Photosynthetic H2 metabolism in Chlamydomonas reinhardtii (unicellular green algae)

Unicellular green algae have the ability to operate in two distinctly different environments (aerobic and anaerobic), and to photosynthetically generate molecular hydrogen (H-2). A recently developed metabolic protocol in the green alga Chlamydomonas reinhardtii permitted separation of photosynthetic O-2-evolution and carbon accumulation from anaerobic consumption of cellular metabolites and concomitant photosynthetic H-2-evolution. The H-2 evolution process was induced upon sulfate nutrient deprivation of the cells, which reversibly inhibits photosystem-II and O-2-evolution in their chloroplast. In the absence of O-2, and in order to generate ATP, green algae resorted to anaerobic photosynthetic metabolism, evolved H-2 in the light and consumed endogenous substrate. This study summarizes recent advances on green algal hydrogen metabolism and discusses avenues of research for the further development of this method. Included is the mechanism of a substantial tenfold starch accumulation in the cells, observed promptly upon S-deprivation, and the regulated starch and protein catabolism during the subsequent H-2-evolution. Also discussed is the function of a chloroplast envelope-localized sulfate permease, and the photosynthesis-respiration relationship in green algae as potential tools by which to stabilize and enhance H-2 metabolism. In addition to potential practical applications of H-2, approaches discussed in this work are beginning to address the biochemistry of anaerobic H-2 photoproduction, its genes, proteins, regulation, and communication with other metabolic pathways in microalgae. Photosynthetic H-2 production by green algae may hold the promise of generating a renewable fuel from nature's most plentiful resources, sunlight and water. The process potentially concerns global warming and the question of energy supply and demand.