Influence of Fermentation Reactions on Continuous Hydrogen Photoproduction by Microalga Chlamydomonas reinhardtii under Sulfur Deficiency

The study investigated the effect of the major fermentation enzyme, pyruvate formate lyase, on hydrogen photoproduction by green microalga Chlamydomonas reinhardtii under mineral deprivation. Cultures of the wild type (CC-125) and mutant pfl1 without pyruvate formate lyase activity were incubated on a medium without sulfur in the sealed photoreactors under constant illumination for 96 h. During the experiment, accumulation of hydrogen in a gas phase of photoreactors and factors affecting hydrogen production, such as photosynthetic activity, respiration, and starch content, in cells were assessed. Both strains showed similar photosynthetic activity and starch content when cultivated in the complete medium. Under sulfur depletion, the cultures of the pfl1 mutant began to evolve hydrogen later than the wild type; however, this process lasted much longer in the mutant. The pfl1 cultures showed higher rates of starch accumulation and breakdown during the initial and late stages of sulfur deprivation, respectively. Moreover, the mutant was characterized by higher photosynthetic activity at the later stage of deprivation. The obtained results indicate close relationships between photosynthetic electron transport, hydrogenase reaction, carbohydrate metabolism and fermentation process, pointing to the prospect of using genetic engineering to modify fermentation reactions in order to improve photosynthetic hydrogen production in biotechnology.