF] Lavaud | In diatoms, the transthylakoid proton gradient regulates the photoprotective non-photochemical fluorescence quenching beyond its control on the xanthophyll cycle[END_REF]

and conversion of Dd to Dt (see below and

[START_REF] Enriquez | Direct determination of the excited state energies of the xanthophylls diadinoxanthin and diatoxanthin from Phaeodactylum tricornutum[END_REF]

. Once disconnected, the Chl a 710- 712 FCP oligomers show a spatial arrangement of pigment-protein complexes that establishes new Chl a-Chl a interactions reducing the lifetime of excited Chl a

[START_REF] Miloslavina | Ultrafast fluorescence study on the location and mechanism of non-photochemical quenching in diatoms[END_REF]

),

Acknowledgments

References Acknowledgments JL thanks the Centre National de la Recherche Scientifique (CNRS), the Université de La Rochelle (ULR), the Contrat Plan Etat Région (CPER) 'Littoral', the Région Poitou-Charentes (CG17), the Deutsche Forschungsgemeinschaft (DFG, grant LA2368/2-1), the Deutscher Akademischer Austausch Dienst (DAAD), and Egide/Campus France (grants 27377TB and 28992UA) for their financial support. RG thanks the DFG (grants Go818/6-1, Go818/7-1) for its financial support. The two authors thank B. Lepetit for his help building and drawing the model of Figure 2. JL dedicates this work to his mentors Pr. J.-C. Duval and Dr. A.-L. Etienne.

B. Diatoms of the Microphytobenthos

The in-situ light response of diatoms of the microphytobenthic (MPB) community has received less attention. This is probably due to the technical challenge of measuring the photosynthetic activity of the MPB community in its major natural habitat, i.e. estuarine mudflats (though MPB organisms can also be found in surf beaches, coral reef lagoons, and shallow coastal waters) [START_REF] Underwood | Primary production by phytoplankton and microphytobenthos in estuaries[END_REF][START_REF] Perkins | The application of variable chlorophyll fluorescence to microphytobenthic biofilms[END_REF]. Because they inhabit sediments, MPB diatoms experience a different light climate than planktonic diatoms.

The fluctuations of irradiance at the surface of mudflats are similar to those on a land surface, with the succession of emersion/immersion periods adding a regular, sharp light/dark photoperiod. Additionally, other environmental cues (especially temperature and salinity) can vary rapidly between extremes [START_REF] Underwood | Primary production by phytoplankton and microphytobenthos in estuaries[END_REF] and generate stressful conditions.

The in-situ pool size of Dd+Dt and XC operation in the MPB diatoms depends strongly on light and temperature as a function of the tidal cycle, season, and latitude (van [START_REF] Van Leeuwe | Photacclimation in microphytobenthos and the role of the xanthophyll pigments[END_REF][START_REF] Chevalier | In situ photosynthetic activity and xanthophyll cycle development of undisturbed microphytobenthos in an intertidal mudflat[END_REF][START_REF] Jordan | Diurnal changes of photoadaptive pigments in microphytobenthos[END_REF]. The DES correlates well with higher daily and latitudinal irradiances [START_REF] Van Leeuwe | Photacclimation in microphytobenthos and the role of the xanthophyll pigments[END_REF][START_REF] Chevalier | In situ photosynthetic activity and xanthophyll cycle development of undisturbed microphytobenthos in an intertidal mudflat[END_REF][START_REF] Jordan | Diurnal changes of photoadaptive pigments in microphytobenthos[END_REF]) and with light distribution in the sediments, i.e. a higher DES is observed at the surface [START_REF] Jesus | Adaptations of microphytobenthos assemblages to sediment type and tidal position[END_REF][START_REF] Cartaxana | Physiological versus behavioral photoprotection in intertidal epipelic and epipsammic benthic diatom communities[END_REF]. The NPQ of the MPB diatoms in situ can reach high values above 5-6 [START_REF] Serôdio | Non-photochemical quenching of chlorophyll fluorescence and operation of the xanthophyll cycle in estuarine microphytobenthos[END_REF][START_REF] Serôdio | Photosynthetic activity, photoprotection and photoinhibition in intertidal microphytobenthos as studied in situ using variable chlorophyll fluorescence[END_REF][START_REF] Serôdio | Efficiency of photoprotection in microphytobenthos: role of vertical migration and the xanthophyll cycle against photoinhbition[END_REF]. In general, NPQ correlates well with DES with an especially clear increase at midday emersion (exposure to high irradiance) in summer [START_REF] Serôdio | Non-photochemical quenching of chlorophyll fluorescence and operation of the xanthophyll cycle in estuarine microphytobenthos[END_REF][START_REF] Chevalier | In situ photosynthetic activity and xanthophyll cycle development of undisturbed microphytobenthos in an intertidal mudflat[END_REF]. The seasonal pattern of NPQ is strongly influenced by both irradiance and temperature in a complex manner [START_REF] Serôdio | Non-photochemical quenching of chlorophyll fluorescence and operation of the xanthophyll cycle in estuarine microphytobenthos[END_REF][START_REF] Serôdio | Efficiency of photoprotection in microphytobenthos: role of vertical migration and the xanthophyll cycle against photoinhbition[END_REF][START_REF] Jesus | Effects of vertical migration by benthic microalgae on fluorescence measurements of photophysiology[END_REF]). These general trends of light and temperature control Inter-species XC-NPQ variability correlates with depth distribution, showing a higher NPQ capacity for the species located on the upper-shore (Harker et al. 1999;[START_REF] Rodrigues | A smaller and impaired xanthophyll cycle makes the deep sea algae Laminaria abyssalis (Phaeophyceae) highly sensitive to daylight when compared with shallow water Laminaria digitata[END_REF][START_REF] Nitschke | Chlorophyll a fluorescence responses of temperate Phaeophyceae under submersion and emersion regimes: a comparison of rapid and steadystate light curves[END_REF]. In this case, the higher NPQ capacity (usually twice as high) correlates with an increase of the pool size of VAZ and a greater synthesis of Z (Harker et al. 1999;[START_REF] Rodrigues | A smaller and impaired xanthophyll cycle makes the deep sea algae Laminaria abyssalis (Phaeophyceae) highly sensitive to daylight when compared with shallow water Laminaria digitata[END_REF]. The depth distribution and the related XC-dependent NPQ acclimation to irradiance also holds true for the same species in a given ecosystem [START_REF] Apprill | Effects of ultraviolet radiation on Laminaria saccharina in relation to depth and tidal height in the Gulf of Maine[END_REF]. All intertidal species increase the level of NPQ during emersion; for instance, the acclimative response of L. digitata to midday emersion involves Z synthesis and increased NPQ resulting from increased thermal energy dissipation [START_REF] Gévaert | Photo-inhibition and seasonal photosynthetic performance of the seaweed Laminaria saccharina during a simulated tidal cycle: chlorophyll fluorescence measurements and pigment analysis[END_REF][START_REF] Gévaert | Laminaria saccharina photosynthesis measured in situ: photoinhibition and xanthophyll cycle during a tidal cycle[END_REF][START_REF] Apprill | Effects of ultraviolet radiation on Laminaria saccharina in relation to depth and tidal height in the Gulf of Maine[END_REF]. Laminaria digitata also exhibits a XC-NPQ seasonal pattern involving an increase of the VAZ pool size in spring compared to winter [START_REF] Gévaert | Photo-inhibition and seasonal photosynthetic performance of the seaweed Laminaria saccharina during a simulated tidal cycle: chlorophyll fluorescence measurements and pigment analysis[END_REF]. The canopy-forming Macrocystis pyrifera provides a striking example [START_REF] Colombo-Pallotta | Photosynthetic performance, light absorption, and pigment composition of Macrocystis pyrifera (Laminariales, Phaeophyceae) blades from different depths[END_REF][START_REF] Konotchick | Transcriptomic analysis of metabolic function in th egiant kelp, Macrocystis pyrifera, across depth and season[END_REF][START_REF] Ocampo-Alvarez | Antagonistic effect between violaxanthin and de-epoxidated pigments in nonphotochemical quenching induction in the qE deficient brown alga Macrocystis pyrifera[END_REF]. Due to its height (about 18-20 m), this alga occupies the entire water column, such that the blades close to the surface are exposed to an underwater light environment with higher irradiance and stronger fluctuations.

Within a single organism, the XC-dependent capacity for NPQ varies dramatically with depth

(5-fold), with NPQ values being linearly related to water depth. As is also the case for intertidal species, NPQ differences are correlated with VAZ pool size and the capacity to synthesize Z and A, and with the differential expression of LI818 isoforms [START_REF] Colombo-Pallotta | Photosynthetic performance, light absorption, and pigment composition of Macrocystis pyrifera (Laminariales, Phaeophyceae) blades from different depths[END_REF][START_REF] Konotchick | Transcriptomic analysis of metabolic function in th egiant kelp, Macrocystis pyrifera, across depth and season[END_REF][START_REF] Ocampo-Alvarez | Antagonistic effect between violaxanthin and de-epoxidated pigments in nonphotochemical quenching induction in the qE deficient brown alga Macrocystis pyrifera[END_REF]. Finally, in pelagic species that live at the air-surface interface, the XC conversions follow the daily course of the sun and NPQ strictly correlates with the synthesis and the accumulation of Z [START_REF] Schofield | Photosystem II quantum yields and xanthophyllcycle pigments of the macroalga Sargassum natans (Phaeophyceae): responses under natural sunlight[END_REF].

Interestingly, the strongest competitors with brown algae in the intertidal zone are the red algae (Wiencke and Bischof 2012). Based on the current state of knowledge, it is not clear if red algae contain an operable XC [START_REF] Carnicas | Effects of changes of irradiance on the pigment composition of Gracilaria tenuistipitata var. liui Zhang et Xia[END_REF][START_REF] Ursi | Intraspecific variation of photosynthesis, respiration and photoprotective carotenoids in Gracilaria birdiae (Gracilariales: Rhodophyta)[END_REF]