Increased photosystem II stability promotes H₂production in sulfur-deprivedChlamydomonas reinhardtii

Abstract

Photobiological H 2 production is an attractive option for renewable solar fuels. Sulfur-deprived cells of Chlamydomonas reinhardtii have been shown to produce hydrogen with the highest efficiency among photobiological systems. We have investigated the photosynthetic reactions during sulfur deprivation and H 2 production in the wild-type and state transition mutant 6 (Stm6) mutant of Chlamydomonas reinhardtii. The incubation period (130 h) was dissected into different phases, and changes in the amount and functional status of photosystem II (PSII) were investigated in vivo by electron paramagnetic resonance spectroscopy and variable fluorescence measurements. In the wild type it was found that the amount of PSII is decreased to 25% of the original level; the electron transport from PSII was completely blocked during the anaerobic phase preceding H 2 formation. This block was released during the H 2 production phase, indicating that the hydrogenase withdraws electrons from the plastoquinone pool. This partly removes the block in PSII electron transport, thereby permitting electron flow from water oxidation to hydrogenase. In the Stm6 mutant, which has higher respiration and H 2 evolution than the wild type, PSII was analogously but much less affected. The addition of the PSII inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea revealed that ∼80% of the H 2 production was inhibited in both strains. We conclude that (i) at least in the earlier stages, most of the electrons delivered to the hydrogenase originate from water oxidation by PSII, (ii) a faster onset of anaerobiosis preserves PSII from irreversible photoinhibition, and (iii) mutants with enhanced respiratory activity should be considered for better photobiological H 2 production. S olar fuels are an attractive concept for development of future renewable energy systems. Among other fuels, H 2 is considered to be one of the most effective and clean fuels (1-3). Solardriven H 2 production by photosynthetic microorganisms (photobio-H 2 production) is a viable alternative that complements the proposed chemical technologies. Green algae and cyanobacteria can, using water as an electron source via photosynthesis, produce H 2 with specific H 2 -evolving enzymes (hydrogenases) coupled to the photosynthetic machinery (4-6).Although green algae possess a very active hydrogenase enzyme compared with other organisms (the turnover rate of the algal FeFe hydrogenase is in thousands per second, 100-fold higher than that of other hydrogenases), direct light-to-H 2 conversion efficiency is very low (7,8). Thus, this is not a main metabolic process. Moreover, H 2 formation requires anaerobic conditions in the cell because the hydrogenase activity is sensitive to the presence of O 2 . The consequence is that oxygenic photosynthesis cannot easily be directly coupled to H 2 evolution in green algae.Melis and coworkers reported a two-stage process based on sulfur (S) deprivation in Chlamydomonas reinhardtii, which allowed the separation of the photosynthetic reactions from H 2 fo...

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“…Additionally, this faster transition from aerobic to anaerobic conditions in sulfur-depleted stm6 cultures was recently proposed to reduce the exposure time of PSII to reactive oxygen species formed in sulfur-deprived cells when the PSII repair cycle is impaired. In this study, a higher residual PSII activity was seen for stm6 within the anaerobic phase and suggested as an explanation for the higher hydrogen production capacity, since both wild type and stm6 maintained electron flow to the hydrogenase by water-splitting and linear electron transport (Volgusheva et al, 2013). However, several studies demonstrated the competition between cyclic electron flow and hydrogen production in C. reinhardtii (Tolleter et al, 2011;Steinbeck et al, 2015), and the inability of stm6 to switch from linear to cyclic electron flow under anaerobic conditions (Kruse et al, 2005) should largely contribute to its elevated hydrogen production capacity.…”

“…Higher rates of dark respiration in acetate-containing media have been observed before (Kruse et al, 2005) and were proposed to represent one of the main reasons for the rapid induction of photosynthetic hydrogen production in this mutant (Nguyen et al, 2011;Volgusheva et al, 2013). Considering the effect of a MOC1 inactivation on mitochondrial nd1 expression (Wobbe and Nixon, 2013;Fig.…”

“…5, E and F). Under anaerobic, hydrogen-producing conditions, stm6 prefers linear photosynthetic electron transport to the hydrogenase to prevent an accumulation of reducing equivalents in the chloroplast (Kruse et al, 2005;Volgusheva et al, 2013). A difference between these two mechanisms that relieve the plastidic redox pressure, caused by a diminished mitochondrial uptake of reducing equivalents, might be the regulation of involved components.…”

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“…As the efficiency of substrate supply is the major bottleneck in H 2 production with microalgae (Kruse and Hankamer, 2010), reduced availability of protons and electrons derived from the water-splitting reaction at PSII should result in a perturbation of hydrogen production activity. Recently, Volgusheva et al (2013) showed that the amount of active PSII during the hydrogen production phase is directly correlated to the amount of H 2 generation. This could mean that in the absence of LHCBM9 and the concomitant higher production of ROS, photoinhibitory damage of PSII is more pronounced, therefore leading to lower overall amounts of H 2 production.…”

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“…Tests are still limited to the lab, but according to some researchers, growing microalgae able to produce H2 has great promise for generating large-scale sustainable energy (e.g. [130][131][132][133][134]). However, many technical limitations exist, hindering the exploitation of this potential resource (e.g., algal physiology, metabolic issues) [130,131].…”