The relationship between photosystem II regulation and hydrogen production in Chlamydomonas reinhardtii under nitrogen or sulfur deprivation

Some microalgae are capable of light-dependent hydrogen production after a period of anaerobic adaptation, thus performing biophotolysis of water. The hydrogen production rate at the initial moment reaches the maximum rate of photosynthesis. However, this process is short-lived: the oxygen released during photosynthesis quickly inactivates the key enzyme of biophotolysis, hydrogenase, and inhibits its expression. Approaches have been developed to achieve sustained hydrogen production by microalgae. The most studied are approaches based on transferring microalgae to nutrient-deficient conditions. However, it is known that hydrogen production under nutrient deficiency is always accompanied by a decrease in the activity of photosystem II (PSII). Several mechanisms of PSII activity suppression have been described in the literature, and there is no consensus on which mechanism is the determining one. The aim of this work was to test the hypothesis that the implementation of a particular mechanism of PSII suppression depends not only on the type of stress but also on the growth conditions. For this purpose, a photoautotrophic culture of the microalga Chlamydomonas reinhardtii was grown under nitrogen or sulfur deficiency under different light regimes, and the implementation of the following mechanisms of PSII activity suppression was analyzed: over-reduction of the plastoquinone pool (coupled with over-reduction of the entire photosynthetic electron transport chain), decoupling of PSII (based on the kinetics of ascorbate accumulation and the JIP test), the violaxanthin cycle, anaerobic stress associated with the creation of a reducing redox potential of the culture suspension. It was found that the key mechanism determining hydrogen production is over-reduction of the plastoquinone pool. Other mechanisms are also implemented under various conditions but do not show a clear correlation with hydrogen production. The results obtained indicate that stress caused by starvation of cultures is a convenient approach for studying hydrogen production by microalgae, but due to the low activity of PSII, it is impractical. New approaches are required to create industrial systems based on microalgae, allowing the full realization of the photosynthetic potential of microalgae.