Chlamydomonas reinhardtii PsbS Protein Is Functional and Accumulates Rapidly and Transiently under High Light

Introducing an Arabidopsis thaliana Thylakoid Thiol/Disulfide-Modulating Protein Into Synechocystis Increases the Efficiency of Photosystem II Photochemistry

Photosynthetic species are subjected to a variety of environmental stresses, including suboptimal irradiance. In oxygenic photosynthetic organisms, a major effect of high light exposure is damage to the Photosystem II (PSII) reaction-center protein D1. This process even happens under low or moderate light. To cope with photodamage to D1, photosynthetic organisms evolved an intricate PSII repair and reassembly cycle, which requires the participation of different auxiliary proteins, including thiol/disulfide-modulating proteins. Most of these auxiliary proteins exist ubiquitously in oxygenic photosynthetic organisms. Due to differences in mobility and environmental conditions, land plants are subject to more extensive high light stress than algae and cyanobacteria. Therefore, land plants evolved additional thiol/disulfide-modulating proteins, such as Low Quantum Yield of PSII 1 (LQY1), to aid in the repair and reassembly cycle of PSII. In this study, we introduced an Arabidopsis thaliana homolog of LQY1 (AtLQY1) into the cyanobacterium Synechocystis sp. PCC6803 and performed a series of biochemical and physiological assays on AtLQY1-expressing Synechocystis. At a moderate growth light intensity (50 µmol photons m-2 s-1), AtLQY1-expressing Synechocystis was found to have significantly higher F v /F m , and lower nonphotochemical quenching and reactive oxygen species levels than the empty-vector control, which is opposite from the loss-of-function Atlqy1 mutant phenotype. Light response curve analysis of PSII operating efficiency and electron transport rate showed that AtLQY1-expressing Synechocystis also outperform the empty-vector control under higher light intensities. The increases in F v /F m , PSII operating efficiency, and PSII electron transport rate in AtLQY1-expressing Synechocystis under such growth conditions most likely come from an increased amount of PSII, because the level of D1 protein was found to be higher in AtLQY1-expressing Synechocystis. These results suggest that introducing AtLQY1 is beneficial to Synechocystis.

Solving the Riddle of the Evolution of Shine-Dalgarno Based Translation in Chloroplasts

Chloroplasts originated from an ancient cyanobacterium and still harbor a bacterial-like genome. However, the centrality of Shine–Dalgarno ribosome binding, which predominantly regulates proteobacterial translation initiation, is significantly decreased in chloroplasts. As plastid ribosomal RNA anti-Shine–Dalgarno elements are similar to their bacterial counterparts, these sites alone cannot explain this decline. By computational simulation we show that upstream point mutations modulate the local structure of ribosomal RNA in chloroplasts, creating significantly tighter structures around the anti-Shine–Dalgarno locus, which in-turn reduce the probability of ribosome binding. To validate our model, we expressed two reporter genes (mCherry, hydrogenase) harboring a Shine–Dalgarno motif in the Chlamydomonas reinhardtii chloroplast. Coexpressing them with a 16S ribosomal RNA, modified according to our model, significantly enhances mCherry and hydrogenase expression compared with coexpression with an endogenous 16S gene.

Light-dependent N-terminal phosphorylation of LHCSR3 and LHCB4 are interlinked in Chlamydomonas reinhardtii

Phosphorylation dynamics of LHCSR3 were investigated in Chlamydomonas reinhardtii by quantitative proteomics and genetic engineering. LHCSR3 protein expression and phosphorylation were induced in high light. Our data revealed synergistic and dynamic N-terminal LHCSR3 phosphorylation. Phosphorylated and nonphosphorylated LHCSR3 associated with PSII-LHCII supercomplexes. The phosphorylation status of LHCB4 was closely linked to the phosphorylation of multiple sites at the N-terminus of LHCSR3, indicating that LHCSR3 phosphorylation may operate as a molecular switch modulating LHCB4 phosphorylation, which in turn is important for PSII-LHCII disassembly. Notably, LHCSR3 phosphorylation diminished under prolonged high light, which coincided with onset of CEF. Hierarchical clustering of significantly altered proteins revealed similar expression profiles of LHCSR3, CRX, and FNR. This finding indicated the existence of a functional link between LHCSR3 protein abundance and phosphorylation, photosynthetic electron flow, and the oxidative stress response.

Regulation of photoprotection gene expression in Chlamydomonas by a putative E3 ubiquitin ligase complex and a homolog of CONSTANS

Photosynthetic organisms use nonphotochemical quenching (NPQ) mechanisms to dissipate excess absorbed light energy and protect themselves from photooxidation. In the model green alga Chlamydomonas reinhardtii, the capacity for rapidly reversible NPQ (qE) is induced by high light, blue light, and UV light via increased expression of LHCSR and PSBS genes that are necessary for qE. Here, we used a forward genetics approach to identify SPA1 and CUL4, components of a putative green algal E3 ubiquitin ligase complex, as critical factors in a signaling pathway that controls light-regulated expression of the LHCSR and PSBS genes in C. reinhardtii The spa1 and cul4 mutants accumulate increased levels of LHCSR1 and PSBS proteins in high light, and unlike the wild type, they express LHCSR1 and exhibit qE capacity even when grown in low light. The spa1-1 mutation resulted in constitutively high expression of LHCSR and PSBS RNAs in both low light and high light. The qE and gene expression phenotypes of spa1-1 are blocked by mutation of CrCO, a B-box Zn-finger transcription factor that is a homolog of CONSTANS, which controls flowering time in plants. CONSTANS-like cis-regulatory sequences were identified proximal to the qE genes, consistent with CrCO acting as a direct activator of qE gene expression. We conclude that SPA1 and CUL4 are components of a conserved E3 ubiquitin ligase that acts upstream of CrCO, whose regulatory function is wired differently in C. reinhardtii to control qE capacity via cis-regulatory CrCO-binding sites at key photoprotection genes.

Photoprotection and growth under different lights of Arabidopsis single and double mutants for energy dissipation (npq4) and state transitions (pph1)

Arabidopsis single and double mutants for energy dissipation (npq4) and state transitions (pph1, blocked in State II) show enhanced growth and flowers + siliques production under controlled low-light conditions. Non-photochemical quenching (NPQ) is a short-term regulation important to maintain efficient photosynthesis and to avoid photooxidative damages by dissipation of excess energy. Full activation of NPQ in plants requires the protonation of the PsbS protein, which is the sensor of the low lumenal pH triggering the thermal dissipation. State transitions are a second important photosynthetic regulation to respond to changes in light quality and unbalanced excitation of photosystems. State transitions allow energy redistribution between PSI and PSII through the reversible exchange of LHCII antenna complexes between photosystems thanks to the opposite action of the STN7 kinase and PPH1 phosphatase: phosphorylation of LHCII promotes its mobilization from PSII to PSI, while dephosphorylation has the opposite effect. In this work, we produced the pph1/npq4 double mutant and characterized some photosynthetic, growth and reproduction properties in comparison with wild-type and single-mutant plants in high- and low-light conditions. Results indicate that in high light, the pph1 mutant maintains good photoprotection ability, while npq4 plants show more susceptibility to photodamages. The pph1/npq4 double mutant showed a resistance to high-light stress similar to that of the single npq4 mutant. In low-light condition, the single mutants showed a significant increase of growth and flowering compared to wild-type plants and this effect was further enhanced in the pph1/npq4 double mutant. Results suggest that photosynthetic optimisation to improve crop growth and productivity might be possible, at least under controlled low-light conditions, by modifying NPQ and regulation of state transitions.

pH dependence, kinetics and light-harvesting regulation of nonphotochemical quenching in Chlamydomonas

Photosynthetic organisms utilize sunlight as a form of energy. Under low light, they maximize their capacity to harvest photons; however, under excess light, they dissipate part of the harvested energy to prevent photodamage, at the expense of light-use efficiency. Optimally balancing light harvesting and energy dissipation in natural (fluctuating light) conditions is considered a target for improving the productivity of both algae and plants. Here we have studied the energy dissipation process in the green alga Chlamydomonas reinhardtii in vivo. We found that it is remarkably different from that of higher plants, highlighting the need of developing tailor-made strategies to optimize the light harvesting–energy dissipation balance in different organisms.

Sunlight drives photosynthesis but can also cause photodamage. To protect themselves, photosynthetic organisms dissipate the excess absorbed energy as heat, in a process known as nonphotochemical quenching (NPQ). In green algae, diatoms, and mosses, NPQ depends on the light-harvesting complex stress-related (LHCSR) proteins. Here we investigated NPQ in Chlamydomonas reinhardtii using an approach that maintains the cells in a stable quenched state. We show that in the presence of LHCSR3, all of the photosystem (PS) II complexes are quenched and the LHCs are the site of quenching, which occurs at a rate of ∼150 ps⁻¹ and is not induced by LHCII aggregation. The effective light-harvesting capacity of PSII decreases upon NPQ, and the NPQ rate is independent of the redox state of the reaction center. Finally, we could measure the pH dependence of NPQ, showing that the luminal pH is always above 5.5 in vivo and highlighting the role of LHCSR3 as an ultrasensitive pH sensor.

Light Acclimation of the Colonial Green Alga Botryococcus braunii Strain Showa

In contrast to single cellular species, detailed information is lacking on the processes of photosynthetic acclimation for colonial algae, although these algae are important for biofuel production, ecosystem biodiversity, and wastewater treatment. To investigate differences between single cellular and colonial species, we studied the regulation of photosynthesis and photoprotection during photoacclimation for the colonial green alga Botryococcus braunii and made a comparison with the properties of the single cellular species Chlamydomonas reinhardtii We show that B. braunii shares some high-light (HL) photoacclimation strategies with C. reinhardtii and other frequently studied green algae: decreased chlorophyll content, increased free carotenoid content, and increased nonphotochemical quenching (NPQ). Additionally, B. braunii has unique HL photoacclimation strategies, related to its colonial form: strong internal shading by an increase of the colony size and the accumulation of extracellular echinenone (a ketocarotenoid). HL colonies are larger and more spatially heterogenous than low-light colonies. Compared with surface cells, cells deeper inside the colony have increased pigmentation and larger photosystem II antenna size. The core of the largest of the HL colonies does not contain living cells. In contrast with C. reinhardtii, but similar to other biofilm-forming algae, NPQ capacity is substantial in low light. In HL, NPQ amplitude increases, but kinetics are unchanged. We discuss possible causes of the different acclimation responses of C. reinhardtii and B. braunii Knowledge of the specific photoacclimation processes for this colonial green alga further extends the view of the diversity of photoacclimation strategies in photosynthetic organisms.

Isolation of photoprotective signal transduction mutants by systematic bioluminescence screening in Chlamydomonas reinhardtii

In photosynthetic organisms, photoprotection to avoid overexcitation of photosystems is a prerequisite for survival. Green algae have evolved light-inducible photoprotective mechanisms mediated by genes such as light-harvesting complex stress-related (LHCSR). Studies on the light-dependent regulation of LHCSR expression in the green alga Chlamydomonas reinhardtii have revealed that photoreceptors for blue light (phototropin) and ultraviolet light perception (UVR8) play key roles in initiating photoprotective signal transduction. Although initial light perception via phototropin or UVR8 is known to result in increased LHCSR3 and LHCSR1 gene expression, respectively, the mechanisms of signal transduction from the input (light perception) to the output (gene expression) remain unclear. In this study, to further elucidate the signal transduction pathway of the photoprotective response of green algae, we established a systematic screening protocol for UV-inducible LHCSR1 gene expression mutants using a bioluminescence reporter assay. Following random mutagenesis screening, we succeeded in isolating mutants deficient in LHCSR1 gene and protein expression after UV illumination. Further characterization revealed that the obtained mutants could be separated into 3 different phenotype groups, the “UV-specific”, “LHCSR1-promoter/transcript-specific” and “general photoprotective” mutant groups, which provided further insight into photoprotective signal transduction in C. reinhardtii.

LHCSR3 is a nonphotochemical quencher of both photosystems in Chlamydomonas reinhardtii

Photosynthetic organisms prevent oxidative stress from light energy absorbed in excess through several photoprotective mechanisms. A major component is thermal dissipation of chlorophyll singlet excited states and is called nonphotochemical quenching (NPQ). NPQ is catalyzed in green algae by protein subunits called LHCSRs (Light Harvesting Complex Stress Related), homologous to the Light Harvesting Complexes (LHC), constituting the antenna system of both photosystem I (PSI) and PSII. We investigated the role of LHCSR1 and LHCSR3 in NPQ activation to verify whether these proteins are involved in thermal dissipation of PSI excitation energy, in addition to their well-known effect on PSII. To this aim, we measured the fluorescence emitted at 77 K by whole cells in a quenched or unquenched state, using green fluorescence protein as the internal standard. We show that NPQ activation by high light treatment in Chlamydomonas reinhardtii leads to energy quenching in both PSI and PSII antenna systems. By analyzing quenching properties of mutants affected on the expression of LHCSR1 or LHCSR3 gene products and/or state 1-state 2 transitions or zeaxanthin accumulation, namely, npq4, stt7, stt7 npq4, npq4 lhcsr1, lhcsr3-complemented npq4 lhcsr1 and npq1, we showed that PSI undergoes NPQ through quenching of the associated LHCII antenna. This quenching event is fast-reversible on switching the light off, is mainly related to LHCSR3 activity, and is dependent on thylakoid luminal pH. Moreover, PSI quenching could also be observed in the absence of zeaxanthin or STT7 kinase activity.

LHC-like proteins involved in stress responses and biogenesis/repair of the photosynthetic apparatus

LHC (light-harvesting complex) proteins of plants and algae are known to be involved both in collecting light energy for driving the primary photochemical reactions of photosynthesis and in photoprotection when the absorbed light energy exceeds the capacity of the photosynthetic apparatus. These proteins usually contain three transmembrane (TM) helices which span the thylakoid membranes and bind several chlorophyll, carotenoid and lipid molecules. In addition, the LHC protein family includes LHC-like proteins containing one, two, three or even four TM domains. One-helix proteins are not only present in eukaryotic photosynthetic organisms but also in cyanobacteria where they have been named high light-inducible proteins. These small proteins are probably the ancestors of the members of the extant LHC protein family which arouse through gene duplications, deletions and fusions. During evolution, some of these proteins have diverged and acquired novel functions. In most cases, LHC-like proteins are induced in response to various stress conditions including high light, high salinity, elevated temperature and nutrient limitation. Many of these proteins play key roles in photoprotection, notably in non-photochemical quenching of absorbed light energy. Moreover, some of these proteins appear to be involved in the regulation of chlorophyll synthesis and in the assembly and repair of Photosystem II and also of Photosystem I possibly by mediating the insertion of newly synthesized pigments into the photosynthetic reaction centers.

Light and Dehydration but Not Temperature Drive Photosynthetic Adaptations of Basal Streptophytes (Hormidiella, Streptosarcina and Streptofilum) Living in Terrestrial Habitats

Streptophyte algae are the ancestors of land plants, and several classes contain taxa that are adapted to an aero-terrestrial lifestyle. In this study, four basal terrestrial streptophytes from the class Klebsormidiophyceae, including Hormidiella parvula; two species of the newly described genus Streptosarcina (S. costaricana and S. arenaria); and the newly described Streptofilum capillatum were investigated for their responses to radiation, desiccation and temperature stress conditions. All the strains showed low-light adaptation (Ik < 70 μmol photons m-2 s-1) but differed in photoprotective capacities (such as non-photochemical quenching). Acclimation to enhanced photon fluence rates (160 μmol photons m-2 s-1) increased photosynthetic performance in H. parvula and S. costaricana but not in S. arenaria, showing that low-light adaptation is a constitutive trait for S. arenaria. This lower-light adaptation of S. arenaria was coupled with a higher desiccation tolerance, providing further evidence that dehydration is a selective force shaping species occurrence in low light. For protection against ultraviolet radiation, all species synthesised and accumulated different amounts of mycosporine-like amino acids (MAAs). Biochemically, MAAs synthesised by Hormidiella and Streptosarcina were similar to MAAs from closely related Klebsormidium spp. but differed in retention time and spectral characteristics in S. capillatum. Unlike the different radiation and dehydration tolerances, Hormidiella, Streptosarcina and Streptofilum displayed preferences for similar thermal conditions. These species showed a temperature dependence of photosynthesis similar to respiration, contrasting with Klebsormidium spp. and highlighting an interspecific diversity in thermal requirements, which could regulate species distributions under temperature changes.

Multiomics resolution of molecular events during a day in the life of Chlamydomonas

The unicellular green alga Chlamydomonas reinhardtii displays metabolic flexibility in response to a changing environment. We analyzed expression patterns of its three genomes in cells grown under light-dark cycles. Nearly 85% of transcribed genes show differential expression, with different sets of transcripts being up-regulated over the course of the day to coordinate cellular growth before undergoing cell division. Parallel measurements of select metabolites and pigments, physiological parameters, and a subset of proteins allow us to infer metabolic events and to evaluate the impact of the transcriptome on the proteome. Among the findings are the observations that Chlamydomonas exhibits lower respiratory activity at night compared with the day; multiple fermentation pathways, some oxygen-sensitive, are expressed at night in aerated cultures; we propose that the ferredoxin, FDX9, is potentially the electron donor to hydrogenases. The light stress-responsive genes PSBS, LHCSR1, and LHCSR3 show an acute response to lights-on at dawn under abrupt dark-to-light transitions, while LHCSR3 genes also exhibit a later, second burst in expression in the middle of the day dependent on light intensity. Each response to light (acute and sustained) can be selectively activated under specific conditions. Our expression dataset, complemented with coexpression networks and metabolite profiling, should constitute an excellent resource for the algal and plant communities.

A New Light on Photosystem II Maintenance in Oxygenic Photosynthesis

Life on earth is sustained by oxygenic photosynthesis, a process that converts solar energy, carbon dioxide, and water into chemical energy and biomass. Sunlight is essential for growth and productivity of photosynthetic organisms. However, exposure to an excessive amount of light adversely affects fitness due to photooxidative damage to the photosynthetic machinery, primarily to the reaction center of the oxygen-evolving photosystem II (PSII). Photosynthetic organisms have evolved diverse photoprotective and adaptive strategies to avoid, alleviate, and repair PSII damage caused by high-irradiance or fluctuating light. Rapid and harmless dissipation of excess absorbed light within antenna as heat, which is measured by chlorophyll fluorescence as non-photochemical quenching (NPQ), constitutes one of the most efficient protective strategies. In parallel, an elaborate repair system represents another efficient strategy to maintain PSII reaction centers in active states. This article reviews both the reaction center-based strategy for robust repair of photodamaged PSII and the antenna-based strategy for swift control of PSII light-harvesting (NPQ). We discuss evolutionarily and mechanistically diverse strategies used by photosynthetic organisms to maintain PSII function for growth and productivity under static high-irradiance light or fluctuating light environments. Knowledge of mechanisms underlying PSII maintenance would facilitate bioengineering photosynthesis to enhance agricultural productivity and sustainability to feed a growing world population amidst climate change.

The evolution of the photoprotective antenna proteins in oxygenic photosynthetic eukaryotes

Photosynthetic organisms require rapid and reversible down-regulation of light harvesting to avoid photodamage. Response to unpredictable light fluctuations is achieved by inducing energy-dependent quenching, qE, which is the major component of the process known as non-photochemical quenching (NPQ) of chlorophyll fluorescence. qE is controlled by the operation of the xanthophyll cycle and accumulation of specific types of proteins, upon thylakoid lumen acidification. The protein cofactors so far identified to modulate qE in photosynthetic eukaryotes are the photosystem II subunit S (PsbS) and light-harvesting complex stress-related (LHCSR/LHCX) proteins. A transition from LHCSR- to PsbS-dependent qE took place during the evolution of the Viridiplantae (also known as ‘green lineage’ organisms), such as green algae, mosses and vascular plants. Multiple studies showed that LHCSR and PsbS proteins have distinct functions in the mechanism of qE. LHCX(-like) proteins are closely related to LHCSR proteins and found in ‘red lineage’ organisms that contain secondary red plastids, such as diatoms. Although LHCX proteins appear to control qE in diatoms, their role in the mechanism remains poorly understood. Here, we present the current knowledge on the functions and evolution of these crucial proteins, which evolved in photosynthetic eukaryotes to optimise light harvesting.

Features of cues and processes during chloroplast-mediated retrograde signaling in the alga Chlamydomonas

Retrograde signaling is an intracellular communication process defined by cues generated in chloroplast and mitochondria which traverse membranes to their destination in the nucleus in order to regulate nuclear gene expression and protein synthesis. The coding and decoding of such organellar message(s) involve gene medleys and metabolic components about which more is known in higher plants than the unicellular organisms such as algae. Chlamydomonas reinhardtii is an oxygenic microalgal model for genetic and physiological studies. It harbors a single chloroplast and is amenable for generating mutants. The focus of this review is on studies that delineate retrograde signaling in Chlamydomonas vis a vis higher plants. Thus, communication networks between chloroplast and nucleus involving photosynthesis- and ROS-generated signals, functional tetrapyrrole biosynthesis intermediates, and Ca²⁺-signaling that modulate nuclear gene expression in this alga are discussed. Conceptually, different signaling components converge to regulate either the same or functionally-overlapping gene products.

A siphonous morphology affects light-harvesting modulation in the intertidal green macroalga Bryopsis corticulans (Ulvophyceae)

The macroalga Bryopsis corticulans relies on a sustained protective NPQ and a peculiar body architecture to efficiently adapt to the extreme light changes of intertidal shores. During low tides, intertidal algae experience prolonged high light stress. Efficient dissipation of excess light energy, measured as non-photochemical quenching (NPQ) of chlorophyll fluorescence, is therefore required to avoid photodamage. Light-harvesting regulation was studied in the intertidal macroalga Bryopsis corticulans, during high light and air exposure. Photosynthetic capacity and NPQ kinetics were assessed in different filament layers of the algal tufts and in intact chloroplasts to unravel the nature of NPQ in this siphonous green alga. We found that the morphology and pigment composition of the B. corticulans body provides functional segregation between surface sunlit filaments (protective state) and those that are underneath and undergo severe light attenuation (light-harvesting state). In the surface filaments, very high and sustained NPQ gradually formed. NPQ induction was triggered by the formation of transthylakoid proton gradient and independent of the xanthophyll cycle. PsbS and LHCSR proteins seem not to be active in the NPQ mechanism activated by this alga. Our results show that B. corticulans endures excess light energy pressure through a sustained protective NPQ, not related to photodamage, as revealed by the unusually quick restoration of photosystem II (PSII) function in the dark. This might suggest either the occurrence of transient PSII photoinactivation or a fast rate of PSII repair cycle.

Molecular basis of autotrophic vs mixotrophic growth in Chlorella sorokiniana

In this work, we investigated the molecular basis of autotrophic vs. mixotrophic growth of Chlorella sorokiniana, one of the most productive microalgae species with high potential to produce biofuels, food and high value compounds. To increase biomass accumulation, photosynthetic microalgae are commonly cultivated in mixotrophic conditions, adding reduced carbon sources to the growth media. In the case of C. sorokiniana, the presence of acetate enhanced biomass, proteins, lipids and starch productivity when compared to autotrophic conditions. Despite decreased chlorophyll content, photosynthetic properties were essentially unaffected while differential gene expression profile revealed transcriptional regulation of several genes mainly involved in control of carbon flux. Interestingly, acetate assimilation caused upregulation of phosphoenolpyruvate carboxylase enzyme, enabling potential recovery of carbon atoms lost by acetate oxidation. The obtained results allowed to associate the increased productivity observed in mixotrophy in C. sorokiniana with a different gene regulation leading to a fine regulation of cell metabolism.

Surprisal analysis of genome-wide transcript profiling identifies differentially expressed genes and pathways associated with four growth conditions in the microalga Chlamydomonas

The usual cultivation mode of the green microalga Chlamydomonas is liquid medium and light. However, the microalga can also be grown on agar plates and in darkness. Our aim is to analyze and compare gene expression of cells cultivated in these different conditions. For that purpose, RNA-seq data are obtained from Chlamydomonas samples of two different labs grown in four environmental conditions (agar@light, agar@dark, liquid@light, liquid@dark). The RNA seq data are analyzed by surprisal analysis, which allows the simultaneous meta-analysis of all the samples. First we identify a balance state, which defines a state where the expression levels are similar in all the samples irrespectively of their growth conditions, or lab origin. In addition our analysis identifies additional constraints needed to quantify the deviation with respect to the balance state. The first constraint differentiates the agar samples versus the liquid ones; the second constraint the dark samples versus the light ones. The two constraints are almost of equal importance. Pathways involved in stress responses are found in the agar phenotype while the liquid phenotype comprises ATP and NADH production pathways. Remodeling of membrane is suggested in the dark phenotype while photosynthetic pathways characterize the light phenotype. The same trends are also present when performing purely statistical analysis such as K-means clustering and differentially expressed genes.

The peculiar NPQ regulation in the stramenopile Phaeomonas sp. challenges the xanthophyll cycle dogma

In changing light conditions, photosynthetic organisms develop different strategies to maintain a fine balance between light harvesting, photochemistry, and photoprotection. One of the most widespread photoprotective mechanisms consists in the dissipation of excess light energy in the form of heat in the photosystem II antenna, which participates to the Non Photochemical Quenching (NPQ) of chlorophyll fluorescence. It is tightly related to the reversible epoxidation of xanthophyll pigments, catalyzed by the two enzymes, the violaxanthin deepoxidase and the zeaxanthin epoxidase. In Phaeomonas sp. (Pinguiophyte, Stramenopiles), we show that the regulation of the heat dissipation process is different from that of the green lineage: the NPQ is strictly proportional to the amount of the xanthophyll pigment zeaxanthin and the xanthophyll cycle enzymes are differently regulated. The violaxanthin deepoxidase is already active in the dark, because of a low luminal pH, and the zeaxanthin epoxidase shows a maximal activity under moderate light conditions, being almost inactive in the dark and under high light. This light-dependency mirrors the one of NPQ: Phaeomonas sp. displays a large NPQ in the dark as well as under high light, which recovers under moderate light. Our results pinpoint zeaxanthin epoxidase activity as the prime regulator of NPQ in Phaeomonas sp. and therefore challenge the deepoxidase-regulated xanthophyll cycle dogma.

Fluorescence lifetime analyses reveal how the high light-responsive protein LHCSR3 transforms PSII light-harvesting complexes into an energy-dissipative state

In green algae, light-harvesting complex stress-related 3 (LHCSR3) is responsible for the pH-dependent dissipation of absorbed light energy, a function vital for survival under high-light conditions. LHCSR3 binds the photosystem II and light-harvesting complex II (PSII-LHCII) supercomplex and transforms it into an energy-dissipative form under acidic conditions, but the molecular mechanism remains unclear. Here we show that in the green alga Chlamydomonas reinhardtii, LHCSR3 modulates the excitation energy flow and dissipates the excitation energy within the light-harvesting complexes of the PSII supercomplex. Using fluorescence decay-associated spectra analysis, we found that, when the PSII supercomplex is associated with LHCSR3 under high-light conditions, excitation energy transfer from light-harvesting complexes to chlorophyll-binding protein CP43 is selectively inhibited compared with that to CP47, preventing excess excitation energy from overloading the reaction center. By analyzing femtosecond up-conversion fluorescence kinetics, we further found that pH- and LHCSR3-dependent quenching of the PSII-LHCII-LHCSR3 supercomplex is accompanied by a fluorescence emission centered at 684 nm, with a decay time constant of 18.6 ps, which is equivalent to the rise time constant of the lutein radical cation generated within a chlorophyll-lutein heterodimer. These results suggest a mechanism in which LHCSR3 transforms the PSII supercomplex into an energy-dissipative state and provide critical insight into the molecular events and characteristics in LHCSR3-dependent energy quenching.

Functional modulation of LHCSR1 protein from Physcomitrella patens by zeaxanthin binding and low pH

Light harvesting for oxygenic photosynthesis is regulated to prevent the formation of harmful photoproducts by activation of photoprotective mechanisms safely dissipating the energy absorbed in excess. Lumen acidification is the trigger for the formation of quenching states in pigment binding complexes. With the aim to uncover the photoprotective functional states responsible for excess energy dissipation in green algae and mosses, we compared the fluorescence dynamic properties of the light-harvesting complex stress-related (LHCSR1) protein, which is essential for fast and reversible regulation of light use efficiency in lower plants, as compared to the major LHCII antenna protein, which mainly fulfills light harvesting function. Both LHCII and LHCSR1 had a chlorophyll fluorescence yield and lifetime strongly dependent on detergent concentration but the transition from long- to short-living states was far more complete and fast in the latter. Low pH and zeaxanthin binding enhanced the relative amplitude of quenched states in LHCSR1, which were characterized by the presence of 80 ps fluorescence decay components with a red-shifted emission spectrum. We suggest that energy dissipation occurs in the chloroplast by the activation of 80 ps quenching sites in LHCSR1 which spill over excitons from the photosystem II antenna system.

Photoreceptor-dependent regulation of photoprotection

In photosynthetic organisms, proteins in the light-harvesting complex (LHC) harvest light energy to fuel photosynthesis, whereas photoreceptor proteins are activated by the different wavelengths of the light spectrum to regulate cellular functions. Under conditions of excess light, blue-light photoreceptors activate chloroplast avoidance movements in sessile plants, and blue- and green-light photoreceptors cause motile algae to swim away from intense light. Simultaneously, LHCs switch from light-harvesting mode to energy-dissipation mode, which was thought to be independent of photoreceptor-signaling up until recently. Recent advances, however, indicate that energy dissipation in green algae is controlled by photoreceptors activated by blue and UV-B light, and new molecular links have been established between photoreception and photoprotection.

Compartmentalisation of [FeFe]-hydrogenase maturation in Chlamydomonas reinhardtii

Molecular hydrogen (H₂ ) can be produced in green microalgae by [FeFe]-hydrogenases as a direct product of photosynthesis. The Chlamydomonas reinhardtii hydrogenase HYDA1 contains a catalytic site comprising a classic [4Fe4S] cluster linked to a unique 2Fe sub-cluster. From in vitro studies it appears that the [4Fe4S] cluster is incorporated first by the housekeeping FeS cluster assembly machinery, followed by the 2Fe sub-cluster, whose biosynthesis requires the specific maturases HYDEF and HYDG. To investigate the maturation process in vivo, we expressed HYDA1 from the C. reinhardtii chloroplast and nuclear genomes (with and without a chloroplast transit peptide) in a hydrogenase-deficient mutant strain, and examined the cellular enzymatic hydrogenase activity, as well as in vivo H₂ production. The transformants expressing HYDA1 from the chloroplast genome displayed levels of H₂ production comparable to the wild type, as did the transformants expressing full-length HYDA1 from the nuclear genome. In contrast, cells equipped with cytoplasm-targeted HYDA1 produced inactive enzyme, which could only be activated in vitro after reconstitution of the [4Fe4S] cluster. This indicates that the HYDA1 FeS cluster can only be built by the chloroplastic FeS cluster assembly machinery. Further, the expression of a bacterial hydrogenase gene, CPI, from the C. reinhardtii chloroplast genome resulted in H₂ -producing strains, demonstrating that a hydrogenase with a very different structure can fulfil the role of HYDA1 in vivo and that overexpression of foreign hydrogenases in C. reinhardtii is possible. All chloroplast transformants were stable and no toxic effects were seen from HYDA1 or CPI expression.

Photoprotection in a monophyletic branch of chlorophyte algae is independent of energy-dependent quenching (qE)

Phototrophic organisms need to ensure high photosynthetic performance whilst suppressing reactive oxygen species (ROS)-induced stress occurring under excess light conditions. The xanthophyll cycle (XC), related to the high-energy quenching component (qE) of the nonphotochemical quenching (NPQ) of excitation energy, is considered to be an obligatory component of photoprotective mechanisms. The pigment composition of at least one representative of each major clade of Ulvophyceae (Chlorophyta) was investigated. We searched for a light-dependent conversion of pigments and investigated the NPQ capacity with regard to the contribution of XC and the qE component when grown under different light conditions. A XC was found to be absent in a monophyletic group of Ulvophyceae, the Bryopsidales, when cultivated under low light, but was triggered in one of the 10 investigated bryopsidalean species, Caulerpa cf. taxifolia, when cultivated under high light. Although Bryopsidales accumulate zeaxanthin (Zea) under high-light (HL) conditions, NPQ formation is independent of a XC and not related to qE. qE- and XC-independent NPQ in the Bryopsidales contradicts the common perception regarding its ubiquitous occurrence in Chloroplastida. Zea accumulation in HL-acclimated Bryopsidales most probably represents a remnant of a functional XC. The existence of a monophyletic algal taxon that lacks qE highlights the need for broad biodiversity studies on photoprotective mechanisms.

Membrane Proteomic Insights into the Physiology and Taxonomy of an Oleaginous Green Microalga

Ettlia oleoabundans is a nonsequenced oleaginous green microalga. Despite the significant biotechnological interest in producing value-added compounds from the acyl lipids of this microalga, a basic understanding of the physiology and biochemistry of oleaginous microalgae is lacking, especially under nitrogen deprivation conditions known to trigger lipid accumulation. Using an RNA sequencing-based proteomics approach together with manual annotation, we are able to provide, to our knowledge, the first membrane proteome of an oleaginous microalga. This approach allowed the identification of novel proteins in E. oleoabundans, including two photoprotection-related proteins, Photosystem II Subunit S and Maintenance of Photosystem II under High Light1, which were considered exclusive to higher photosynthetic organisms, as well as Retinitis Pigmentosa Type 2-Clathrin Light Chain, a membrane protein with a novel domain architecture. Free-flow zonal electrophoresis of microalgal membranes coupled to liquid chromatography-tandem mass spectrometry proved to be a useful technique for determining the intracellular location of proteins of interest. Carbon-flow compartmentalization in E. oleoabundans was modeled using this information. Molecular phylogenetic analyses of protein markers and 18S ribosomal DNA support the reclassification of E. oleoabundans within the trebouxiophycean microalgae, rather than with the Chlorophyceae class, in which it is currently classified, indicating that it may not be closely related to the model green alga Chlamydomonas reinhardtii A detailed survey of biological processes taking place in the membranes of nitrogen-deprived E. oleoabundans, including lipid metabolism, provides insights into the basic biology of this nonmodel organism.

UV-B photoreceptor-mediated protection of the photosynthetic machinery in Chlamydomonas reinhardtii

Life on earth is dependent on the photosynthetic conversion of light energy into chemical energy. However, absorption of excess sunlight can damage the photosynthetic machinery and limit photosynthetic activity, thereby affecting growth and productivity. Photosynthetic light harvesting can be down-regulated by nonphotochemical quenching (NPQ). A major component of NPQ is qE (energy-dependent nonphotochemical quenching), which allows dissipation of light energy as heat. Photodamage peaks in the UV-B part of the spectrum, but whether and how UV-B induces qE are unknown. Plants are responsive to UV-B via the UVR8 photoreceptor. Here, we report in the green alga Chlamydomonas reinhardtii that UVR8 induces accumulation of specific members of the light-harvesting complex (LHC) superfamily that contribute to qE, in particular LHC Stress-Related 1 (LHCSR1) and Photosystem II Subunit S (PSBS). The capacity for qE is strongly induced by UV-B, although the patterns of qE-related proteins accumulating in response to UV-B or to high light are clearly different. The competence for qE induced by acclimation to UV-B markedly contributes to photoprotection upon subsequent exposure to high light. Our study reveals an anterograde link between photoreceptor-mediated signaling in the nucleocytosolic compartment and the photoprotective regulation of photosynthetic activity in the chloroplast.

Cytochrome b 6 f function and localization, phosphorylation state of thylakoid membrane proteins and consequences on cyclic electron flow

Both the structure and the protein composition of thylakoid membranes have an impact on light harvesting and electron transfer in the photosynthetic chain. Thylakoid membranes form stacks and lamellae where photosystem II and photosystem I localize, respectively. Light-harvesting complexes II can be associated to either PSII or PSI depending on the redox state of the plastoquinone pool, and their distribution is governed by state transitions. Upon state transitions, the thylakoid ultrastructure and lateral distribution of proteins along the membrane are subject to significant rearrangements. In addition, quinone diffusion is limited to membrane microdomains and the cytochrome b 6 f complex localizes either to PSII-containing grana stacks or PSI-containing stroma lamellae. Here, we discuss possible similarities or differences between green algae and C3 plants on the functional consequences of such heterogeneities in the photosynthetic electron transport chain and propose a model in which quinones, accepting electrons either from PSII (linear flow) or NDH/PGR pathways (cyclic flow), represent a crucial control point. Our aim is to give an integrated description of these processes and discuss their potential roles in the balance between linear and cyclic electron flows.

Biochemical and Spectroscopic Characterization of Highly Stable Photosystem II Supercomplexes from Arabidopsis

Photosystem II (PSII) is a large membrane supercomplex involved in the first step of oxygenic photosynthesis. It is organized as a dimer, with each monomer consisting of more than 20 subunits as well as several cofactors, including chlorophyll and carotenoid pigments, lipids, and ions. The isolation of stable and homogeneous PSII supercomplexes from plants has been a hindrance for their deep structural and functional characterization. In recent years, purification of complexes with different antenna sizes was achieved with mild detergent solubilization of photosynthetic membranes and fractionation on a sucrose gradient, but these preparations were only stable in the cold for a few hours. In this work, we present an improved protocol to obtain plant PSII supercomplexes that are stable for several hours/days at a wide range of temperatures and can be concentrated without degradation. Biochemical and spectroscopic properties of the purified PSII are presented, as well as a study of the complex solubility in the presence of salts. We also tested the impact of a large panel of detergents on PSII stability and found that very few are able to maintain the integrity of PSII. Such new PSII preparation opens the possibility of performing experiments that require room temperature conditions and/or high protein concentrations, and thus it will allow more detailed investigations into the structure and molecular mechanisms that underlie plant PSII function.