Relationship between the Photosystem II Regulation Mechanisms and Hydrogen Production in 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 rate of hydrogen production the start of illumination has the rate equal to the maximum rate of photosynthesis. However, this process is short-lived: oxygen produced during photosynthesis quickly inactivates the key enzyme of biophotolysis, hydrogenase, and inhibits its expression. To date, approaches have been developed to achieve sustained hydrogen production by microalgae. The most studied are those based on transferring microalgae to nutrient-deficient conditions. However, it is known that hydrogen production under nutrient deficiency is always accompanied by the decrease in activity of photosystem II (PSII). Several mechanisms for suppression of PSII activity 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 realization of a particular mechanism of PSII suppression depends not only on the type of stress but also on the growth conditions. For this purpose, the photoautotrophic culture of the microalga was grown under nitrogen or sulfur deficiency under different light regimes, and realization 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) with water-oxidizing complex, 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 the over-reduction of the plastoquinone pool. Other mechanisms are also realized under various conditions but do not show clear correlation with hydrogen production. The obtained results indicate that induction of stress through 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 full realization of their photosynthetic potential.