Transcriptomic analysis of hydrogen photoproduction in Chlorella pyrenoidosa under nitrogen deprivation

Abstract H2 photoproduction by green algae is a very attractive way to produce clean and renewable energy. In our previous study, Chlorella pyrenoidosa strain IOAC707S was found to produce hydrogen under nitrogen deprivation in seawater medium; however the mechanism is not entirely clear. In this work, it was found that under nitrogen deprivation, the efficiency of photosystem II photochemical activity and the photosynthetic O2 evolution rate of C. pyrenoidosa strain IOAC707S decreased, while the respiratory rate increased, which were favorable for the establishment of anoxia and H2 photoproduction. Transcriptomic analysis indicated that photosynthetic activity did not simply decline but underwent a complex adjustment to prepare for H2 photoproduction. The activities of glycolysis and the pentose phosphate pathway were up-regulated, suggesting more electron transport from endogenous substrate to H2 photoproduction. The decrease of the tricarboxylic acid cycle activity resulted in decreased levels of CO2, which would limit Calvin cycle activity, and subsequently lead to the accumulation of NADPH and a reduction of the photosynthetic electron transport chain. Under these conditions, the induced hydrogenase served as an electron valve, and H2 was produced.