Photosystem II Function Research Articles

Storm-generated sediment deposition on rocky shores: Simulating burial effects on the physiology and morphology of Saccharina latissima sporophytes

Kelps inhabiting wave-exposed coasts are frequently exposed to disturbance such as storm-generated sediment deposition on blades which can be resuspended and removed by local hydrodynamic processes. However, in extreme cases, whole thalli can be buried under sediment for a period of time. The objectives of the present study are to determine the proximate effects of long-term sediment burial on kelp physiology and to confirm whether transient sediment load have functional significance in mitigating the negative effects of saturating light intensity normally encountered in the field. We simulated sediment burial to evaluate its impact on the vitality, in terms of photosystem II (PSII) function, pigments and morphology, of Saccharina latissima on different time scales. The effect was compared to the negative controls, kelps without sediment cover exposed to the whole radiation treatment. Results of our study showed that short-term burial under different sediment types (gravel, sand, and silt and clay) has no negative effect on the physiology and morphology of S. latissima, whereas sediment-free algal discs exposed to high PAR and UVR were bleached and photoinhibited. Conversely, long-term burial under silt and clay showed adverse smothering effect leading to bleaching, loss of PSII function and tissue decay. We observed that burial under gravel and sand (up to a period of 7 days), e.g. after storm slowdown and settlement of sediment particles, showed some protective function in mitigating the negative effect of photoinhibiting high light intensities naturally encountered by most intertidal and upper subtidal macroalgae.

Photoinhibition: molecular mechanisms and physiological significance

Photoinhibition: molecular mechanisms and physiological significance

Psb30 is a photosystem II reaction center subunit and is required for optimal growth in high light in Chlamydomonas reinhardtii

The psb30 ( ycf12) gene is conserved in a wide variety of oxygenic-photosynthetic organisms except angiosperms and some marine cyanobacteria. Psb30 protein is found in cyanobacterial photosystem II (PSII) core complexes and is dispensable for PSII structure and function. The most recent three-dimensional structure of cyanobacterial PSII core complex has revealed that Psb30 is located in proximity of PsbJ, PsbK, and PsbZ. However Psb30 has not yet been detected in PSII complexes from eukaryotic photosynthetic organisms. Here we found the expression of the chloroplast psb30 gene in the green alga Chlamydomonas reinhardtii by immunoblotting and Psb30 is exclusively co-purifies with PSII core complex and is significantly reduced in PSII-deficient mutants. Partial disintegration of PSII core complex and subsequent fractionation of the resulting subcomplexes revealed that Psb30 is exclusively associated with PSII reaction center. We have generated chloroplast transformants in which the psb30 gene is disrupted and the resulting ΔPsb30 cells showed decreased oxygen evolution activity by 15%, grew photosynthetically under moderate light, and displayed increased sensitivity to high light relative to wild type. We conclude that Psb30 is a PSII reaction center subunit and is required for optimal PSII function under high light environments.

Physiological basis for high resistance to photoinhibition under nitrogen depletion in Emiliania huxleyi

We quantified and compared the photophysiological characteristics of Emiliania huxleyi with two marine diatoms Thalassiosira pseudonana and Coscinodiscus sp. under nitrogen‐ and phosphorus‐replete and ‐depleted conditions. Under nitrogen‐depleted conditions, E. huxleyi maintained significant photosystem II (PSII) function over at least 38 d, whereas for both diatoms, PSII function declined to marginal levels within 8‐10 d of nitrogen depletion. In contrast, under phosphorus‐depleted conditions, PSII function in E. huxleyi declined sharply within 7 d and in T. pseudonana within 3 d. Under nutrient‐replete conditions, E. huxleyi had one of the highest PSII repair rate constants among phytoplankton examined to date. Under nitrogen depletion, E. huxleyi exhibited a decrease in susceptibility to photoinactivation of PSII, while the diatoms showed no significant change in susceptibility. The superior ability of E. huxleyi relative to the diatoms to limit PSII photoinactivation as well as to maintain PSII repair for many days of nitrogen depletion could help to explain why E. huxleyi is able to form blooms under the low‐nitrate, high‐light conditions that frequently occur in stratified surface waters.

Heat stress results in loss of chloroplast Cu/Zn superoxide dismutase and increased damage to Photosystem II in combined drought-heat stressed Lotus japonicus

Drought and heat stress have been studied extensively in plants, but most reports involve analysis of response to only one of these stresses. Studies in which both stresses were studied in combination have less commonly been reported. We report the combined effect of drought and heat stress on Photosystem II (PSII) of Lotus japonicus cv. Gifu plants. Photochemistry of PSII was not affected by drought or heat stress alone, but the two stresses together decreased PSII activity as determined by fluorescence emission. Heat stress alone resulted in degradation of D1 and CP47 proteins, and D2 protein was also degraded by combined drought-heat stress. None of these proteins were degraded by drought stress alone. Drought alone induced accumulation of hydrogen peroxide but the drought-heat combination led to an increase in superoxide levels and a decrease in hydrogen peroxide levels. Furthermore, combined drought-heat stress was correlated with an increase in oxidative damage as determined by increased levels of thiobarbituric acid reactive substances. Heat also induced degradation of chloroplast Cu/Zn superoxide dismutase (SOD: EC 1.15.1.1) as shown by reduced protein levels and isozyme-specific SOD activity. Loss of Cu/Zn SOD and induction of catalase (CAT: EC 1.11.1.6) activity would explain the altered balance between hydrogen peroxide and superoxide in response to drought vs combined drought-heat stress. Degradation of PSII could thus be caused by the loss of components of chloroplast antioxidant defence systems and subsequent decreased function of PSII. A possible explanation for energy dissipation by L. japonicus under stress conditions is discussed.

LPA19, a Psb27 Homolog in Arabidopsis thaliana, Facilitates D1 Protein Precursor Processing during PSII Biogenesis

The biogenesis and assembly of photosystem II (PSII) are mainly regulated by the nuclear-encoded factors. To further identify the novel components involved in PSII biogenesis, we isolated and characterized a high chlorophyll fluorescence low psii accumulation19 (lpa19) mutant, which is defective in PSII biogenesis. LPA19 encodes a Psb27 homolog (At1g05385). Interestingly, another Psb27 homolog (At1g03600) in Arabidopsis was revealed to be required for the efficient repair of photodamaged PSII. These results suggest that the Psb27 homologs play distinct functions in PSII biogenesis and repair in Arabidopsis. Chloroplast protein labeling assays showed that the C-terminal processing of D1 in the lpa19 mutant was impaired. Protein overlay assays provided evidence that LPA19 interacts with D1, and coimmunoprecipitation analysis demonstrated that LPA19 interacts with mature D1 (mD1) and precursor D1 (pD1). Moreover, LPA19 protein was shown to specifically interact with the soluble C terminus present in the precursor and mature D1 through yeast two-hybrid analyses. Thus, these studies suggest that LPA19 is involved in facilitating the D1 precursor protein processing in Arabidopsis.

Characterization of photosystem II photochemistry in transgenic tobacco plants with lowered Rubisco activase content

Rubisco activase plays an important role in the regulation of CO 2 assimilation. However, it is unknown how activase regulates photosystem II (PSII) photochemistry. To investigate the effects of Rubisco activase on PSII photochemistry, we obtained transgenic tobacco ( Nicotiana tabacum) plants with 50% (i7), 25% (i28), and 5% (i46) activase levels as compared to wild type plants by using a gene encoding tobacco activase for the RNAi construct. Both CO 2 assimilation and PSII activity were significantly reduced only in transgenic i28 and i46 plants, suggesting that activase deficiency led to decreased PSII activity. Flash-induced fluorescence kinetics indicated that activase deficiency resulted in a slow electron transfer between Q A (primary quinine electron acceptor of PSII) and Q B (secondary quinone electron acceptor of PSII). Thermoluminescence measurements revealed that activase deficiency induced a shift of S 2Q A − and S 2Q B − recombinations to higher temperatures in parallel, and a decrease in the intensities of the thermoluminescence emissions. Activase deficiency also dampened the period-four oscillation of the thermoluminescence B-band. Protein gel blot analysis showed that activase deficiency resulted in a significant decrease in the content of D1, D2, CP43, CP47, and PsbO proteins. Transmission electron microscopy analysis demonstrated that activase deficiency induced a significant decrease in the number of grana stacks per chloroplast and discs per grana stack. Our results suggest that activase plays an important role in maintaining PSII function and chloroplast development.

Plant experiments with light-emitting diode module in Svet space greenhouse

Light is necessary for photosynthesis and shoot orientation in the space plant growth facilities. Light modules (LM) must provide sufficient photosynthetic photon flux for optimal efficiency of photosynthetic processes and also meet the constraints for power, volume and mass. A new LM for Svet space greenhouse using Cree® XLamp® 7090 XR light-emitting diodes (LEDs) was developed. Monochromic LEDs emitting in the red, green, and blue regions of the spectrum were used. The LED-LM contains 36 LED spots – 30 LED spots with one red, green and blue LED and 6 LED spots with three red LEDs. Digital Multiplex Control Unit controls the LED spots and can set 231 levels of light intensity thus achieving Photosynthetic Photon Flux Density (PPFD) in the range 0–400 μmol m −2 s −1 and different percentages of the red, green and blue light, depending on the experimental objectives. Two one-month experiments with plants – lettuce and radicchio were carried out at 400 μmol m −2 s −1 PPFD (high light – HL) and 220 μmol m −2 s −1 PPFD (low light – LL) and 70% red, 20% green and 10% blue light composition. To evaluate the efficiency of photosynthesis, in vivo modulated chlorophyll fluorescence was measured by Pulse Amplitude Modulation (PAM) fluorometer on leaf discs and the following parameters: effective quantum yield of Photosystem II ( Φ PSII) and non-photochemical quenching (NPQ) were calculated. Both lettuce and radicchio plants grown at LL express higher photochemical activity of Photosystem II (PSII) than HL grown plants, evaluated by Φ PSII. Accelerated rise in NPQ in both LL grown plants was observed, while steady state NPQ values were higher in LL grown lettuce plants and did not differ in LL and HL grown radicchio plants. The extent of photoinhibition process in both plants was evaluated by changes in malonedialdehyde (MDA) concentration, peroxidase (POX) activity and hydrogen peroxide (H 2O 2) content. Accumulation of high levels of MDA and increased POX activity correlating with decreased H 2O 2 content were observed in both HL grown plants. These biochemical indicators revealed higher sensitivity to photodamage in HL grown lettuce and radicchio plants. LL conditions resulted in more effective functioning of PSII than HL when lettuce and radicchio plants were grown at 70% red, 20% green and 10% blue light composition.

Recent Progress in the Crystallographic Studies of Photosystem II

The photosynthetic oxygen-evolving photosystem II (PSII) is the only known biochemical system that is able to oxidize water molecules and thereby generates almost all oxygen in the Earth's atmosphere. The elucidation of the structural and mechanistic aspects of PSII keeps scientists all over the world engaged since several decades. In this Minireview, we outline the progress in understanding PSII based on the most recent crystal structure at 2.9 A resolution. A likely position of the chloride ion, which is known to be required for the fast turnover of water oxidation, could be determined in native PSII and is compared with work on bromide and iodide substituted PSII. Moreover, eleven new integral lipids could be assigned, emphasizing the importance of lipids for the perfect function of PSII. A third plastoquinone molecule (Q(C)) and a second quinone transfer channel are revealed, making it possible to consider different mechanisms for the exchange of plastoquinone/plastoquinol molecules. In addition, possible transport channels for water, dioxygen and protons are identified.

Spectroscopic and Functional Characterizations of Cyanobacterium Synechocystis PCC 6803 Mutants on and near the Heme Axial Ligand of Cytochrome b559 in Photosystem II

The functional role of cytochrome (cyt) b(559) in photosystem II (PSII) was investigated in H22K alpha and Y18S alpha cyt b(559) mutants of the cyanobacterium Synechocystis sp. PCC6803. H22K alpha and Y18S alpha cyt b(559) mutant carries one amino acid substitution on and near one of heme axial ligands of cyt b(559) in PSII, respectively. Both mutants grew photoautotrophically, assembled stable PSII, and exhibited the normal period-four oscillation in oxygen yield. However, both mutants showed several distinct chlorophyll a fluorescence properties and were more susceptible to photoinhibition than wild type. EPR results indicated the displacement of one of the two axial ligands to the heme of cyt b(559) in H22K alpha mutant reaction centers, at least in isolated reaction centers. The maximum absorption of cyt b(559) in Y18S alpha mutant PSII core complexes was shifted to 561 nm. Y18S alpha and H22K alpha mutant PSII core complexes contained predominately the low potential form of cyt b(559). The findings lend support to the concept that the redox properties of cyt b(559) are strongly influenced by the hydrophobicity and ligation environment of the heme. When the cyt b(559) mutations placed in a D1-D170A genetic background that prevents assembly of the manganese cluster, accumulation of PSII is almost completely abolished. Overall, our data support a functional role of cyt b(559) in protection of PSII under photoinhibition conditions in vivo.

Physico-chemical Property of Rare Earths—Effects on the Energy Regulation of Photosystem II in Arabidopsis thaliana

Photosystem II (PSII) from Arabidopsis thaliana treated by lanthanum (La(3+)), cerium (Ce(3+)), and neodymium (Nd(3+)) were isolated to investigate the effects of 4f electron characteristics and alternation valence of rare earth elements (REEs) on PSII function regulation comparatively. Results showed that REE treatment could induce the generous expression of LhcII b in A. thaliana and increase the content of light-harvesting complex II and its trimer on the thylakoid membrane significantly. Meanwhile, the light absorption in the red and blue region and fluorescence quantum yield near 683 nm were obviously increased; oxygen evolution rate was greatly improved too, suggesting that REEs could enhance the efficiency of light absorption, regulate excitation energy distribution from photosystem I (PSI) to PSII, and thus increase the activity of photochemical reaction and oxygen evolution accordingly. The efficiency order of the four treatments was Ce(3+) > Nd(3+) > La(3+) > control.

Charge Separation, Stabilization, and Protein Relaxation in Photosystem II Core Particles with Closed Reaction Center

The fluorescence kinetics of cyanobacterial photosystem II (PSII) core particles with closed reaction centers (RCs) were studied with picosecond resolution. The data are modeled in terms of electron transfer (ET) and associated protein conformational relaxation processes, resolving four different radical pair (RP) states. The target analyses reveal the importance of protein relaxation steps in the ET chain for the functioning of PSII. We also tested previously published data on cyanobacterial PSII with open RCs using models that involved protein relaxation steps as suggested by our data on closed RCs. The rationale for this reanalysis is that at least one short-lived component could not be described in the previous simpler models. This new analysis supports the involvement of a protein relaxation step for open RCs as well. In this model the rate of ET from reduced pheophytin to the primary quinone QA is determined to be 4.1 ns−1. The rate of initial charge separation is slowed down substantially from ∼170 ns−1 in PSII with open RCs to 56 ns−1 upon reduction of QA. However, the free-energy drop of the first RP is not changed substantially between the two RC redox states. The currently assumed mechanistic model, assuming the same early RP intermediates in both states of RC, is inconsistent with the presented energetics of the RPs. Additionally, a comparison between PSII with closed RCs in isolated cores and in intact cells reveals slightly different relaxation kinetics, with a ∼3.7 ns component present only in isolated cores.

Inorganic Cofactor Stabilization and Retention: The Unique Functions of the Two PsbO Subunits of Eukaryotic Photosystem II

Eukaryotic PsbO, the photosystem II (PSII) manganese-stabilizing protein, has two N-terminal sequences that are required for binding of two copies of the protein to PSII [Popelkova, H., et al. (2002) Biochemistry 41, 10038-10045; Popelkova, H., et al. (2003) Biochemistry 42, 6193-6200]. In the work reported here, a set of selected N-terminal truncation mutants of PsbO that affect subunit binding to PSII were used to determine the effects of PsbO stoichiometry on the Mn, Ca(2+), and Cl(-) cofactors and to characterize the roles of each of the PsbO subunits in PSII function. Results of the experiments with the PsbO-depleted PSII membranes reconstituted with the PsbO deletion mutants showed that the presence of PsbO does not affect Ca(2+) retention by PSII in steady-state assays of activity, nor is it required for Ca(2+) to protect the Mn cluster against reductive inhibition in darkness. In contrast to the results with Ca(2+), PsbO increases the affinity of Cl(-) for the active site of the O(2)-evolving complex (OEC) as expected. These results together with other data on activity retention suggest that PsbO can stabilize the Mn cluster by facilitating retention of Cl(-) in the OEC. The data presented here indicate that each of two copies of PsbO has a distinctive function in PSII. Binding of the first PsbO subunit fully stabilizes the Mn cluster and enhances Cl(-) retention, while binding of the second subunit optimizes Cl(-) retention, which in turn maximizes O(2) evolution activity. Nonspecific binding of some PsbO truncation mutants to PSII has no functional significance.

Dynamic flexibility in the structure and function of photosystem II in higher plant thylakoid membranes: the grana enigma

Grana are not essential for photosynthesis, yet they are ubiquitous in higher plants and in the recently evolved Charaphyta algae; hence grana role and its need is still an intriguing enigma. This article discusses how the grana provide integrated and multifaceted functional advantages, by facilitating mechanisms that fine-tune the dynamics of the photosynthetic apparatus, with particular implications for photosystem II (PSII). This dynamic flexibility of photosynthetic membranes is advantageous in plants responding to ever-changing environmental conditions, from darkness or limiting light to saturating light and sustained or intermittent high light. The thylakoid dynamics are brought about by structural and organizational changes at the level of the overall height and number of granal stacks per chloroplast, molecular dynamics within the membrane itself, the partition gap between appressed membranes within stacks, the aqueous lumen encased by the continuous thylakoid membrane network, and even the stroma bathing the thylakoids. The structural and organizational changes of grana stacks in turn are driven by physicochemical forces, including entropy, at work in the chloroplast. In response to light, attractive van der Waals interactions and screening of electrostatic repulsion between appressed grana thylakoids across the partition gap and most probably direct protein interactions across the granal lumen (PSII extrinsic proteins OEEp-OEEp, particularly PsbQ-PsbQ) contribute to the integrity of grana stacks. We propose that both the light-induced contraction of the partition gap and the granal lumen elicit maximisation of entropy in the chloroplast stroma, thereby enhancing carbon fixation and chloroplast protein synthesizing capacity. This spatiotemporal dynamic flexibility in the structure and function of active and inactive PSIIs within grana stacks in higher plant chloroplasts is vital for the optimization of photosynthesis under a wide range of environmental and developmental conditions.

High Sensitivity Proteomics Assisted Discovery of a Novel Operon Involved in the Assembly of Photosystem II, a Membrane Protein Complex

Photosystem II (PSII) is a large membrane protein complex that performs the water oxidation reactions of photosynthesis in cyanobacteria, algae, and plants. The unusual redox reactions in PSII often lead to damage, degradation, and reassembly of this molecular machine. To identify novel assembly factors, high sensitivity proteomic analysis of PSII purified from the cyanobacterium Synechocystis sp. PCC 6803 was performed. This analysis identified six PSII-associated proteins that are encoded by an operon containing nine genes, slr0144 to slr0152. This operon encodes proteins that are not essential components of the PSII holocomplex but accumulate to high levels in pre-complexes lacking any of the lumenal proteins PsbP, PsbQ, or PsbV. The operon contains genes with putative binding domains for chlorophylls and bilins, suggesting these proteins may function as a reservoir for cofactors needed during the PSII lifecycle. Genetic deletion of this operon shows that removal of these protein products does not alter photoautotrophic growth or PSII fluorescence properties. However, the deletion does result in decreased PSII-mediated oxygen evolution and an altered distribution of the S states of the catalytic manganese cluster. These data demonstrate that the proteins encoded by the genes in this operon are necessary for optimal function of PSII and function as accessory proteins during assembly of the PSII complex. Thus, we have named the products of the slr0144-slr0152 operon Pap (Photosystem II assembly proteins).

Designing photosystem II: molecular engineering of photo-catalytic proteins

Biological photosynthesis utilizes membrane-bound pigment/protein complexes to convert light into chemical energy through a series of electron-transfer events. In the unique photosystem II (PSII) complex these electron-transfer events result in the oxidation of water to molecular oxygen. PSII is an extremely complex enzyme and in order to exploit its unique ability to convert sunlight into chemical energy it will be necessary to make a minimal model. Here we will briefly describe how PSII functions and identify those aspects that are essential in order to catalyze the oxidation of water into O(2), and review previous attempts to design simple photo-catalytic proteins and summarize our current research exploiting the E. coli bacterioferritin protein as a scaffold into which multiple cofactors can be bound, to oxidize a manganese metal center upon illumination. Through the reverse engineering of PSII and light driven water splitting reactions it may be possible to provide a blueprint for catalysts that can produce clean green fuel for human energy needs.

Effects of phospholipase and lipase treatments on photosystem II core dimer from a thermophilic cyanobacterium

Lipids are important components of transmembrane protein complexes. In order to study the roles of lipids in photosystem II (PSII), we treated the PSII core dimer complex from a thermophilic cyanobacterium Thermosynechococcus vulcanus with phospholipase A(2) (PLA(2)) and lipase, and examined their effects on PSII structure and function. PLA(2)-treatment decreased the content of phospholipid, phosphatidylglycerol (PG) by 59%, leading to a decrease of oxygen evolution by 40%. On the other hand, although treatment with lipase specifically decreased the content of monogalactosyldiacylglycerol (MGDG) by 52%, it decreased oxygen evolution only by 16%. This indicates that PG plays a more important role in PSII than MGDG. Both PLA(2)- and lipase-treatments induced neither the dissociation of PSII dimer, nor any loss of polypeptides. The degradation of PG resulted in a damage to the Q(B)-binding site as demonstrated from photoreduction activity of 2,6-dichlorophenolindophenol and chlorophyll fluorescence yields in the absence or presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, and the dependencies of oxygen evolution on various electron acceptors before and after PLA(2)- or lipase-treatments. However, there were approximately three and five molecules of PG and MGDG per PSII reaction center left in the PSII dimeric complex after the PLA(2)- and lipase-treatments. These lipids are therefore bound to the interior of the protein matrix and resistant to the lipase treatments. The resistance of these lipids against PLA(2)- and lipase-treatments may be a specific feature of PSII from the thermophilic cyanobacterium, suggesting a possible correlation between binding of lipids and thermostability of PSII.

The Psb27 Protein Facilitates Manganese Cluster Assembly in Photosystem II

Photosystem II (PSII) is a large membrane protein complex that uses light energy to convert water to molecular oxygen. This enzyme undergoes an intricate assembly process to ensure accurate and efficient positioning of its many components. It has been proposed that the Psb27 protein, a lumenal extrinsic subunit, serves as a PSII assembly factor. Using a psb27 genetic deletion strain (Deltapsb27) of the cyanobacterium Synechocystis sp. PCC 6803, we have defined the role of the Psb27 protein in PSII biogenesis. While the Psb27 protein was not essential for photosynthetic activity, various PSII assembly assays revealed that the Deltapsb27 mutant was defective in integration of the Mn(4)Ca(1)Cl(x) cluster, the catalytic core of the oxygen-evolving machinery within the PSII complex. The other lumenal extrinsic proteins (PsbO, PsbU, PsbV, and PsbQ) are key components of the fully assembled PSII complex and are important for the water oxidation reaction, but we propose that the Psb27 protein has a distinct function separate from these subunits. We show that the Psb27 protein facilitates Mn(4)Ca(1)Cl(x) cluster assembly in PSII at least in part by preventing the premature association of the other extrinsic proteins. Thus, we propose an exchange of lumenal subunits and cofactors during PSII assembly, in that the Psb27 protein is replaced by the other extrinsic proteins upon assembly of the Mn(4)Ca(1)Cl(x) cluster. Furthermore, we show that the Psb27 protein provides a selective advantage for cyanobacterial cells under conditions such as nutrient deprivation where Mn(4)Ca(1)Cl(x) cluster assembly efficiency is critical for survival.

Dynamics of the xanthophyll cycle and non-radiative dissipation of absorbed light energy during exposure of Norway spruce to high irradiance

The response of Norway spruce saplings (Picea abies [L.] Karst.) was monitored continuously during short-term exposure (10 days) to high irradiance (HI; 1000micromol m(-2)s(-1)). Compared with plants acclimated to low irradiance (100micromol m(-2)s(-1)), plants after HI exposure were characterized by a significantly reduced CO(2) assimilation rate throughout the light response curve. Pigment contents varied only slightly during HI exposure, but a rapid and strong response was observed in xanthophyll cycle activity, particularly within the first 3 days of the HI treatment. Both violaxanthin convertibility under HI and the amount of zeaxanthin pool sustained in darkness increased markedly under HI conditions. These changes were accompanied by an enhanced non-radiative dissipation of absorbed light energy (NRD) and the acceleration of induction of both NRD and de-epoxidation of the xanthophyll cycle pigments. We found a strong negative linear correlation between the amount of sustained de-epoxidized xanthophylls and the photosystem II (PSII) photochemical efficiency (F(V)/F(M)), indicating photoprotective down-regulation of the PSII function. Recovery of F(V)/F(M) at the end of the HI treatment revealed that Norway spruce was able to cope with a 10-fold elevated irradiance due particularly to an efficient NRD within the PSII antenna that was associated with enhanced violaxanthin convertibility and a light-induced accumulation of zeaxanthin that persisted in darkness.

Photoproduction of hydrogen by sulfur-deprived C. reinhardtii mutants with impaired Photosystem II photochemical activity

Photoproduction of H2 was examined in a series of sulfur-deprived Chlamydomonas reinhardtii D1-R323 mutants with progressively impaired PSII photochemical activity. In the R323H, R323D, and R323E D1 mutants, replacement of arginine affects photosystem II (PSII) function, as demonstrated by progressive decreases in O2-evolving activity and loss of PSII photochemical activity. Significant changes in PSII activity were found when the arginine residue was replaced by negatively charged amino acid residues (R323D and R323E). However, the R323H (positively charged or neutral, depending on the ambient pH) mutant had minimal changes in PSII activity. The R323H, R323D, and R323E mutants and the pseudo-wild-type (pWt) with restored PSII function were used to study the effects of sulfur deprivation on H2-production activity. All of these mutants exhibited significant changes in the normal parameters associated with the H2-photoproduction process, such as a shorter aerobic phase, lower accumulation of starch, a prolonged anaerobic phase observed before the onset of H2-production, a shorter duration of H2-production, lower H2 yields compared to the pWt control, and slightly higher production of dark fermentation products such as acetate and formate. The more compromised the PSII photochemical activity, the more dramatic was the effect of sulfur deprivation on the H2-production process, which depends both on the presence of residual PSII activity and the amount of stored starch.