Photosynthesis Research

Flash-induced absorption changes in the Soret region, which originate from the [PD1PD2]+ state, the chlorophyll cation radical formed upon Photosystem II (PSII) excitation, were investigated in Mn-depleted Photosystem II. In wild-type PSII from Thermosynechococcus elongatus, the [PD1PD2]+-minus-[PD1PD2] difference spectrum shows a main negative feature at 434 nm and a smaller negative feature at 446 nm [Boussac et al. Photosynth Res (2023), https://doi.org/10.1007/s11120-023-01049-3]. While the main feature at 434 nm is associated with PD1+ formation, the origin of the dip at 446 nm remains to be identified. For that, we have compared the [PD1PD2]+-minus-[PD1PD2] difference spectra from the PsbA3/H198Q PSII mutant in T. elongatus and D2/H197A PSII mutant in Synechocystis sp. PCC 6803 with their respective wild type strains. By modifying the PD1 axial ligand with the H198Q mutation in the D1 protein in T. elongatus, the contribution at 434 nm was shifted to 431 nm, while the contribution at 446 nm was hardly affected. In Synechocystis sp. PCC 6803, by modifying the PD2 axial ligand with the H197A mutation in the D2 protein, the contribution at 446 nm was downshifted by [~] 3 nm to [~] 443 nm, while the main contribution at 432 nm was only slightly shifted upwards to 433 nm. This result suggests that the bleaching seen at 446 nm involves PD2. This could reflects a change in the [PD1+PD2]{longleftrightarrow}[PD1PD2+] equilibrium or a more complex mechanism.

Euglena gracilis, a phototrophic flagellate, possesses light-harvesting complexes (LHCs) with a pigment composition distinct from that of land plants and green algae, despite notable similarities in LHC polypeptide sequences to green algae. Here, we purified photosystem I-LHCI (PSI-LHCI) supercomplexes from E. gracilis strain Z and characterized their biochemical and spectroscopic properties. The purified complex exhibited a unique pigment profile, notably including diadinoxanthin that is typically found in red-lineage organisms, setting E. gracilis apart from green-lineage organisms. The absorption spectrum displayed the Qy band of chlorophyll a at 675{square}nm, while the 77-K fluorescence-emission spectrum revealed a prominent peak at 732{square}nm, closely resembling those in land plants. These features suggest that long-wavelength chlorophylls bound to LHCI may be evolutionarily conserved. Nevertheless, the absence of neoxanthin, lutein, and violaxanthin further differentiates the Euglena LHCIs from other oxyphototrophs. Together, these results illuminate the evolutionary diversification of PSI-LHCI supercomplexes and offer insights into the unique pigment-binding features of the Euglena light-harvesting system.

Red algae are photosynthetic eukaryotes whose light-harvesting complexes (LHCs) associate with photosystem I (PSI). In this study, we examined characteristics of PSI-LHCI, PSI, and LHCI isolated from the red alga Galdieria sulphuraria NIES-3638. The PSI-LHCI supercomplexes were purified using anion-exchange chromatography followed by hydrophobic interaction chromatography, and finally by trehalose density gradient centrifugation. PSI and LHCI were similarly prepared following the dissociation of PSI-LHCI with Anzergent 3-16. Polypeptide analysis of PSI-LHCI revealed the presence of PSI and LHC proteins, along with a red-lineage chlorophyll a/b-binding-like protein (RedCAP), which is distinct from LHC proteins within the LHC protein superfamily. RedCAP, rather than LHC proteins, exhibited tight binding to PSI. Carotenoid analysis of LHCI identified zeaxanthin, {beta}-cryptoxanthin, and {beta}-carotene, with zeaxanthin particularly enriched, which is consistent with other red algal LHCIs. A Qy peak of chlorophyll a in the LHCI absorption spectrum was blue-shifted compared with those of PSI-LHCI and PSI, and a fluorescence emission peak was similarly shifted to shorter wavelengths. Based on these results, we discuss the diversity of LHC proteins, including RedCAP, in red algal PSI-LHCI supercomplexes.

The acclimation of cyanobacteria to iron deficiency is crucial for their survival in natural environments. In response to iron deficiency, many cyanobacterial species induce the production of a pigment-protein complex called IsiA. IsiA proteins associate with photosystem I (PSI) and can function as light-harvesting antennas or dissipate excess energy. They may also serve as Chl storage during iron limitation. In this study we examined the functional role of IsiA in cells of Synechocystis sp. PCC 6803 grown under iron limitation conditions by measuring the cellular IsiA content and its capability to transfer energy to PSI. We specifically test the effect of the oligomeric state of PSI by comparing wild-type (WT) Synechocystis sp. PCC 6803 to mutants lacking specific subunits of PSI, namely PsaL/PsaI ({Delta}psaL mutant) and PsaF/PsaJ ({Delta}FIJL). Time-resolved fluorescence spectroscopy revealed that IsiA formed functional PSI3-IsiA18 supercomplexes, wherein IsiA effectively transfers energy to PSI on a timescale of 10 ps at room temperature - measured in isolated complexes and in vivo - confirming the primary role of IsiA as an accessory light-harvesting antenna to PSI. However, a significant fraction (40%) remained unconnected to PSI, supporting the notion of a dual functional role of IsiA. Cells with monomeric PSI under iron deficiency contained only 3-4 IsiA complexes bound to PSI. Together the results show that IsiA is capable of transferring energy to trimeric and monomeric PSI but to varying degrees and that the acclimatory production of IsiA under iron stress is controlled by its ability to perform its light-harvesting function.

Marine diatoms effectively photosynthesize by acclimating to the wide range of growth irradiance in the ocean. Using a pennate diatom Phaeodactylum tricornutum and a centric diatom Chaetoceros gracilis, we evaluated differences in the photosynthetic machinery under a wide range of growth irradiances. The chlorophyll a-specific amounts of the major accessory pigments remained relatively constant irrespective of growth irradiance in both diatoms. However, fluorescence spectra at 77K differed drastically depending on the growth irradiance: In P. tricornutum, fluorescence from photosystem II was dominant in high-light-grown cells and negligible in low-light-grown cells, while in C. gracilis, the opposite trend was observed. These drastic changes in fluorescence spectra were slow processes. The amounts of the two reaction centers, as assessed by specific antibodies and absorption changes in P700, remained almost constant under different irradiances. These results indicate that under dim growth irradiance, more excitation energy is diverted to photosystem I in the pennate diatom, and to photosystem II in the centric diatom. Therefore, the light-harvesting antennas balance excitation energy distribution by changing their association between photosystems I and II in different manners between P. tricornutum and C. gracilis, depending on irradiance. This phenomenon is similar to state transitions, but differs in its magnitude and duration. Differences in the preference of energy distribution in the two diatoms suggest that the dynamic state conversion--an antenna rearrangement during the long-term acclimation process in diatoms--is the species-specific strategy to achieve effective photosynthesis under the wide range of growth irradiances in the ocean.

Phosphorylation of the core subunits of photosystem II (PSII) is largely governed by a protein kinase and an antagonistic protein phosphatase. In plants the respective mutants show alterations in the architecture of thylakoid membranes and in the repair of PSII after photo-inhibition. However the protein kinase targets several subunits of PSII, as well as other proteins. To specifically investigate the role of phosphorylation of the different PSII subunits, we used site-directed mutagenesis and chloroplast transformation in Chlamydomonas reinhardtii. Major, evolutionarily-conserved sites of phosphorylation in three components of PSII (CP43, D2 and PsbH) were mutated to replace the corresponding serine or threonine residues with alanine. The alanine substitution mutant of D2 had no apparent phenotype, while the mutant of CP43 presented a minor delay in recovery from photo-inhibition. Alanine substitutions of the phosphorylation sites in PsbH had significant effects on the accumulation of PSII or on its recovery from photo-inhibition. When mutations in two of the target subunits were combined through a second cycle of chloroplast transformation, the strongest phenotype was observed in the mutant lacking phosphorylation of both PsbH and CP43, which showed delayed recovery from photo-inhibition. Surprisingly this phenotype was reversed in the mutant defective for phosphorylation of all three subunits. Our analysis indicates a prominent role for the N-terminus of PsbH in the stable accumulation of PSII and of PsbH phosphorylation in its repair cycle.\n\nSIGNIFICANCE STATEMENTTo specifically investigate the role of PSII phosphorylation, alanine-substitution mutants of the major phospho-sites in the subunits of PSII were generated individually or in combinations using chloroplast transformation. PSII assembly was defective in some of the PsbH mutants. PSII repair after photo-inhibition was delayed most strongly in the mutant lacking phosphorylation of both PsbC (CP43) and PsbH.

Photosynthetic organisms harvest light using pigment-protein super-complexes. In cyanobacteria, these are water-soluble antennae known as phycobilisomes (PBSs). The light absorbed by PBS is transferred to the photosystems in the thylakoid membrane to drive photosynthesis. The energy transfer between these super-complexes implies that protein-protein interactions allow the association of PBS with the photosystems. However, the specific proteins involved in the interaction of PBS with the photosystems are not fully characterized. Here, we show that the newly discovered PBS linker protein ApcG interacts specifically with photosystem II through its N-terminal region. Growth of cyanobacteria is impaired in apcG deletion strains under light-limiting conditions. Furthermore, complementation of these strains using a phospho-mimicking version of ApcG exhibit reduced growth under normal growth conditions. Interestingly, the interaction of ApcG with photosystem II is affected when a phospho-mimicking version of ApcG is used, targeting the positively charged residues interacting with thylakoid membrane suggesting a regulatory role mediated by phosphorylation of ApcG. Low temperature fluorescence measurements showed increased photosystem I fluorescence in apcG deletion and complementation strains. The photosystem I fluorescence was the highest in the phospho-mimicking complementation strain while pull-down experiment showed no interaction of ApcG with PSI under any tested condition. Our results highlight the importance of ApcG for selectively directing energy harvested by the PBS and implies that the phosphorylation status of ApcG plays a role in regulating energy transfer from PSII to PSI.

In the last decade, studies have revealed the importance of PSI-driven cyclic electron flow (CEF) in stress acclimation in model organisms like C. reinhardtii; however, these studies focused on transient, short-term stress. In addition, PSI-supercomplexes are associated with CEF during state transition response to short-term stress. On the other hand, the role of CEF during long-term stress acclimation is still largely unknown. In this study, we elucidate the involvement of CEF in acclimation response to long-term high salinity in three different Chlamydomonas species displaying varying salinity tolerance. We compared CEF rates, capacity for state transitions, and formation of supercomplexes after salinity acclimation in the model mesophile C. reinhardtii and two psychrophilic green algae C. priscuii (UWO241) and C. sp. ICE-MDV. CEF was activated under high salt in all three species, with the psychrophilic Chlamydomonas spp. exhibiting the highest CEF rates. High salt acclimation was also correlated with reduced state transition capacity and a PSI-supercomplex was associated with high CEF. We propose that under long-term stress, CEF is constitutively activated through assembly of a stable PSI-supercomplex. The proteomic composition of the long-term PSI-supercomplex is distinct from the supercomplex formed during state transitions, and its presence attenuates the state transition response.

Carbonic anhydrase (CA) activity, associated with Photosystem II (PSII) from Pisum sativum, has been shown to enhance water oxidation. But, the nature of the CA activity, its origin and role in photochemistry has been under debate, since the rates of CA reactions, measured earlier, were less than the rates of photochemical reactions. Here, we demonstrate high CA activity in PSII from Pisum sativum, measured by HCO3- dehydration at pH 6.5 (i.e. under optimal condition for PSII photochemistry), with kinetic parameters Km of 2.7 mM; Vmax of 2.74{middle dot}10-2 mM{middle dot}sec-1; kcat of 1.16{middle dot}103 sec-1 and kcat/Km of 4.1{middle dot}105 M-1 sec-1, showing the enzymatic nature of this activity, which kcat exceeds by [~]13 times the rate of PSII, as measured by O2 evolution. The similar dependence of HCO3- dehydration, of the maximal quantum yield of photochemical reactions and of O2 evolution on the ratio of chlorophyll/photochemical reaction center II demonstrate the interconnection of these processes on the electron donor side of PSII. Since the removal of protons is critical for fast water oxidation, and since HCO3- dehydration consumes a proton, we suggest that CA activity, catalyzing very fast removal of protons, supports efficient water oxidation in PSII and, thus, photosynthesis in general.

A diverse subset of cyanobacteria is capable of transiently modifying their photosynthetic machinery in a process known as far-red light photoacclimation to drive photosynthesis with less energetic photons (700 nm - 800 nm). To achieve this all the main light-driven components of the photosynthetic apparatus, including their allophycocyanin antenna, are replaced with red-shifted paralogues. Recent studies based on the structure of an incomplete complex provided some insights into the tuning of the far-red phycobiliproteins. Here, we solved the structure of the intact bicylindrical allophycocyanin complex from the cyanobacterium Chroococcidiopsis thermalis PCC 7203 at a resolution of 2.61 [A] determined by Cryo-electron microscopy single particle analysis. A comparison between far-red and white light allophycocyanin cores provides insight on the evolutionary adaptations needed to optimize excitation energy transfer in the far-red light adapted photosynthetic apparatus. The reduction in antenna size in far-red photosynthesis, suggests a need to optimize membrane packing to increase the number of photosystems, while the wider spread in the absorption range of the bilins suggests faster and more efficient excitation energy transfer to far-red Photosystem II by limiting backflow of excitation from the reaction centres to the far-red bilin pigments.

Efficient and safe harvesting of sunlight by photosynthesis in plant thylakoid membranes requires that both photosystems (PS)I and PSII operate with similar electron turnover rates. This is realized by state transition encompassing the redistribution of light-harvesting complexes II (LHCII) between the spatially separated PSII (mainly in stacked grana thylakoids) and PSI (mainly in unstacked domains). Here, we provide a quantitative holistic view on lateral protein and pigment reorganizations within the thylakoid membrane network induced by state transitions and the role of reversible protein phosphorylation for this process. The data reveals that plants can perfectly balance electron fluxes through both photosystems by the redistribution of a certain pool of hyperphosphorylated LHCII from PSII in stacked to PSI in unstacked thylakoid membranes. Force balance analysis predicts that the photosystems antenna reorganization is not realized by a phosphorylation induced stimulation of lateral mobility of LHCII but likely by vertical unstacking. Shuffling of phospho-LHCII during state transition results in remodeling of the PSI supercomplex landscape but not of the PSII landscape supporting the notion that only a loosely bound pool of LHCIIs is involved in state transition.

Non-photochemical quenching (NPQ) is the process that protects photosynthetic organisms from photodamage by dissipating the energy absorbed in excess as heat. In the model green alga Chlamydomonas reinhardtii, NPQ was abolished in the knock-out mutants of the pigment-protein complexes LHCSR3 and LHCBM1. However, while LHCSR3 was shown to be a pH sensor and switching to a quenched conformation at low pH, the role of LHCBM1 in NPQ has not been elucidated yet. In this work, we combine biochemical and physiological measurements to study short-term high light acclimation of npq5, the mutant lacking LHCBM1. We show that while in low light in the absence of this complex, the antenna size of PSII is smaller than in its presence, this effect is marginal in high light, implying that a reduction of the antenna is not responsible for the low NPQ. We also show that the mutant expresses LHCSR3 at the WT level in high light, indicating that the absence of this complex is also not the reason. Finally, NPQ remains low in the mutant even when the pH is artificially lowered to values that can switch LHCSR3 to the quenched conformation. It is concluded that both LHCSR3 and LHCBM1 need to be present for the induction of NPQ and that LHCBM1 is the interacting partner of LHCSR3. This interaction can either enhance the quenching capacity of LHCSR3 or connect this complex with the PSII supercomplex.

Responding to stress caused by various environmental changes is essential for living organisms. In cyanobacteria that perform oxygenic photosynthesis, the highly conserved histidine kinase Hik33/NblS homologs respond to diverse stressors such as high light, low temperature, high salts, high osmolarity, and reactive oxygen species. However, how this unique protein kinase responds to such divergent stresses remains unknown. This study has focused on the underlying stress sensing mechanism of NblS in Synechococcus elongatus PCC 7942. First, the sensory response by NblS was analyzed in vivo by monitoring the NblS-regulated hliA transcript accumulation with treatment of various benzoquinone reagents known as photosystem II (PSII) electron acceptors. It was found that molecular responses induced by various stresses were diminished in the presence of 2,6-dichloro-1,4-benzoquinone, which accepts electrons specifically from the PSII-bound plastoquinone QB. Cell fractionation analysis indicated that NblS was localized in the thylakoid membrane, which was consistent with its predicted membrane-spanning structure. In the thylakoid membrane, NblS was found in approximately 400 kDa and 800 kDa unknown complexes in clear native PAGE (CN-PAGE). Immunoprecipitation analysis of the cross-linked thylakoid membrane revealed that NblS is associated with D2 and CP47 proteins but not with CP43 protein, and thus it was suggested that dimeric NblS is associated with RC47-like complex, an assembly intermediate complex of PSII, as [RC47like-NblS2] (370 kDa) or [RC47like-NblS2]2 (740 kDa). We propose that the redox status of an RC47-bound plastoquinone molecule is a cue for the NblS response.

Phycobilisomes are versatile cyanobacterial antenna complexes that harvest light energy to drive photosynthesis. These complexes can also adapt to various light conditions, dismantling under high light to prevent photo-oxidation and arranging in rows under low light to increase light harvesting efficiency. Light quality also influences phycobilisome structure and function, as observed under far-red light exposure. Here we describe a new, phycobilisome linker protein, ApcI (previously hypothetical protein sll1911), expressed specifically under red light. We characterized ApcI in Synechocystis sp. PCC 6803 using mutant strain analyses, phycobilisome binding experiments, and protein interaction studies. Mutation of apcI conferred high light tolerance to Synechocystis sp. PCC 6803 compared to wild type with reduced energy transfer from phycobilisomes to the photosystems. Binding experiments revealed that ApcI replaces the linker protein ApcG at the membrane-facing side of the phycobilisome core using a paralogous C-terminal domain. Additionally, the N-terminal extension of ApcI was found to interact with photosystem II. Our findings highlight the importance of phycobilisome remodeling for adaptation under different light conditions. The characterization of ApcI provides new insights into the mechanisms by which cyanobacteria optimize light-harvesting in response to varying light environments.

Photosynthesis needs light energy, but that exceeding the maximal capacity of photosynthesis enhances formation of reactive oxygen species, which potentially causes photodamages. Therefore, light-harvesting complexes (Lhc) in phototrophs harbor various proteins and pigments to function in both light capture and energy dissipation. Diatom Lhcx proteins are reported to be a critical component for thermal dissipation of excess light energy, but the molecular mechanism of photoprotection is still not fully understood and the functions of each Lhcx isoform are not yet differentiated. Here, we focused on two types of Lhcx isoforms in Thalassiosira pseudonana: TpLhcx1/2, putative major components for energy-dependent fluorescence quenching (qE); and TpLhcx6_1, functionally unknown isoform uniquely conserved in Thalassiosirales. TpLhcx1/2 proteins accumulated more under high light than under low light, while the TpLhcx6_1 protein level was constitutive irrespective of light intensities and CO2 concentrations. High-light induced photodamage of photosystem II was increased in the genome-editing transformants of these Lhcx isoforms relative to the wild-type. Transformants lacking TpLhcx1/2 showed significantly lowered qE capacities, strongly suggesting that these proteins are important for the fast thermal energy dissipation. While in contrast, genome-editing transformants lacking the TpLhcx6_1 protein rather increased the qE capacity. TpLhcx6_1 transformants were further evaluated by the low-temperature time-resolved chlorophyll fluorescence measurement, showing the longer fluorescence lifetime in transformants than that in the wild type cells even at the dark-acclimated state of these cells. These results suggest that TpLhcx6_1 functions in photoprotection through non-photochemical energy dissipation in the different way from qE. One sentence summaryThe marine diatom Thalassiosira pseudonana dissipates excess light energy for photoprotection via two types of mechanisms supported by different Lhc isofoms.

Photosynthetic organisms rely on finely tuned mechanisms to optimize photosynthesis under different light conditions. While these processes are well-characterized in land plants, the adaptive strategies of marine plants remain largely unexplored. The Mediterranean seagrass Posidonia oceanica (Alismatales), a key ecosystem engineer thriving from the surface up to 40m depth and one of the largest long-term blue carbon sinks in coastal environments. Here, we investigate how P. oceanica adjusts its photosynthetic apparatus in response to varying light spectra encountered at different seawater depths. Contrary to land plants, P. oceanica maintains a relatively high PSI/PSII ratio and a high content of the major light-harvesting complex II (LHCII), regardless of depth. Notably, the antenna size of the photosystems remains stable across depths, although we document significant depth-dependent reorganization of the thylakoid membrane ultrastructure. Moreover, we identify a novel large PSI-LHCII supercomplex (L-PSI-LHCII) in P. oceanica, characterized by additional Lhca proteins, reduced red-shifted absorption, and increased chlorophyll b content. Ultrafast spectroscopy reveals the distinct energy transfer dynamics within this complex. The presence of a similar supercomplex in other marine plants, such as Zostera marina, suggests a conserved adaptive strategy among seagrasses.

Cyanobacteria play a key role in primary production in both oceans and fresh waters and hold great potential for sustainable production of a large number of commodities. During their life, cyanobacteria cells need to acclimate to a multitude of challenges, including shifts in intensity and quality of incident light. Despite our increasing understanding of metabolic regulation under various light regimes, detailed insight into fitness advantages and limitations under shifting light quality has been missing. Here, we study photo-physiological acclimation in the cyanobacterium Synechocystis sp. PCC 6803 through the whole range of photosynthetically active radiation (PAR). Using LEDs with qualitatively different narrow spectra, we describe wavelength dependence of light capture, electron transport and energy transduction to main cellular pools. In addition, we describe processes fine-tuning light capture such as state transitions and efficiency of energy transfer from phycobilisomes to photosystems. We show that growth was the most limited under blue light due to inefficient light harvesting, and that many cellular processes are tightly linked to the redox state of the PQ pool, which was the most reduced under red light. The PSI-to-PSII ratio was low under blue photons, however, it was not the main growth-limiting factor, since it was even more reduced under violet and near far-red lights, where Synechocystis grew faster compared to blue light. Our results provide insight into the spectral dependence of phototrophic growth and can provide the foundation for future studies of molecular mechanisms underlying light acclimation in cyanobacteria, leading to light optimization in controlled cultivations.

The marine cyanobacterium, Acaryochloris marina, uses the red-shifted chlorophyll d as its primary pigment, allowing it to absorb photons >700 nm. However, the widely studied type strain, A. marina MBIC11017, is atypical compared to most cyanobacteria, due to the absence of low energy chlorophylls ( red forms) within its Photosystem I complex. Consequently, Photosystem I and Photosystem II in the MBIC11017 strain share similar absorption spectra and are incapable of absorbing photons >740 nm. Recently, it has been discovered that the absorption and emission spectra from other A. marina strains are significantly more red-shifted than the MBIC11017 strain. Here, we have combined advanced spectroscopy and high-resolution cryo-EM to characterize Photosystem I from Acaryochloris marina NIES-2412, a red-shifted strain that is more representative of the A. marina species. The structure resolves all 96 chlorophylls and cofactors and indicates the location of the red chlorophyll forms. Spectroscopic analysis reveals two distinct types of red forms: one arising from the classical mechanism of charge transfer-exciton mixing, and the other from purely excitonic interactions. Furthermore, we have identified PsaX2 as a critical subunit that fine-tunes the pigment geometries and energies to enable the formation of these red forms. Together, these findings reveal how NIES-2412 PSI balances far-red light harvesting and energy trapping, highlighting its distinct strategy for adaptation in far-red light environments and redefining A. marina MBIC11017 as an atypical representative of the species.

We report here a technical advancement that enables time-resolved fluorescence spectroscopy in spatially resolved domains of a living cell at low temperatures. The technique is based on a combination of the self-developed cryo-confocal microscope system and the streak-camera technology. An instrumental response time of ca. 24 ps was achieved. This technique was applied to reveal the light-harvesting dynamics in local domains within single Chlamydomonas reinhardtii cells. Organisms performing oxygenic photosynthesis, like Chlamydomonas, have evolved a regulation mechanism called state transitions (ST), which maintains the excitation balance between PSI and PSII. ST relies on the shuttling of light-harvesting chlorophyll protein complex II (LHCII) between the two PSs. In the present experiment, cells were induced either to state1, where LHCII is bound to PSII, or state2, where LHCII moved and is bound to PSI. After the induction of ST, cells were immediately cooled to ca. 80 K, where PSI and PSII show clearly separated fluorescence emission bands, enabling the visualization of these components separately. Based on kinetic analyses of the time-resolved fluorescence spectra in both PSI-rich and PSII-rich local domains, we concluded that (1) the intracellular inhomogeneity in the PSII/PSI fluorescence ratio comes from that in the PSII/PSI stoichiometry, not from that in the antenna sizes of the PSs, and (2) the antenna size of PSI in state2 cells may larger in intact cells than that of the isolated PSI-LHCI-LHCII super-complex reported so far.

Photosynthetic organisms require acclimation mechanisms to regulate photosynthesis in response to light conditions. Here, two mutant alleles of ACCLIMATION OF PHOTOSYNTHESIS TO THE ENVIRONMENT 1 (ape1) have been characterized in Chlamydomonas reinhardtii. The ape1 mutants are photosensitive and show PSII photoinhibition during high light acclimation or under high light stress. The ape1 mutants retain more PSII super-complexes and have changes to thylakoid stacking relative to control strains during photosynthetic growth at different light intensities. The APE1 protein is found in all oxygenic phototrophs and encodes a 25 kDa thylakoid protein that interacts with the Photosystem II core complex as monomers, dimers and supercomplexes. We propose a model where APE1 bound to PSII supercomplexes releases core complexes and promotes PSII heterogeneity influencing the stacking of Chlamydomonas thylakoids. APE1 is a regulator in light acclimation and its function is to reduce over-excitation of PSII centres and avoid PSII photoinhibition to increase the resilience of photosynthesis to high light.