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Green hydrogen, produced during microalgal photosynthesis, is regarded as one of the most promising sustainable energy sources. It utilizes sunlight and water, which are essentially unlimited, and its combustion results in only water as a waste product. In microalgal hydrogen energy production systems, the sensitivity of hydrogenase to O2 poses a significant challenge, limiting sustained photosynthetic H2 production in microalgae. Additionally, efficient photosynthetic H2 production in anaerobic microalgal cells is hindered by impaired electron source (photosystem II) and electron loss through the Calvin‐Benson cycle, cyclic electron transfer around photosystem I, and O2 photoreduction, which are identified as the other key challenges. Over the past eight decades, considerable progress has been made in addressing these challenges and regulating electron transfer to achieve sustainable and efficient photosynthetic H2 production in microalgae. In this review, we discuss a range of regulatory methods for achieving sustainable and efficient photosynthetic H2 production in microalgae. Emphasizing the significant progress made over the past eight decades, we also address current challenges and propose potential future solutions. [ABSTRACT FROM AUTHOR]

Abstract: In cyanobacteria, treatment with low concentrations of NaHSO3 can enhance photosynthetic efficiency, whereas NaHSO3 in high amounts often inhibits cell growth and photosynthesis may even cause death. In the present study, our results showed that treatment with moderate concentrations of NaHSO3 considerably improved the yield of photobiological H2 production in the filamentous N2-fixing cyanobacterium Anabaena sp. strain PCC 7120. Under steady state conditions, the accumulated H2 levels in cells treated with 1 mM NaHSO3 were approximately 10 times higher than that in untreated cells. Such improvement occurred in heterocysts and was most likely caused by increases in the expression and activity of nitrogenase. The effects of treatment with low, moderate, and high concentrations of NaHSO3 in cyanobacteria were proposed on the basis of the results obtained in the present study and from previous knowledge. [ABSTRACT FROM AUTHOR]

Abstract: Treatment with NaHSO3 induces a 10-fold increase in H2 photoproduction in the filamentous N2-fixing cyanobacterium Anabaena sp. strain PCC 7120. However, it is unclear whether this treatment also increases H2 photoproduction in green alga. In this study, treatment with 13mM NaHSO3 resulted in about a 200-fold increase in H2 production in Chlamydomonas reinhardtii, and this increase was most probably the result of reduced O2 content and enhanced hydrogenase activity. Compared to the conventional strategy of sulfur deprivation, NaHSO3 treatment results in a higher maximum rate of H2 photoproduction, greater efficiency of conversion of light energy into H2, shorter half-time to produce the maximum accumulated H2 levels, and reduced costs because no centrifugation is involved. We therefore conclude that NaHSO3 treatment is an efficient, rapid, and economic strategy for improving photobiological H2 production in the green alga C. reinhardtii. [Copyright &y& Elsevier]

NdhO, a regulatory oxygenic photosynthesis-specific subunit, is close to the ferredoxin-binding site of cyanobacterial NDH-1, and its levels are negatively associated with the rates of cyclic electron transfer around PSI mediated by NDH-1 (NDH-CET). However, the effect of NdhO levels on cyanobacterial cell death triggered by high temperature remains elusive. Here, our results uncovered a synergistic effect of NdhO levels on the cell death and reactive oxygen species (ROS) accumulation when cyanobacterial cells grown at 30°C for 1 day were transferred to 45°C for 2 days. Such synergistic effect was found to be closely associated with the activities of NDH-CET and CO2 assimilation during high temperature. Collectively, we propose that the effect of NdhO levels on the cyanobacterial cell bleaching and cell death triggered by high temperature is a result of influencing production of ROS by NDH-CET, which is considered to be vital to balance the ATP/NADPH ratio and improve the Calvin-Benson cycle.

NaHSO 3 addition greatly increases the yield of H 2 photoproduction in a unicellular green alga Chlamydomonas reinhardtii through removing O 2 and activating hydrogenase but significantly impairs the activity of PSII, an electron source for H 2 photoproduction. Here, a stepwise addition mode of total 13 mM NaHSO 3 , an optimal concentration for H 2 photoproduction of C. reinhardtii identified in a previous one step addition method, significantly improved H 2 photoproduction. Such improvement was believed to be the result of increased residual PSII activity in an anaerobic background, but was at least independent of two alternative electron sinks for H 2 photoproduction, cyclic electron transport around PSI and CO 2 assimilation. Based on the above results, we propose that increased residual PSII activity in an anaerobic environment is an efficient strategy to enhance H 2 photoproduction in C. reinhardtii , and the stepwise NaHSO 3 addition mode is a case study in the strategy.