Hydrogen production by Chlamydomonas reinhardtii: an elaborate interplay of electron sources and sinks

The unicellular green alga Chlamydomonas reinhardtii possesses a [FeFe]-hydrogenase HydA1 (EC 1.12.7.2), which is coupled to the photosynthetic electron transport chain. Large amounts of H-2 are produced in a light-dependent reaction for several days when C. reinhardtii cells are deprived of sulfur. Under these conditions, the cells drastically change their physiology from aerobic photosynthetic growth to an anaerobic resting state. The understanding of the underlying physiological processes is not only important for getting further insights into the adaptability of photosynthesis, but will help to optimize the biotechnological application of algae as H-2 producers. Two of the still most disputed questions regarding H-2 generation by C. reinhardtii concern the electron source for H-2 evolution and the competition of the hydrogenase with alternative electron sinks. We analyzed the H-2 metabolism of S-depleted C. reinhardtii cultures utilizing a special mass spectrometer setup and investigated the influence of photosystem II (PSII)- or ribulosebisphosphate-carboxylase/oxygenase (Rubisco)-deficiency. We show that electrons for H-2-production are provided both by PSII activity and by a non-photochemical plastoquinone reduction pathway, which is dependent on previous PSII activity. In a Rubisco-deficient strain, which produces H-2 also in the presence of sulfur, H-2 generation seems to be the only significant electron sink for PSII activity and rescues this strain at least partially from a light-sensitive phenotype. The latter indicates that the down-regulation of assimilatory pathways in S-deprived C. reinhardtii cells is one of the important prerequisites for a sustained H-2 evolution.