Claudia Büchel
email: c.buechel@bio.uni-frankfurt.de

Reimund Goss
email: rgoss@rz.uni-leipzig.de

Benjamin Bailleul

Douglas A Campbell

Johann Lavaud

Bernard Lepetit

Photosynthetic Light Reactions in Diatoms. I. The Lipids and Light-Harvesting Complexes of the Thylakoid Membrane

Keywords: diatoms, FCP, LHC, light harvesting, lipids, thylakoids, xanthophyll cycle Cyclotella meneghiniana Ch. gracilis: Chaetoceros gracilis Chl: chlorophyll Dd: diadinoxanthin diacylglycerylhydroxymethyl-N, N, N-trimethyl-β- intramolecular charge transfer LHC: light-harvesting complex photosystem II PUFAs: polyunsaturated fatty acids sulphoquinovosyldiacylglycerol T. pseudonana: Thalassiosira pseudonana XC: xanthophyll cycle

Table of content

a molecules in the PS, the so-called reaction center Chls, where charge separation takes place. This fuels the photosynthetic electron transport, where electrons are subtracted from water and eventually used for the generation of NADPH (Fig. 1A). These photosynthetic light reactions take place in the thylakoid membranes, and the coupling of the electron transfer to a unidirectional transport of protons leads to accumulation of protons in the lumen that is used by an ATP-Synthase to generate ATP (for details see "Photosynthetic light reactions in diatoms.

II. The dynamic regulation of the various light reactions"). NADPH and ATP are then mainly used by the Calvin cycle in the chloroplast stroma to fix CO2. Thus, diatoms share the same elementary photosynthetic modules with other organisms performing oxygenic photosynthesis [START_REF] Falkowski | The microbial engines that drive earth's biogeochemical cycles[END_REF], which are fairly similar in all eukaryotes, with one exception: Lightharvesting systems differ significantly amongst the different taxa [START_REF] Neilson | Structural and functional diversification of the light-harvesting complexes in photosynthetic eukaryotes[END_REF][START_REF] Büchel | Evolution and function of light harvesting proteins[END_REF][START_REF] Büchel | Light harvesting complexes in chlorophyll c-containing algae[END_REF][START_REF] Croce | Light harvesting in oxygenic photosynthesis: Structural biology meets spectroscopy[END_REF]. In addition, there are further subtle changes concerning photosystems and their functional organization. For example, the PSII core recently crystallized from Chaetoceros gracilis contains four more Chl a molecules than the plant PSII core, and one additional subunit shielding the oxygen evolving complex (Nagao et al. 2019a). With more than 100,000 species of diatoms present on earth, exhibiting a huge phylogenetic diversity [START_REF] Mann | An inordinate fondness? The number, distributions, and origins of diatom species[END_REF], we may also assume some diversity among diatom species even within these elementary modules.

The photosynthetic modules are embedded in the thylakoid membrane. In contrast to the green lineage, diatom thylakoids are organized in homogeneous stacks of three which run along the whole diatom plastid [START_REF] Berkaloff | Subunit organization of PSI particles from brown algae and diatoms: polypeptide and pigment analysis[END_REF]) (Fig. 1B). They do not show the grana stroma differentiation which imposes the lateral segregation of the PSII and PSI in plants [START_REF] Anderson | Insights into the consequences of grana stacking of thylakoid membranes in vascular plants: a personal perspective[END_REF]). In diatoms, immunolocalization studies, lipid analyses, and 3D reconstruction showed that PSI are mostly found in the stroma-facing external thylakoids whereas PSII are embedded in the core of the stack, at the interface of two thylakoids [START_REF] Pyszniak | Immunocytochemical localization of photosystem I and the fucoxanthinchlorophyll-a/c light-harvesting complex in the diatom Phaeodactylum tricornutum[END_REF][START_REF] Lepetit | Molecular dynamics of the diatom thylakoid membrane under different light conditions[END_REF][START_REF] Flori | Plastid thylakoid architecture optimizes photosynthesis in diatoms[END_REF]). This heterogeneity is enforced under red light conditions, which induce an unusual stacking of diatom thylakoids. Here, large areas were revealed which are exclusively occupied by PSI supercomplexes consisting of PSI cores with their PSI-specific antenna of Lhcr proteins [START_REF] Bína | Novel structural aspect of the diatom thylakoid membrane: lateral segregation of photosystem I under red-enhanced illumination[END_REF]. In line with this, clusters of PSII cores including their FCP antennae have been revealed by cryo-electron tomography in Phaeodactylum tricornutum recently [START_REF] Levitan | Structural and functional analyses of photosystem II in the marine diatom Phaeodactylum tricornutum[END_REF]. For steric reasons the ATP synthase is located in the outer thylakoids. The vicinity of PSII and PSI, as well as some connections between thylakoids, ensures a fast diffusion of electron shuttles between the two photosystems [START_REF] Flori | Plastid thylakoid architecture optimizes photosynthesis in diatoms[END_REF]), whereas their diffusion can limit the overall rate of the linear electron flow in plants [START_REF] Kirchhoff | Plastocyanin redox kinetics in spinach chloroplasts: evidence for disequilibrium in the high potential chain[END_REF]Kirchhoff et al. 2011). This peculiar segregation also has important consequences for the regulation of the light capture by the two photosystems, by preventing the share of excitons between them [START_REF] Flori | Plastid thylakoid architecture optimizes photosynthesis in diatoms[END_REF]), a process found in cyanobacteria and red algae, and called spillover [START_REF] Biggins | Regulation of excitation energy transfer in organisms containing phycobilins[END_REF]. Whether based on immunoblot quantification or functional measurements, diatoms seem to possess more PSII than PSI, in contrast to plants and green algae where PSI occurs at least in the same amount as PSII [START_REF] Smith | Photochemical apparatus organization in the diatom Cylindrotheca fusiformis: Photosystem stoichiometry and excitation distribution in cells grown under high and low irradiance[END_REF][START_REF] Strzepek | Photosynthetic architecture differs in coastal and oceanic diatoms[END_REF][START_REF] Thamatrakoln | Deathspecific protein in a marine diatom regulates photosynthetic responses to iron and light availability[END_REF]. With this higher amount of PSII, diatoms may compensate their slightly lower maximum PSII efficiency, which is usually around 0.6-0.7, while in plants it reaches values of ~0.84 (Kalaji et al. 2014).

Lipids and pigmented proteins are the major components of the diatom thylakoid membrane.

Usually, the proteins occupy an area of 70-80 % of the thylakoid membrane, while lipids correspondingly contribute to 20-30 % [START_REF] Kirchhoff | Diffusion of molecules and macromolecules in thylakoid membranes[END_REF]. Compared to the green lineage, both the respective amounts of the different thylakoid lipid classes and the respective fatty acids composition is considerably different in diatoms. Moreover, the structure and composition of the light harvesting system, the dominant protein constituent of the thylakoid membranes, shows various diatom specific peculiarities. Certainly one reason for these differences is based on the complex evolutionary history of diatoms -with plastids originating from secondary endosymbiosis with red algae, while considerable amounts of 'green' genes are also found in the diatom genome (Dorrell et al. 2017).

In this review article, we will summarize current knowledge on lipid composition (part 1) and antennae structures of diatom thylakoids (part 2).

Thylakoid lipids

The lipid composition of thylakoid membranes

Before the lipid and fatty acid composition of diatom thylakoid membranes is presented in detail, it should be mentioned that most of the depicted results have been derived from laboratory experiments under defined growth conditions (for details please refer to the cited references). However, in their natural environment diatoms are exposed to extreme differences in temperature and light intensities. To cope with these extreme abiotic conditions diatoms adjust the lipid and fatty acid composition of the thylakoids in order to maintain the membrane in a fluid working state (see also 1.2 and 1.3).

The diatom thylakoid membranes are composed of the two neutral galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), the negatively charged sulfolipid sulphoquinovosyldiacylglycerol (SQDG) and the anionic phospholipid phosphatidylglycerol (PG) [START_REF] Vieler | The lipid composition of the unicellular green alga Chlamydomonas reinhardtii and the diatom Cyclotella meneghiniana investigated by MALDI-TOF MS and TLC[END_REF][START_REF] Goss | The lipid dependence of diadinoxanthin de-epoxidation presents new evidence for a macrodomain organization of the diatom thylakoid membrane[END_REF][START_REF] Lepetit | Molecular dynamics of the diatom thylakoid membrane under different light conditions[END_REF][START_REF] Abida | Membrane glycerolipid remodeling triggered by nitrogen and phosphorus starvation in Phaeodactylum tricornutum[END_REF]. These lipids are also found in higher plants, but in addition the diatom thylakoid membranes can contain low amounts of betaine lipids like diacylglycerylcarboxyhydroxymethylcholine (DGGC) and diacylglycerylhydroxymethyl-N,N,N-trimethyl-β-alanine (DGTA) [START_REF] Vieler | The lipid composition of the unicellular green alga Chlamydomonas reinhardtii and the diatom Cyclotella meneghiniana investigated by MALDI-TOF MS and TLC[END_REF][START_REF] Canavate | Exploring occurrence and molecular diversity of betaine lipids across taxonomy of marine microalgae[END_REF]. A third betaine lipid, namely diacylgyceryl-N-trimethylhomoserine (DGTS), which acts as precursor in the synthesis of DGTA, can be found in trace amounts [START_REF] Canavate | Exploring occurrence and molecular diversity of betaine lipids across taxonomy of marine microalgae[END_REF]). Phosphatidylcholine, which in higher plant thylakoid preparations is supposed to represent a contamination with chloroplast envelope membranes, seems to be a general constituent of diatom thylakoid membranes [START_REF] Vieler | The lipid composition of the unicellular green alga Chlamydomonas reinhardtii and the diatom Cyclotella meneghiniana investigated by MALDI-TOF MS and TLC[END_REF][START_REF] Goss | The lipid dependence of diadinoxanthin de-epoxidation presents new evidence for a macrodomain organization of the diatom thylakoid membrane[END_REF][START_REF] Lepetit | Molecular dynamics of the diatom thylakoid membrane under different light conditions[END_REF].

Although the main lipids of diatom thylakoids are comparable to those of the green lineage, their contribution to the overall lipid content of the membrane is significantly different. While in higher plants and green algae the neutral galactolipids dominate the lipid content of the thylakoid membrane and amount to 70 to 80% of the total lipid [START_REF] Murata | Lipids in photosynthesis: an overview[END_REF], the concentrations of MGDG and DGDG are strongly reduced in the thylakoids of pennate (P. tricornutum) and centric (Cyclotella meneghiniana) diatoms [START_REF] Goss | The lipid dependence of diadinoxanthin de-epoxidation presents new evidence for a macrodomain organization of the diatom thylakoid membrane[END_REF][START_REF] Lepetit | Molecular dynamics of the diatom thylakoid membrane under different light conditions[END_REF][START_REF] Abida | Membrane glycerolipid remodeling triggered by nitrogen and phosphorus starvation in Phaeodactylum tricornutum[END_REF]. The reduction of MGDG and DGDG is accompanied by simultaneous increases of the two negatively charged lipids SQDG and PG. This is especially obvious in thylakoid membranes of P. tricornutum or C. meneghiniana purified from high light grown cultures [START_REF] Lepetit | Molecular dynamics of the diatom thylakoid membrane under different light conditions[END_REF]. They contain SQDG as the most abundant lipid, and the combined negatively charged lipids SQDG and PG contribute to more than 50% of the total thylakoid lipids. The ratio of neutral to negatively charged lipids lies between one and two and is thus significantly lower than the typical values between three and four observed in higher plant and green algal thylakoid membranes. High concentrations of the negatively charged lipids SQDG and PG in diatom thylakoid membranes are supported by the study of [START_REF] Yan | Profiles of photosynthetic glycerolipids in three strains of Skeletonema determined by UPLC-Q-TOF-MS[END_REF] who determined the photosynthetic lipid and fatty acid profile of three different strains of the marine diatom Skeletonema sp.. Two of the three strains show high SQDG concentrations (35-40% of the photosynthetic lipids) and, in combination with PG, the negatively charged lipids amount to almost 50% of the total photosynthetic lipids of Skeletonema.

Fatty acid composition of thylakoid membrane lipids

Diatoms are characterized by the synthesis of very long-chain polyunsaturated fatty acids (PUFAs) with chain lengths of up to 28 C atoms [START_REF] Guschina | Lipids and lipid metabolism in eukaryotic algae[END_REF]. The presence of long-chain PUFAs is also reflected by the fatty acid composition of the thylakoid membrane lipids. In centric diatoms (Skeletonema marinoi, Thalassiosira weissflogii) and the pennate P.

tricornutum MGDG contains the main long-chain PUFA of diatoms, i.e. eicosapentaenoic acid (EPA, 20:5) [START_REF] Yongmanitchai | Positional distribution of fatty acids, and molecular species of polar lipids, in the diatom Phaeodactylum tricornutum[END_REF][START_REF] Yan | Profiles of photosynthetic glycerolipids in three strains of Skeletonema determined by UPLC-Q-TOF-MS[END_REF][START_REF] Dodson | Mono-and digalactosyldiacylglycerol composition of the marennine-producing diatom, Haslea ostrearia: Comparison to a selection of pennate and centric diatoms[END_REF][START_REF] Dodson | The long and short of it: temperature-dependent modifications of fatty acid chain length and unsaturation in the galactolipid profiles of the diatoms Haslea ostrearia and Phaeodactylum tricornutum[END_REF][START_REF] Abida | Membrane glycerolipid remodeling triggered by nitrogen and phosphorus starvation in Phaeodactylum tricornutum[END_REF]. EPA is preferentially bound to the sn-1 position of the glycerol backbone whereas C16 fatty acids with different amounts of double bonds (16:1, 16:2, 16:3, 16:4) can be observed at the sn-2 position. MGDG with C20:5 and C16:3 fatty acids seems to represent the majority of MGDG molecules in diatom thylakoids. Interestingly, EPA at the sn-1 position of the MGDG molecule can be replaced by other C20 fatty acids with a lower amount of double bonds like eicosatrienoic acid (20:3) or eicosatetraenoic acid (20:4). In addition to MGDG molecules with C20/C16 fatty acids, the recent analyses have presented evidence for the presence of MGDG molecules with C16 fatty acids at both the sn-1 and sn-2 position.

Longer-chain PUFAs, like docosahexaenoic acid (DHA, 22:6), or C18 fatty acids are usually not found or detected in only minor concentrations in MGDG of centric diatoms and P.

tricornutum.

The analyses by [START_REF] Yongmanitchai | Positional distribution of fatty acids, and molecular species of polar lipids, in the diatom Phaeodactylum tricornutum[END_REF], [START_REF] Yan | Profiles of photosynthetic glycerolipids in three strains of Skeletonema determined by UPLC-Q-TOF-MS[END_REF], [START_REF] Dodson | Mono-and digalactosyldiacylglycerol composition of the marennine-producing diatom, Haslea ostrearia: Comparison to a selection of pennate and centric diatoms[END_REF], [START_REF] Dodson | The long and short of it: temperature-dependent modifications of fatty acid chain length and unsaturation in the galactolipid profiles of the diatoms Haslea ostrearia and Phaeodactylum tricornutum[END_REF][START_REF] Abida | Membrane glycerolipid remodeling triggered by nitrogen and phosphorus starvation in Phaeodactylum tricornutum[END_REF] also pointed out that the fatty acid composition of the main galactolipids MGDG and DGDG is comparable. EPA is the main fatty acid species at the sn-1 position of the DGDG molecule. The sn-2 position of DGDG is usually occupied by C16 fatty acids, and DGDG forms with C16 fatty acids at both the sn-1 and sn-2 positions can be observed. Like MGDG, DGDG molecules with the two fatty acids EPA (C20:5) and hexadecatrienoic acid (C16:3) seem to represent the majority of DGDG molecules in the diatom thylakoid membrane. Other pennate diatoms (Haslea ostrearia and Navicula perminuta) seem to exhibit a different fatty acid composition of the MGDG and DGDG molecules compared with the centric diatoms and P. tricornutum [START_REF] Dodson | Mono-and digalactosyldiacylglycerol composition of the marennine-producing diatom, Haslea ostrearia: Comparison to a selection of pennate and centric diatoms[END_REF]. In these species, an enrichment of EPA cannot be detected and C18/C16 and C18/C18 forms of MGDG and DGDG are typically observed. The major forms of MGDG and DGDG seem to contain linolenic acid (18:3) and hexadecatrienoic acid (16:3) at the sn-1 and sn-2 positions, respectively. The fatty acid composition of these pennate diatoms thus is comparable to the fatty acid composition of MGDG and DGDG of higher plants and green algae [START_REF] Browse | Fluxes through the prokaryotic and eukaryotic pathways of lipid synthesis in the '16:3' plant Arabidopsis thaliana[END_REF][START_REF] Cho | On the metabolic relationships between monogalactosyldiacylglycerol and digalactosyldiacylglycerol molecular species in Dunaliella salina[END_REF]. However, this different lipid profile may be related to acclimation to environmental temperatures. Indeed, [START_REF] Dodson | The long and short of it: temperature-dependent modifications of fatty acid chain length and unsaturation in the galactolipid profiles of the diatoms Haslea ostrearia and Phaeodactylum tricornutum[END_REF] could detect significant amounts of EPA in H. ostrearia at the sn-1 position of the MGDG and DGDG molecules under normal temperature conditions, whereas at high temperatures (30 °C) no EPA or other C20 fatty acids, but high concentrations of C18 fatty acids could be observed at the sn-1 position of MGDG and DGDG both in H. ostrearia and also P. tricornutum.

With regard to the anionic membrane lipids of diatoms, the fatty acid composition of SQDG of P. tricornutum [START_REF] Yongmanitchai | Positional distribution of fatty acids, and molecular species of polar lipids, in the diatom Phaeodactylum tricornutum[END_REF][START_REF] Abida | Membrane glycerolipid remodeling triggered by nitrogen and phosphorus starvation in Phaeodactylum tricornutum[END_REF] and that of SQDG and PG of three Skeletonema species has been analyzed [START_REF] Yan | Profiles of photosynthetic glycerolipids in three strains of Skeletonema determined by UPLC-Q-TOF-MS[END_REF]. In contrast to MGDG and DGDG, the main part of SQDG present in diatom thylakoid membranes is enriched in fatty acids with a shorter chain length and C14 and C16 fatty acids are usually observed in both the sn-1 and sn-2 positions. The C14 and C16 fatty acids include C14:0, C16:0, C16:1 and C16:3 species. PG in Skeletonema, on the other hand, contains C18:1 fatty acids as the main molecular species.

Function of thylakoid membrane lipids

Thylakoid membrane lipids exert their functions on different structural levels. First, they build up the membrane bilayer in which the photosynthetic pigment protein complexes and the components of the electron transport chain are incorporated. Changes in the lipid and/or fatty acid composition of the thylakoid membrane as a reaction to changes in the environmental conditions, like e.g. temperature or light fluctuations, keep the membrane in a fluid state and thus maintain efficient electron transport and other membrane related processes such as the operation of the diadinoxanthin cycle (see below). The importance of a fluid state for the operation of biological membranes has first been described by [START_REF] Singer | The Fluid Mosaic Model of the Structure of Cell Membranes[END_REF].

Recently, advances in our understanding of the fluid mosaic model with a special focus of different lipid phases in thylakoid membranes have been presented by [START_REF] Wilhelm | The fluid-mosaic membrane theory in the context of photosynthetic membranes: Is the thylakoid membrane more like a mixed crystal or like a fluid?[END_REF].

With respect to the environmental factor light, [START_REF] Mock | Photosynthetic energy conversion under extreme conditions -I: important role of lipids as structural modulators and energy sink under N-limited growth in Antarctic sea ice diatoms[END_REF] observed an increase of the non-bilayer lipid MGDG accompanied by higher concentrations of EPA in sea ice diatoms which were grown under low light illumination. They proposed that the high contents of EPA were responsible for membrane fluidity and the velocity of the photosynthetic electron transport. [START_REF] Rousch | Changes in fatty acid profiles of thermo-intolerant and thermo-tolerant marine diatoms during temperature stress[END_REF] observed that the thermo-tolerant diatom Chaetoceros muelleri reacted with stronger changes in the fatty acid profile to temperature changes than the thermo-intolerant P. tricornutum. Recently, [START_REF] Bojko | Acclimatization of Thalassiosira pseudonana photosynthetic membranes to environmental temperature changes[END_REF] could show that the centric diatom Thalassiosira pseudonana reacted with an increase of PUFAs to a decrease of the cultivation temperature. The higher content of PUFAs resulted in a stabilization of the membrane fluidity and the PSII quantum yield at low temperatures. In addition to their role as membrane building blocks, thylakoid lipids serve as structural elements of the pigment proteins and play a role in the oligomerization of photosystems and light-harvesting complexes. Through an enrichment at the monomer-monomer interface MGDG seems to play a role in the establishment and maintenance of PSII dimers which represent the native state of PSII in the thylakoid membrane [START_REF] Kern | Lipids in photosystem II: Multifunctional cofactors[END_REF]Nagao et al. 2019a). PG is most likely involved in the trimerization of the PSII light-harvesting complex of higher plants, the LHCII, whereas DGDG is important for the structural integrity of the complex [START_REF] Kühlbrandt | Atomic model of plant light-harvesting complex by electron crystallography[END_REF].

Recently, the first molecular structure of a diatom pigment protein complex determined by xray crystallography has been published, namely that of the dimeric fucoxanthin chlorophyll binding protein (FCP) complex of P. tricornutum [START_REF] Pi | The pigment-protein network of a diatom photosystem II-light-harvesting antenna supercomplex[END_REF]) (see "2.5 Molecular structure of FCPs"). Within the dimeric FCP two lipid molecules have been resolved, a PG and a DGDG molecule. Although the PG and DGDG molecules have not been assigned specific roles in the dimerization of the FCP, it was proposed that the two lipid molecules serve to stabilize the FCP dimers. The determination of the structure of a PSII-FCP supercomplex from the pennate diatom Ch. gracilis by cryo-electron microscopy revealed the presence of more than 100 lipid molecules in the PSII-FCP dimer [START_REF] Pi | The pigment-protein network of a diatom photosystem II-light-harvesting antenna supercomplex[END_REF][START_REF] Wang | Structural features of the diatom photosystem II-light-harvesting antenna complex[END_REF]. The majority of the lipids, comprising 20 DGDG, 42 MGDG, 16 SQDG and 30 PG molecules, are located at the interfaces of the PSII protein subunits which, according to [START_REF] Wang | Structural features of the diatom photosystem II-light-harvesting antenna complex[END_REF], argues for an important role in the establishment and stabilization of protein subunit interactions. A stabilizing role of thylakoid membrane lipids for the native structure of FCP complexes has also been reported in another study [START_REF] Schaller-Laudel | Influence of thylakoid membrane lipids on the structure of aggregated light-harvesting complexes of the diatom Thalassiosira pseudonana and the green alga Mantoniella squamata[END_REF]. In these experiments, the neutral galactolipids MGDG and DGDG exhibited the highest capacity for the stabilization of FCP complexes. For higher plants and cyanobacteria it has been shown that MGDG plays a role in targeting QB to its binding site at the stromal side of the PSII reaction center and that DGDG is important for the stabilization of the Oxygen Evolving Complex at the lumenal side of PSII [START_REF] Kern | Lipids in photosystem II: Multifunctional cofactors[END_REF]. Although the first results concerning the structure of the diatom PSII core complex have been published [START_REF] Wang | Structural features of the diatom photosystem II-light-harvesting antenna complex[END_REF], a detailed assignment of the lipid molecules within the core complex to specific functions is still missing. However, the structural and functional similarities between the diatom and higher plant and cyanobacterial PSII and PSI core complexes suggest a similar role of the thylakoid lipids in photosystems of diatoms.

Besides the lipids that build up the main lipid phase of the thylakoid membrane and those that serve as structural lipids embedded into the protein matrix, a third class of membrane lipids exist. These lipids are closely associated with the pigment protein complexes and form a lipid shield surrounding the complexes. In the LHCII of higher plants [START_REF] Schaller | The main thylakoid membrane lipid monogalactosyldiacylglycerol (MGDG) promotes the de-epoxidation of violaxanthin associated with the light-harvesting complex of photosystem II (LHCII)[END_REF]) and the FCP complexes of the diatoms P. tricornutum and C. meneghiniana [START_REF] Lepetit | Evidence for the existence of one antennaassociated, lipid-dissolved, and two protein-bound pools of diadinoxanthin cycle pigments in diatoms[END_REF]) the lipid shields are highly enriched in MGDG. MGDG serves to solubilize the hydrophobic xanthophyll cycle (XC) pigments violaxanthin, antheraxanthin and zeaxanthin in higher plants and diadinoxanthin (Dd) and diatoxanthin (Dt) in diatoms, thereby making these pigments accessible to the enzymes violaxanthin or diadinoxanthin de-epoxidase [START_REF] Goss | Role of hexagonal structureforming 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]. (Side note: The diadinoxanthin de-epoxidase is usually annotated as a violaxanthin de-epoxidase in diatom genomes. This reflects the fact that this enzyme can also de-epoxidize violaxanthin, which is a precursor in synthesis of the main diatom carotenoids fucoxanthin and Dd [START_REF] Lohr | Algae displaying the diadinoxanthin cycle also possess the violaxanthin cycle[END_REF].) The XC has a dominant role in photoprotection of plants and diatoms (see "Photosynthetic light reactions in diatoms. II. The dynamic regulation of the various light reactions"). It has also been proposed that the local enrichment of MGDG leads to the establishment of a special lipid phase, the so-called inverted hexagonal (HII) phase. This non-bilayer phase seems to be essential for the efficient conversion of XC pigments [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] Latowski | Violaxanthin de-epoxidase, the xanthophyll cycle enzyme, requires lipid inverted hexagonal structures for its activity[END_REF][START_REF] Goss | Role of hexagonal structureforming 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]). The HII phase together with the LHCII/FCP, the de-epoxidases and the XC pigments can be described as a special thylakoid membrane domain with a specific lipid and protein composition and a specific function, namely the enzymatic de-epoxidation of the XC pigments [START_REF] Goss | Direct isolation of a functional violaxanthin cycle domain from thylakoid membranes of higher plants[END_REF].

The XC pigments also influence the properties of the lipid phase of the membrane. The conversion of Dd to Dt leads to a stable increase of the rigidity of the hydrophobic core of the diatom thylakoid membrane and a dynamic stabilization of the peripheral membrane parts which can be observed during the ongoing de-epoxidation reaction [START_REF] Bojko | Diadinoxanthin deepoxidation as important factor in the short-term stabilization of diatom photosynthetic membranes exposed to different temperatures[END_REF]). The stabilization of diatom thylakoids by XC pigments has been suggested to play a role in the adaptation of the membrane to rapid changes in temperature.

Localization of thylakoid membrane lipids

Based on the findings that SQDG strongly inhibits the de-epoxidation of Dd to Dt [START_REF] Goss | The lipid dependence of diadinoxanthin de-epoxidation presents new evidence for a macrodomain organization of the diatom thylakoid membrane[END_REF]) and that MGDG forms a lipid shield around the FCP complexes, which incorporates a large part of the XC pigments [START_REF] Lepetit | Evidence for the existence of one antennaassociated, lipid-dissolved, and two protein-bound pools of diadinoxanthin cycle pigments in diatoms[END_REF], a model for the lipid and protein distribution in the thylakoid membranes of diatoms was proposed [START_REF] Lepetit | Molecular dynamics of the diatom thylakoid membrane under different light conditions[END_REF]). The model predicts

Light harvesting

Diatoms, like vascular plants, harvest light using membrane intrinsic proteins that belong to the light-harvesting complex (Lhc) protein family. Lhcs are characterized by three membrane spanning α-helices, whereby helix 1 and 3 form a cross-like superhelical structure. These proteins non-covalently attach chlorophylls (Chl) as well as carotenoids. Lhcs are connected with the photosystem core complexes, forming so-called photosystem supercomplexes. Major differences compared to plants can be found in the number of Lhcs associated with the photosystems, their specific structure, and the number and identity of pigments bound to the diatom light harvesting apparatus. In recent years there have been major advances in understanding the light harvesting machinery of diatoms which we will review here.

The diatom light harvesting systems

In diatoms Lhcs, Chl a and Chl c1 (and little Chl c2) [START_REF] Fawley | A new form of chlorophyll c involved in light-harvesting[END_REF][START_REF] Kraay | Separation of chlorophylls c1, c2, and c3 of marine phytoplancton by reversed-phase-C18-high-performance liquid chromatography[END_REF]) are accompanied by the major carotenoid fucoxanthin (Fx), and usually by minor amounts of Dd and Dt. The diatom genome encodes large numbers of Lhc genes whose translated proteins fall into three major and some minor groups. The major groups include i) the main group of proteins working in light harvesting, named Lhcf, ii) Lhcs most closely related to those of red algae called Lhcr, and iii) the photoprotective proteins called LI818 or Lhcx, related to green algal LhcSR [START_REF] Eppard | The genes encoding light-harvesting subunits of Cyclotella cryptica (Bacillariophyceae) constitute a complex and heterogeneous family[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] Ghazaryan | Involvement of the Lhcx protein Fcp6 of the diatom Cyclotella meneghiniana in the macro-organisation and structural flexibility of thylakoid membranes[END_REF]. The minor groups comprise iv) a small group of genes called Lhcz, v) the so called RedCaps [START_REF] Engelken | The extended light-harvesting complex (LHC) protein superfamily: Classification and evolutionary dynamics[END_REF], and vi) some other sequences that do not belong to either group. Lhcf and Lhcr genes constitute the biggest groups with 8-17 and 9-14 members, respectively, when comparing the three best studied diatoms P. tricornutum (Bowler et al. 2008), T. pseudonana (Armbrust et al. 2004), and Cyclotella meneghiniana [START_REF] Gundermann | Fucoxanthin-chlorophyll protein complexes of the centric diatom Cyclotella meneghiniana differ in Lhcx1 and Lhcx6_1 content[END_REF]. Concerning Lhcx, numbers are smaller with 4-6 genes. However, numbers are even higher for e.g. Fragilariopsis cylindrus (Mock et al. 2017). For P. tricornutum as well as for T. pseudonana, expression of all those genes was proven either on mRNA or on protein level in cells and pigment-protein complexes (Nymark et al. 2009;[START_REF] Lepetit | Evidence for the existence of one antennaassociated, lipid-dissolved, and two protein-bound pools of diadinoxanthin cycle pigments in diatoms[END_REF][START_REF] Grouneva | The thylakoid membrane proteome of two marine diatoms outlines both diatom-specific and species-specific features of the photosynthetic machinery[END_REF][START_REF] Schober | Organelle studies and proteome analyses of mitochondria and plastids fractions from the diatom Thalassiosira pseudonana[END_REF][START_REF] Kansy | Pre-purification of diatom pigment protein complexes provides insight into the heterogeneity of FCP complexes[END_REF], and for C. meneghiniana all Lhcf and some Lhcx proteins were found in different light harvesting complexes [START_REF] Gundermann | Fucoxanthin-chlorophyll protein complexes of the centric diatom Cyclotella meneghiniana differ in Lhcx1 and Lhcx6_1 content[END_REF]. Most of the proteins encoded for by Lhc genes assemble into multi-subunit complexes that are then called fucoxanthin-chlorophyllprotein complexes (FCP). Note that single Lhc proteins of diatoms sometimes are also called FCP mainly for historical reasons, since the Lhc nomenclature was only introduced when the first whole genome sequences became available.

Subunit compositions of light harvesting complexes

As in vascular plants, the photosystems of diatoms are surrounded by light harvesting complexes, whereby some are more tightly bound to either PSII or PSI, forming PSII-FCP or PSI-FCP supercore complexes/supercomplexes, respectively. Besides, there is an additional pool of more loosely bound FCPs, which will be considered first. Early on pools of FCPs were isolated and characterized from the pennate diatom P. tricornutum [START_REF] Alberte | Light harvesting systems of brown algae and diatoms. Isolation and characterization of chlorophyll a/c and chlorophyll a/fucoxanthin pigment protein complexes[END_REF][START_REF] Friedman | A diatom light-harvesting pigment-protein complex[END_REF][START_REF] Gugliemelli | Isolation and characterization of pigment-protein particles from the lightharvesting complex of Phaeodactylum tricornutum[END_REF][START_REF] Fawley | Polypeptides of a light-harvesting complex of the diatom Phaeodactylum tricornutum are synthesized in the cytoplasm of the cell as precursors[END_REF][START_REF] Owens | Light-harvesting function in the diatom Phaeodactylum tricornutum: I. Isolation and characterization of pigment-protein-complexes[END_REF][START_REF] Caron | Chlorophyll-carotinoid protein complexes from the diatom Phaeodactylum tricornutum:spectrophotometric, pigment and polypeptide analyses[END_REF][START_REF] Owens | Light-harvesting antenna systems in the chlorophyll a/c-containing algae[END_REF][START_REF] Berkaloff | Subunit organization of PSI particles from brown algae and diatoms: polypeptide and pigment analysis[END_REF][START_REF] Lavaud | Enrichment of the light-harvesting complex in diadinoxanthin and implications for the nonphotochemical fluorescence quenching in diatoms[END_REF][START_REF] Guglielmi | The light-harvesting antenna of the diatom Phaeodactylum tricornutum -Evidence for a diadinoxanthin-binding subcomplex[END_REF]. Later, centric diatoms like C. meneghiniana or T. pseudonana were studied as well, and sub-populations of FCP complexes were differentiated. In the pennate P. tricornutum, only trimeric FCP complexes were found [START_REF] Joshi-Deo | Characterization of a trimeric light-harvesting complex in the diatom Phaeodactylum tricornutum built of FcpA and FcpE proteins[END_REF][START_REF] Grouneva | The thylakoid membrane proteome of two marine diatoms outlines both diatom-specific and species-specific features of the photosynthetic machinery[END_REF]) that, nonetheless, could interact to form yet larger complexes [START_REF] Lepetit | Spectroscopic and molecular characterization of the oligomeric antenna of the diatom Phaeodactylum tricornutum[END_REF][START_REF] Gardian | Supramolecular organization of fucoxanthin-chlorophyll proteins in centric and pennate diatoms[END_REF]. Further separation demonstrated the presence of three major trimers, with Lhcf5, Lhcf10 and Lhcf2, or Lhcf4 as main subunits, respectively, together with different other Lhcf polypeptides [START_REF] Gundermann | Identification of several subpopulations in the pool of light harvesting proteins in the pennate diatom Phaeodactylum tricornutum[END_REF]. Later, homodimers of Lhcf4 were found in crystals used for X-ray crystallography (Wang et al. 2019, see below). No members of the other Lhc families (Lhcr or Lhcx) were found in purified trimeric FCPs of pennates so far [START_REF] Gundermann | Identification of several subpopulations in the pool of light harvesting proteins in the pennate diatom Phaeodactylum tricornutum[END_REF], although the photoprotective Lhcx as well as Lhcr proteins are present when isolating the whole pool of FCP complexes [START_REF] Lepetit | Evidence for the existence of one antennaassociated, lipid-dissolved, and two protein-bound pools of diadinoxanthin cycle pigments in diatoms[END_REF]Nagao et al. 2013a;Taddei et al. 2018). In contrast, trimeric and specific higher oligomeric complexes were isolated from C. meneghiniana, a centric diatom [START_REF] Büchel | Fucoxanthin-chlorophyll proteins in diatoms: 18 and 19 kDa subunits assemble into different oligomeric states[END_REF], and two different complexes were later verified for other centrics as well, T. pseudonana [START_REF] Grouneva | The thylakoid membrane proteome of two marine diatoms outlines both diatom-specific and species-specific features of the photosynthetic machinery[END_REF] and Ch. gracilis [START_REF] Nagao | Proteases are associated with a minor fucoxanthin chlorophyll a/c-binding protein from the diatom, Chaetoceros gracilis[END_REF]Nagao et al. 2013a), respectively. In the centrics C. meneghiniana and T. pseudonana, Lhcf proteins accompanied by Lhcx1 proteins defined the major trimeric complex, named FCPa. The oligomeric complex with distinct Lhcf composition, named FCPb, was proven to be a nonamer [START_REF] Röding | The structure of FCPb, a light-harvesting complex in the diatom Cyclotella meneghiniana[END_REF]). In Ch. gracilis, the oligomeric complex was named FCP-A and the trimeric complex FCP-B/C due to their subunit composition [START_REF] Nagao | Proteases are associated with a minor fucoxanthin chlorophyll a/c-binding protein from the diatom, Chaetoceros gracilis[END_REF]Nagao et al. 2013a). Recently, FCPa and FCPb complexes of C. meneghiniana were further divided into sub-complexes [START_REF] Gundermann | Fucoxanthin-chlorophyll protein complexes of the centric diatom Cyclotella meneghiniana differ in Lhcx1 and Lhcx6_1 content[END_REF]. Four different trimeric FCPa complexes were shown to exist, whereby FCPa3 and FCPa4 were the most abundant forms. Lhcf1 was the major polypeptide in both, accompanied by Lhcx1 in case of FCPa4. For FCPb two sub-complexes were distinguished: FCPb1 that is built almost solely of Lhcf3, and FCPb2 that in addition contains Lhcx1 and Lhcx6_1 when isolated from cells grown under high light. When using sucrose gradient centrifugation to separate solubilized thylakoid membrane proteins, two bands were obtained, whereby the upper band contained associations of FCPa4, FCPa1 and FCPb2, i.e. those complexes that contain Lhcx1, whereas the lower band was an association of the remaining FCPa trimers and FCPb nonamers devoid of Lhcx1.

Lhcx1 is very similar in sequence in centrics and pennates, but only in centrics it was found in FCP trimers so far. It was proven to be involved in photoprotection in whole cells in both groups [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] Ghazaryan | Involvement of the Lhcx protein Fcp6 of the diatom Cyclotella meneghiniana in the macro-organisation and structural flexibility of thylakoid membranes[END_REF][START_REF] Buck | Lhcx proteins provide photoprotection via thermal dissipation of absorbed light in the diatom Phaeodactylum tricornutum[END_REF]) (see Photosynthetic light reactions in diatoms. II. The dynamic regulation of the various light reactions"). For C. meneghiniana it could also be shown that suppression of the fluorescence quenching of FCPa depends on Lhcx1 content, pH and aggregation [START_REF] Gundermann | Factors determining the fluorescence yield of fucoxanthin-chlorophyll complexes (FCP) involved in non-photochemical quenching in diatoms[END_REF], whereby Lhcx1 in vivo does not seem to act as direct quencher, but probably promotes FCPa aggregation [START_REF] Ghazaryan | Involvement of the Lhcx protein Fcp6 of the diatom Cyclotella meneghiniana in the macro-organisation and structural flexibility of thylakoid membranes[END_REF], in line with the proposed uncoupling of the peripheral antennae upon induction of qE in pennates [START_REF] Buck | Lhcx proteins provide photoprotection via thermal dissipation of absorbed light in the diatom Phaeodactylum tricornutum[END_REF]. Regarding the other Lhcx proteins, very little is known about their specific localization in the thylakoid membrane.

For Lhcx3 of P. tricornutum, some evidence exists for a binding to the FCP complexes and to PSI, but further experiments are needed to corroborate this result (Taddei et al. 2018).

Only pennate diatoms like P. tricornutum have a 'red antenna' when cultivated under red light [START_REF] Herbstová | Molecular basis of chromatic adaptation in pennate diatom Phaeodactylum tricornutum[END_REF][START_REF] Herbstová | Red-light phenotype in a marine diatom involves a specialized oligomeric red-shifted antenna and altered cell morphology[END_REF], characterized by a long wavelength fluorescence at around 710 nm at room temperature that is attributed to PSII. The main component of this red fluorescing antenna is Lhcf15, a protein which is not closely related to the other main Lhcf proteins that constitute the trimers and which has no close homologue in centrics. This redshifted antenna was interpreted as an evolutionary adaptation towards survival in shaded environments. Recently, however, results demonstrated that the 'red antenna' may also be induced under weak green and yellow light [START_REF] Oka | Adaptation of light-harvesting and energy-transfer processes of a diatom Phaeodactylum tricornutum to different light qualities[END_REF].

FCPs associated with PSI

PSI-FCP supercores were isolated before sequences of Lhcs became known, and accordingly no detailed attribution of protein isoforms to complexes was initially possible [START_REF] Berkaloff | Subunit organization of PSI particles from brown algae and diatoms: polypeptide and pigment analysis[END_REF][START_REF] Brakemann | Association of fucoxanthin chlorophyll a/c-binding polypeptides with photosystems and phosphorylation in the centric diatom Cyclotella cryptica[END_REF][START_REF] Veith | The monomeric photosystem I-complex of the diatom Phaeodactylum tricornutum binds specific fucoxanthin chlorophyll proteins (FCPs) as light-harvesting complexes[END_REF][START_REF] Ikeda | Photosystem I complexes associated with fucoxanthin-chlorophyll-binding proteins from a marine centric diatom, Chaetoceros gracilis[END_REF]. Later, the homology of Lhcr with the red algal PSI antenna proteins gave rise to the assumption that Lhcr polypeptides fulfill the same function. Indeed, Lhcr proteins can be found in PSI isolates [START_REF] Veith | Identification of a specific fucoxanthinchlorophyll 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 antennaassociated, lipid-dissolved, and two protein-bound pools of diadinoxanthin cycle pigments in diatoms[END_REF][START_REF] Grouneva | The thylakoid membrane proteome of two marine diatoms outlines both diatom-specific and species-specific features of the photosynthetic machinery[END_REF]. Depending on the isolation procedure, Lhcr are the sole PSI antennae components (Lepetit et al. 2010), or Lhcf (and sometimes Lhcx) proteins were present in the PSI antenna as well [START_REF] Brakemann | Association of fucoxanthin chlorophyll a/c-binding polypeptides with photosystems and phosphorylation in the centric diatom Cyclotella cryptica[END_REF][START_REF] Veith | The monomeric photosystem I-complex of the diatom Phaeodactylum tricornutum binds specific fucoxanthin chlorophyll proteins (FCPs) as light-harvesting complexes[END_REF][START_REF] Ikeda | Photosystem I complexes associated with fucoxanthin-chlorophyll-binding proteins from a marine centric diatom, Chaetoceros gracilis[END_REF][START_REF] Veith | Identification of a specific fucoxanthinchlorophyll protein in the light harvesting complex of photosystem I in the diatom Cyclotella meneghiniana[END_REF][START_REF] Grouneva | The thylakoid membrane proteome of two marine diatoms outlines both diatom-specific and species-specific features of the photosynthetic machinery[END_REF][START_REF] Juhas | Properties of photosystem I antenna protein complexes of the diatom Cyclotella meneghiniana[END_REF][START_REF] Ikeda | Two types of fucoxanthin-chlorophyll-binding proteins I tightly bound to the photosystem I core complex in marine centric diatoms[END_REF][START_REF] Calvaruso | Specific Lhc proteins are bound to PSI or PSII supercomplexes in the diatom Thalassiosira pseudonana[END_REF][START_REF] Oka | Adaptation of light-harvesting and energy-transfer processes of a diatom Phaeodactylum tricornutum to different light qualities[END_REF]. As in vascular plants, PSI of diatoms is a monomer [START_REF] Veith | The monomeric photosystem I-complex of the diatom Phaeodactylum tricornutum binds specific fucoxanthin chlorophyll proteins (FCPs) as light-harvesting complexes[END_REF][START_REF] Ikeda | Photosystem I complexes associated with fucoxanthin-chlorophyll-binding proteins from a marine centric diatom, Chaetoceros gracilis[END_REF][START_REF] Oka | Adaptation of light-harvesting and energy-transfer processes of a diatom Phaeodactylum tricornutum to different light qualities[END_REF]. In Ch. gracilis, the antenna complexes that serve PSI are characterized by long wavelength absorbing Chls that rapidly equilibrate with Chls in the PSI core through uphill energy excitation transfer [START_REF] Nagao | Low-energy chlorophylls in fucoxanthin chlorophyll a/c-binding protein conduct excitation energy transfer to photosystem I in diatoms[END_REF]Nagao et al. 2019b).

From the same organism the structure of a PSI-FCP supercomplex at 2.4 Å resolution became available recently [START_REF] Oka | Adaptation of light-harvesting and energy-transfer processes of a diatom Phaeodactylum tricornutum to different light qualities[END_REF]. Sixteen monomeric Lhcs surround the highly conserved core, i.e. much more Lhc subunits than in any other organism studied so far. Ten of those are Lhcr proteins and the others belong to a group that was named Lhcq and is closely related to Lhcf. The Lhc subunits are called Fcpa in the corresponding publication [START_REF] Oka | Adaptation of light-harvesting and energy-transfer processes of a diatom Phaeodactylum tricornutum to different light qualities[END_REF]), but the respective Lhcr/Lhcq names can be found in the protein data bank file (pdb file 6L4U).

Nine Fcpa are surrounding the core in a circle that is almost closed (Fcpa1-9), whereby only the PsaL/I side is not binding any Lhc. These Lhc all belong to the Lhcr family. On the PsaA side, Fcpa10-16 are found in addition, whereby only Fcpa10 belongs to the Lhcr group, and the others are Lhcq proteins. Pigment binding differs strongly between the Fcpa subunits: five to thirteen Chl a, none to seven Chl c, one to seven Fx and one to four Dd are found. For example, the subunit called Fcpa15 has the highest pigment load with 13 Chl a:0 Chl c:7 Fx:2 Dd, whereas Fcpa1 carries only a small amount of pigments (6 Chl a:2 Chl c:2 Fx:1 Dd). Fcpa13 is very unusual with a high Chl c and low Fx content (6:6:2:1). More recently, the structure of an even bigger PSI-FCP complex from Ch. gracilis was solved [START_REF] Xu | Structural basis for energy transfer in a huge diatom PSI-FCPI supercomplex[END_REF]). Here, 24 monomeric FCP surround the monomeric PSI cores. Unfortunately, neither protein nor pigment attribution are identical for the FCP complexes present in both structures.

FCPs associated with PSII

A lot of detail is also known about PSII-FCP supercores, since the structure of such a complex at high resolution from Ch. gracilis became also available recently (Nagao et al. 2019a;[START_REF] Pi | The pigment-protein network of a diatom photosystem II-light-harvesting antenna supercomplex[END_REF]. The core dimer of PSII is highly homologous to that of vascular plants and especially red algae, but the organization of the surrounding FCPs differs. Diatoms lack homologues to the minor antenna proteins CP24, CP26, and CP29 of plants, but nonetheless monomeric Lhc are found, albeit in totally different positions. Comparable to the situation in vascular plants, where light-harvesting complex II (LHCII) is found in the supercores, two strongly bound (S) and two less tightly bound (M) oligomers of FCPs are present in the dimeric PSII supercores (Fig. 2A). However, these FCPs are tetrameric in Ch. gracilis, in contrast to the trimers or nonamers reported for the free pool of FCPs in the same species. Since Ch. gracilis is not sequenced, only some Lhc sequences could be used for model building. According to [START_REF] Pi | The pigment-protein network of a diatom photosystem II-light-harvesting antenna supercomplex[END_REF], the tetramers consist of FCP-A, i.e. the polypeptide found in the nonamers of the free FCP pool before. Due to the lack of sequences, the homologous sequence from T. pseudonana (Lhcf8) was used for modelling. In contrast, Nagao et al. (2019a) attributed the subunits of the tetramer to Lhcf1, a constituent of the trimeric complexes in other centrics. However, since the resolution of the PSII-FCP supercore structures is at 3.0 and 3.6 Å, respectively, slight variations in the electron densities between monomers might suggest that the tetramers consist of several, very similar Lhc proteins. Concerning the monomeric subunits, one (FCP-D) turned out to be an antenna protein of the Lhcr group [START_REF] Pi | The pigment-protein network of a diatom photosystem II-light-harvesting antenna supercomplex[END_REF]). The others could not be unambiguously identified. Very recently, a low resolution structure of the PSII-FCP complex of T. pseudonana was published [START_REF] Arshad | Revealing the architecture of the photosynthetic apparatus in the diatom Thalassiosira pseudonana[END_REF]. The overall arrangement with three monomeric and two oligomeric FCP complexes resembled closely the structure of PSII from Ch. gracilis. However, the oligomeric FCPs appeared as trimers, not as tetramers, in accordance with the trimeric FCPs from the free FCP pool of T. pseudonana.

In summary, surprisingly, three different oligomeric states of FCPs exist besides monomeric Lhcs when considering pennate and centric diatoms together, whereas in plants only trimeric complexes (LHCII) and dimers (LHCI) are found. The recently available structures of a supercore from centrics (Chaetoceros) (Nagao et al. 2019a;[START_REF] Pi | The pigment-protein network of a diatom photosystem II-light-harvesting antenna supercomplex[END_REF]) and an FCP from pennates (Phaeodactylum) [START_REF] Pi | The pigment-protein network of a diatom photosystem II-light-harvesting antenna supercomplex[END_REF] were combined by [START_REF] Wang | Structural features of the diatom photosystem II-light-harvesting antenna complex[END_REF] into a model where the supercore having tetrameric FCPs bound is surrounded by a pool of dimeric FCPs. However, for the centrics C. meneghiniana as well as T. pseudonana trimeric FCPs were demonstrated by imaging methods to constitute the pool of peripheral FCPs and the PSII associated FCPs, respectively [START_REF] Röding | The structure of FCPb, a light-harvesting complex in the diatom Cyclotella meneghiniana[END_REF][START_REF] Arshad | Revealing the architecture of the photosynthetic apparatus in the diatom Thalassiosira pseudonana[END_REF]. In addition, no data are available about the oligomeric state of the remaining FCPs in pennates (built of other subunits than Lhcf4), and no structure of the PSII supercomplex is available. Using biochemical analyses, trimers were found in both groups of diatoms. In addition, in centric diatoms nonamers of specific polypeptide composition are present [START_REF] Beer | Subunit composition and pigmentation of fucoxanthin-chlorophyll proteins in diatoms: Evidence for a subunit involved in diadinoxanthin and diatoxanthin binding[END_REF][START_REF] Grouneva | The thylakoid membrane proteome of two marine diatoms outlines both diatom-specific and species-specific features of the photosynthetic machinery[END_REF]Nagao et al. 2013a). For PSI the situation is much more clear, since only monomeric Lhcs are attached [START_REF] Veith | The monomeric photosystem I-complex of the diatom Phaeodactylum tricornutum binds specific fucoxanthin chlorophyll proteins (FCPs) as light-harvesting complexes[END_REF]Nagao et al. 2019b;[START_REF] Oka | Adaptation of light-harvesting and energy-transfer processes of a diatom Phaeodactylum tricornutum to different light qualities[END_REF][START_REF] Xu | Structural basis for energy transfer in a huge diatom PSI-FCPI supercomplex[END_REF].

Figure 2

A: Scheme of the PSII-FCP supercore complex according to Nagao et al. (2019a) and [START_REF] Pi | The pigment-protein network of a diatom photosystem II-light-harvesting antenna supercomplex[END_REF]. The core of PSII, with the reaction center proteins D1 and D2 and the inner antenna proteins CP43 and CP47, is almost identical in plants and diatoms and shown in grey. PSII is a dimer, surrounded by FCP complexes on both sides. FCP-A, the subunit of both tetramers (ST and MT) was modelled using T. pseudonana Lhcf8 in case of [START_REF] Pi | The pigment-protein network of a diatom photosystem II-light-harvesting antenna supercomplex[END_REF] and identified as Lhcf1 by Nagao et al. (2019a). Note that in T. pseudonana these complexes are trimeric [START_REF] Arshad | Revealing the architecture of the photosynthetic apparatus in the diatom Thalassiosira pseudonana[END_REF]. FCP-D corresponds to Lhca2, a protein of the Lhcr group. FCP-E and FCP-F could not be identified.

Molecular structure of FCPs

The first molecular structure at high resolution (1.8 Å) of a dimeric FCP of Lhcf4 from P. tricornutum became available recently [START_REF] Pi | The pigment-protein network of a diatom photosystem II-light-harvesting antenna supercomplex[END_REF], and the overall features were confirmed for the FCPs within the PSII-FCP and PSI-FCP supercomplexes (Nagao et al. 2019a;[START_REF] Pi | The pigment-protein network of a diatom photosystem II-light-harvesting antenna supercomplex[END_REF][START_REF] Oka | Adaptation of light-harvesting and energy-transfer processes of a diatom Phaeodactylum tricornutum to different light qualities[END_REF]) (Fig. 2B). The overall protein scaffold structure of FCP monomers is similar to that of LHCII monomers (for an overview see [START_REF] Gundermann | Fucoxanthin-chlorophyll protein complexes of the centric diatom Cyclotella meneghiniana differ in Lhcx1 and Lhcx6_1 content[END_REF]. The helices where sequence conservation is high, i.e. helix 1 and 3, adopt an almost identical configuration in FCP vs. LHCII, whereas helix 2 has a slightly different location and tilt that also differs between the FCP-dimer from P. tricornutum and the FCP-tetramers from Ch. gracilis found in PSII-supercomplexes. The complex from P. tricornutum was crystallized as a dimer, whereby the connection between the monomers was due to strong interactions between helices 2. This 'head to head' configuration has not been shown before for any member of the Lhc family, since e.g. Lhca1/Lhc4 dimers of plant PSI interact in a 'head to tail' configuration [START_REF] Ben-Shem | Crystal structure of plant photosystem I[END_REF]. Similarly, the monomers of the M-and S-tetramer in the PSII-FCP supercore complexes are associated 'head to tail'. In contrast to LHCII, where helices 3 come close to the stromal surface, the subunits of the tetramers are turned by 180° in comparison to LHCII, so that helices 1 approach closest to the stromal side (Nagao et al. 2019a;[START_REF] Pi | The pigment-protein network of a diatom photosystem II-light-harvesting antenna supercomplex[END_REF]. Although helices 1 and 3 are almost identical in FCP and LHCII, the dimer and the tetramers are arranged in a way that these helices adopt a slightly different tilt with respect to the membrane plane compared to the arrangement in the trimeric LHCII [START_REF] Liu | Crystal structure of spinach major light-harvesting complex at 2.72 angstrom resolution[END_REF][START_REF] Standfuss | Mechanisms of photoprotection and nonphotochemical quenching in pea light-harvesting complex at 2.5A resolution[END_REF][START_REF] Gundermann | Fucoxanthin-chlorophyll protein complexes of the centric diatom Cyclotella meneghiniana differ in Lhcx1 and Lhcx6_1 content[END_REF]).

Seven Fx, one Dd, seven Chl a, and two Chl c per monomer were fitted into the electron density map of the P. tricornutum FCP-dimer [START_REF] Pi | The pigment-protein network of a diatom photosystem II-light-harvesting antenna supercomplex[END_REF], whereas in the tetrameric FCPs of Ch. gracile only six Fx, but nine (M-tetramer) or ten Chl molecules (S-tetramer) were found [START_REF] Pi | The pigment-protein network of a diatom photosystem II-light-harvesting antenna supercomplex[END_REF][START_REF] Pi | The pigment-protein network of a diatom photosystem II-light-harvesting antenna supercomplex[END_REF]) (Fig. 2B), and, as stated above, the variability is even larger in PSI associated Lhc. In general, the Chl:carotenoid ratio is much lower than in plant LHCII.

As already anticipated from sequence comparisons and based on spectroscopy data [START_REF] Premvardhan | Pigment organization in fucoxanthin chlorophyll a/c(2) proteins (FCP) based on resonance Raman spectroscopy and sequence analysis[END_REF], six of the Chl binding sites are highly conserved between LHCII and FCP, with the central Fx molecules arranged close to helix 1 and 3 similarly to the luteins in LHCII [START_REF] Liu | Crystal structure of spinach major light-harvesting complex at 2.72 angstrom resolution[END_REF][START_REF] Standfuss | Mechanisms of photoprotection and nonphotochemical quenching in pea light-harvesting complex at 2.5A resolution[END_REF]. One Dd was fitted in a more peripherally located density in the P. tricornutum FCP-dimer, but was not found in any of the Ch. gracilis tetramers (Fig. 2B). The porphyrin rings of Chl c and Chl a are rather similar in structure, so Chl c can only be distinguished from Chl a due to the planarity of the C18=C17 double bonds and the lack of phytol in Chl c, which requires a high resolution electron density map to resolve.

In the diatom FCP-tetramers close to PSII, three Chl c were assigned, the additional one being Chl c405 (nomenclature according to [START_REF] Pi | The pigment-protein network of a diatom photosystem II-light-harvesting antenna supercomplex[END_REF], a binding site that had been assigned to Chl a in the FCP-dimer of P. tricornutum only for reasons of pigment stoichiometry [START_REF] Pi | The pigment-protein network of a diatom photosystem II-light-harvesting antenna supercomplex[END_REF]. Surprisingly, Chl c403 and Chl c408 reside in the strongly conserved binding sites 603 (Chl c403) and 612 (Chl c408), occupied by Chl a in case of LHCII. Both Chl c's are in close contact with Fx molecules, which are bound in places where no carotenoids are found in case of LHCII. All carotenoid molecules in FCP are in all-trans configuration, and all have Chl a molecules in close vicinity (3.1 to 3.6 Å). Whereas the Chl binding sites are highly conserved between the FCP-dimer and the FCP-tetramer structures, only four Fx are found in identical places to LHC carotenoids (Fig. 2B), including those in the lutein binding sites. The Fx interacting with one of the Chl c in the Chl a/Chl c/Chl a clusters (Fx306) is oriented almost perpendicular to the membrane plane in the FCP-dimers and in some subunits of the tetramers, whereas it is oriented in an angle of almost 45° in others. Two Fx are found in totally different locations in tetramers compared to the dimer. The most peculiar Fx is Fx304 in the FCP-dimer, running almost parallel to the membrane plane on the stromal side. This Fx is missing in the FCP-tetramers.

Excitation energy transfer in FCPs

Pigment ratios of around 6-8 Chl a : 2-3 Chl c : 6-8 Fx were reported for the different diatom FCP complexes (which represented rather the main, free pool of FCP complexes), depending on growth condition, isolation procedure and species [START_REF] Papagiannakis | Spectroscopic characterization of the excitation energy transfer in the fucoxanthin-chlorophyll protein of diatoms[END_REF][START_REF] Beer | Subunit composition and pigmentation of fucoxanthin-chlorophyll proteins in diatoms: Evidence for a subunit involved in diadinoxanthin and diatoxanthin binding[END_REF][START_REF] Lepetit | Spectroscopic and molecular characterization of the oligomeric antenna of the diatom Phaeodactylum tricornutum[END_REF][START_REF] Joshi-Deo | Characterization of a trimeric light-harvesting complex in the diatom Phaeodactylum tricornutum built of FcpA and FcpE proteins[END_REF][START_REF] Lepetit | Evidence for the existence of one antennaassociated, lipid-dissolved, and two protein-bound pools of diadinoxanthin cycle pigments in diatoms[END_REF][START_REF] Gundermann | Factors determining the fluorescence yield of fucoxanthin-chlorophyll complexes (FCP) involved in non-photochemical quenching in diatoms[END_REF]Nagao et al. 2013b). More evident differences were reported concerning the PSI associated Lhc antennae proteins: For both centric and pennate diatoms the Chl c/Fx ratio was lower, whereas the (Dd+Dt)/Fx ratio was strongly increased [START_REF] Lepetit | Spectroscopic and molecular characterization of the oligomeric antenna of the diatom Phaeodactylum tricornutum[END_REF][START_REF] Veith | The monomeric photosystem I-complex of the diatom Phaeodactylum tricornutum binds specific fucoxanthin chlorophyll proteins (FCPs) as light-harvesting complexes[END_REF][START_REF] Veith | Identification of a specific fucoxanthinchlorophyll 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 antennaassociated, lipid-dissolved, and two protein-bound pools of diadinoxanthin cycle pigments in diatoms[END_REF][START_REF] Juhas | Properties of photosystem I antenna protein complexes of the diatom Cyclotella meneghiniana[END_REF]. The Chl c/Fx ratio was even lower when P. tricornutum was grown under red light [START_REF] Bína | Novel structural aspect of the diatom thylakoid membrane: lateral segregation of photosystem I under red-enhanced illumination[END_REF]. Whereas this pigmentation is obviously due to the presence of the different Lhcf and Lhcr polypeptides, respectively, nothing is known so far about the pigmentation of the gene products of RedCaps, Lhcy or Lhcz that have been solely analyzed on genetic level so far. Even for Lhcx proteins the question of whether they are pigmented remains open, since they have not been isolated as single proteins so far. However, their high similarity to the pigmented LhcSR proteins of green algae [START_REF] Bonente | Analysis of Lhcsr3, a protein essential for feedback de-excitation in the green alga Chlamydomonas reinhardtii[END_REF]) is a strong argument for diatom Lhcx proteins binding pigments (see "Photosynthetic light reactions in diatoms. II. The dynamic regulation of the various light reactions").

In FCPa and FCPb complexes of C. meneghiniana, two differently bound Chl c molecules were identified by Resonance Raman spectroscopy [START_REF] Premvardhan | Pigment organization in fucoxanthin chlorophyll a/c(2) proteins (FCP) based on resonance Raman spectroscopy and sequence analysis[END_REF] and also in 2D spectroscopy by their different transfer times (Songaila et al. 2013;Gelzinis et al. 2015), in agreement with the numbers although not the locations found in the crystal structure for P. tricornutum Lhcf4 [START_REF] Pi | The pigment-protein network of a diatom photosystem II-light-harvesting antenna supercomplex[END_REF]. In all FCPs isolated so far, the QY absorption of FCPs is at relatively short wavelength (~671 nm), excluding close excitonic interactions between Chl a molecules as verified by CD spectra, where no excitonic interactions are visible in the QY [START_REF] Büchel | Fucoxanthin-chlorophyll proteins in diatoms: 18 and 19 kDa subunits assemble into different oligomeric states[END_REF][START_REF] Szábo | Structurally flexible macroorganization of the pigment-protein complexes of the diatom Phaeodactylum tricornutum[END_REF][START_REF] Joshi-Deo | Characterization of a trimeric light-harvesting complex in the diatom Phaeodactylum tricornutum built of FcpA and FcpE proteins[END_REF]. The fact that the strong Chl a interaction of a610/a611/a612 visible in LHCII [START_REF] Novoderezhkin | Energy-transfer dynamics in the LHCII complex of higher plants: modified redfield approach[END_REF]) is broken by Chl c in FCPs, also due to slightly different arrangement of Chl a401 (FCP) compared to Chl a611 (LHCII), is also in line with the lack of excitonic interactions. Thus, delocalized excited states are missing, which contribute to the robustness of energy transfer in the flexible protein environment e.g. in LHCII of vascular plants. FCP complexes, in contrast, seem to use subtle changes in protein scaffold conformation to switch frequently into low-energy states with improved light-harvesting properties as shown for FCPa from C. meneghiniana [START_REF] Krüger | How reduced excitonic coupling enhances light harvesting in the main photosynthetic antennae of diatoms[END_REF].

Fx is actively involved in energy transfer to Chl a. Fx is special since it has a carbonyl moiety in conjugation with the polyene backbone. Carotenoids are in principle able to transfer energy from both their lowest singlet excited states, i.e. S1 and S2, but direct absorption into S1 is forbidden due to their symmetry. In Fx an additional excited state with an intramolecular charge transfer (ICT) character exists. This ICT state can be coupled to the S1 state, and plays a major role in carotenoid-Chl energy transfer [START_REF] Bautista | Excited state properties of peridinin: observation of a solvent dependence of the lowest excited singlet state lifetime and spectral behavior unique among carotenoids[END_REF][START_REF] Vaswani | Quantum chemical evidence for an intramolecular charge-transfer state in the carotenoid peridinin of peridinin-chlorophyllprotein[END_REF][START_REF] Zigmantas | Effect of a conjugated carbonyl group on the photophysical properties of carotenoids[END_REF][START_REF] Papagiannakis | Spectroscopic characterization of the excitation energy transfer in the fucoxanthin-chlorophyll protein of diatoms[END_REF][START_REF] Premvardhan | The charge-transfer character of the S0→ S2 transition in the carotenoid peridinin is revealed by Stark spectroscopy[END_REF]Gelzinis et al. 2015;[START_REF] West | Ultrafast multi-pulse transient absorption spectroscopy of fucoxanthin chlorophyll a protein from Phaeodactylum tricornutum[END_REF]). In addition, Fx displays an extreme bathochromic shift upon protein binding, extending the absorption from 390 nm up to 580 nm. More 'blue', 'green' and 'red' absorbing Fx molecules were detected in FCPa as well as in FCPb from C. meneghiniana [START_REF] Premvardhan | The charge-transfer properties of the S state of fucoxanthin in solution and in fucoxanthin chlorophyll-a/c protein (FCP) based on stark spectroscopy and molecular-orbital theory[END_REF][START_REF] Premvardhan | Carotenoid structures and environments in trimeric and oligomeric fucoxanthin chlorophyll a/c proteins from Resonance Raman Spectroscopy[END_REF][START_REF] Premvardhan | Pigment organization in fucoxanthin chlorophyll a/c(2) proteins (FCP) based on resonance Raman spectroscopy and sequence analysis[END_REF] using Stark and Resonance Raman spectroscopy, but could so far not be attributed to certain Fx molecules in the structure. 'Blue' and 'red' Fx were also demonstrated for whole cells of C. meneghiniana and P. tricornutum using electrochromic shift measurements [START_REF] Szábo | Functional heterogeneity of the fucoxanthins and fucoxanthin-chlorophyll proteins in diatom cells revealed by their electrochromic response and fluorescence and linear dichroism spectra[END_REF]. Excitation energy transfer from Fx to Chl a proceeds mainly via the S1/SICT state in FCPs, whereby the transfer to the QY state of Chl a has a time constant of 0.6 ps in FCPa of C. meneghiniana [START_REF] Papagiannakis | Spectroscopic characterization of the excitation energy transfer in the fucoxanthin-chlorophyll protein of diatoms[END_REF]Gildenhoff et al. 2010a). The transfer for the Fx S2 state directly into the QX state of Chl a is even faster with <150 fs (Gildenhoff et al. 2010a). For P. tricornutum, two routes using the S1/SICT state were determined recently, whereby the slower (~ 6 ps) route uses the S1 part of the potential surface of the S1/SICT equilibrium [START_REF] West | Ultrafast multi-pulse transient absorption spectroscopy of fucoxanthin chlorophyll a protein from Phaeodactylum tricornutum[END_REF]. The fast Fx to Chl a transfer is possible due to the arrangement of Fx and Chl a: each Fx has an Chl a molecule in close vicinity as stated above. Fx also acts as an efficient quencher of Chl a triplets [START_REF] Valentin | Chlorophyll triplet quenching by fucoxanthin in the fucoxanthin-chlorophyll protein from the diatom Cyclotella meneghiniana[END_REF].

Fx-Fx transfer with a time constant of 25 ps was measured using FCPa of C. meneghiniana (Gildenhoff et al. 2010a;Gildenhoff et al. 2010b). However, no pair of Fx molecules can be pinpointed in the structures responsible for this route of excitation energy transfer.

No energy transfer from Fx to Chl c was observed within the limit of instrumentation of pumpprobe as well as 2D spectroscopy for C. meneghiniana FCPs [START_REF] Papagiannakis | Spectroscopic characterization of the excitation energy transfer in the fucoxanthin-chlorophyll protein of diatoms[END_REF]Gildenhoff et al. 2010a;Songaila et al. 2013;Gelzinis et al. 2015). 2D spectroscopy also revealed that the two Chl c have different time constants for energy transfer into Chl a, both reactions being extremely fast with 60 fs and less, respectively (Songaila et al. 2013;Gelzinis et al. 2015). This is in agreement with the close contact of Chl c and Chl a, allowing fast excitation energy transfer between these pigments. Judging from the structure, also the Fx/Chl c arrangement is, however, perfectly suited for fast excitation energy transfer. If present, it has to be even faster than Chl c-Chl a transfer in order to have escaped detection.

The differences might be due to the fact that spectroscopically mainly the trimeric FCPs from C. meneghiniana were analyzed, whereas structural data come from the dimeric FCP of P.

tricornutum. The contradictions between the structure and the wealth of spectroscopic data were recently highlighted (Gelzinis et al. 2020). More structures are evidently needed in order to fully understand excitation energy transfer in FCP.

In the PSII-FCP supercore complex, energy pathways have only been deduced from the structure so far. The distances between Chls imply that the S-tetramers funnel energy into CP47, one of the inner antenna proteins of PSII, via a Chl a binding subunit called PsbG that is not present in vascular plant PSII, and into FCP-D (Fig. 2A). From the latter the energy can be transferred to CP43, the inner antenna on the opposite side of the PSII reaction center compared to CP47. The M-tetramer might transfer energy via FCP-E and FCP-D also into the Chls bound to CP43. FCP-F is rather peripheral and thus probably delivers energy mainly to the M-tetramer. Also for the PSI-FCP complexes an excitation energy transfer network was proposed, with pathways in between the inner ring of Lhc proteins as well as pathways from the more peripheral antenna into the inner ring and from there to the core. Since the methods to prepare supercore complexes in sufficient amounts for ultrafast spectroscopy are available now, these pathways will probably be elucidated in more detail in future.

  photosynthesis, whereby the basic reactions are identical in all eukaryotes: light energy is absorbed by the pigments bound to photosystem (PS) I and II and their associated light-harvesting complexes (LHC). It is transferred to special chlorophyll(Chl)

Figure 1 :

 1 Figure 1: (A) General, simplified scheme of the light reactions of photosynthesis. Only PSII (red) cytb6/f complex (yellow), PSI (green) and ATPsynthase (blue) are shown. Lightharvesting systems have been omitted since they differ significantly between the different algal groups and higher plants. For more detail on electron and proton transfer refer to Figure 1 in chapter "Photosynthetic light reactions in diatoms. II. The dynamic regulation of the various light reactions". (B) Scheme of the thylakoid membrane structure of diatoms (B). The thylakoids are organized in stacks of three that span the whole plastid. The outer membranes of such stack are enriched in PSI, whereas PSII is predominantly found in the inner four membranes. Cytb6/f complexes that are not preferentially localized (Flori et al. 2017) have been omitted as well as connections between thylakoids of one stack. In diatoms the light-harvesting complexes are called fucoxanthin-chlorophyll proteins (FCP). FCPs tightly bound to PSII are shown in light green, FCPs belonging to PSI are shown in dark green.

B:

  Overlay of one of the monomers of the S-tetramer (taken from pdb 6JLU, 3.0 Å resolution, Pi et al. 2019) with one monomer of the dimeric FCP out of Lhcf4 of P. tricornutum (pdb 6A2W, 1.8 Å resolution, Wang et al. 2019). Since the structure of the M-tetramers as well as the tetramers by Nagao et al. (2019a) (pdb 6J40, 3.6 Å) closely resembles the shown structure, they are omitted from the comparison. The protein backbone is depicted in grey (6JLU) and white (6A2W), respectively. The pigments in (nearly) identical sites in both complexes are omitted for the dimeric FCP and shown in colors for the ST-FCP: Chl a in green and Chl c in blue, whereby Chl 405, which was attributed to Chl a in the dimeric FCP and to Chl c in the tetramers, is shown in blue-green. The Chl a not found in the dimeric FCP (and M-tetramers), but in S-tetramers is depicted in light-green. The four Fx molecules present in identical locations in the dimer and the tetramers are shown in orange, whereas the Fx molecule present only in the tetrameric FCPs is shown in yellow. Fx306 is oriented more perpendicular to the membrane plane in the FCP dimer, whereas it adopts a different orientation in some subunits of the tetramers; both orientations are given in brown color. The additional Fx molecules and the Dd only present in the dimer are shown in red and black, respectively. The left panel shows the same complexes as the right panel, but turned by 180° around the y-axis.

Acknowledgements

BB acknowledges financial support from the European Research Council (ERC) under the European Union Horizon 2020 research and innovation program (grant agreement no. 715579). BL thanks the Deutsche Forschungsgemeinschaft (LE3358/3-2) and the Baden-Württemberg Stiftung (Elite program) for financial support. CB acknowledges support by the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 675006 and from the Deutsche Forschungsgemeinschaft, grant Bu 812 10-1. DAC thanks the Canada Research Chairs and Natural Science and Engineering Research Council of Canada for support. JL thanks the Centre National de la Recherche Scientifique-CNRS, the Natural Sciences and Engineering Research Council of Canada-NSERC (Discovery and Northern Supplement grants), the Canada First Research Excellence Fund-Sentinelle Nord, and the strategic research cluster Québec-Océan for their financial support. All authors thank the reviewer for valuable suggestions.

that the inner membranes of the typical stacks of three thylakoids are enriched in PSII and its associated FCP complexes (Fig. 1). The lipid composition of the inner membranes is dominated by MGDG and represents the place where efficient operation of the XC is taking place. SQDG, on the other hand, is enriched in the outer membranes of the thylakoid stacks so as not to interfere with the diadinoxanthin de-epoxidase. Outer membranes and margin regions of the stacks contain mainly PSI and the ATP synthase which thus gains access to the chloroplast stroma. Although the model was mainly based on physiological and biochemical data, recent structural data have confirmed several important aspects and predictions [START_REF] Flori | Plastid thylakoid architecture optimizes photosynthesis in diatoms[END_REF]) (see also "Introduction"). Revealing also connections between the different thylakoid lamellae, the authors concluded that the three-dimensional network of the thylakoid membrane of diatoms is far more complex than the simple layout of three loosely connected membranes.

Outlook

The recently obtained structures of PSII and PSI supercomplexes allow for the first time detailed insights into particular steric features of the whole light harvesting system and the excitation energy routes in diatoms. We need more of these structures, obtained from diatoms in different acclimation states but also from different species. This way, we may reveal the localization of the Lhcx proteins as well of those proteins which so far have only been found on the genetic level, such as Lhcz or Redcap. More of these studies will also enable us to resolve some so far existing discrepancies about the composition and oligomerisation states of FCPs obtained either by classical biochemical preparations or by crystallization approaches. In addition, the dynamic interaction between thylakoid lipid domains and thylakoidal protein complexes in diatoms is a field which is as yet largely understudied and would also profit from sophisticated imaging approaches. In this regard, we also need to better understand biogenesis of thylakoid membranes and photosynthetic complexes. Recently, first insights regarding the insertion of FCPs in P. tricornutum have been obtained. Alb3b, a protein functioning in the insertion and assembly of thylakoid membrane protein complexes in plants, was demonstrated to be essential for FCP assembly [START_REF] Nymark | Loss of ALBINO3b insertase results in truncated light-harvesting antenna in diatoms[END_REF]. On the other hand, the chloroplast signal recognition particle protein CpSRP54, involved in targeting LHC to the chloroplast membrane in plants and green algae, is not involved in FCP accumulation but important for the insertion of plastid encoded thylakoid membrane proteins (Nymark et al 2021). This paves the way for future studies on FCP assembly, but to fully understand this process, also the enzymes working in Fx and Chl c synthesis still need to be identified.