Recent studies indicate that [Fe]-hydrogenases and H-2 metabolism are widely distributed among green algae. The enzymes are simple structured and catalyze H-2 evolution with similar rates than the more complex [Fe]-hydrogenases from bacteria. Different green algal species developed diverse strategies to survive under sulfur deprivation. Chlamydomonas reinhardtii evolves large quantities of hydrogen gas in the absence of sulfur. In a scaled culture of C reinhardtii, the photosynthetic O-2 evolution rate drops below the rate of respiratory O-2 consumption due to a reversible inhibition of photosystem II, thus leading to an intracellular anaerobiosis. The algal cells survive under these anaerobic conditions by switching their metabolism to a kind of photo-fermentation. Although possessing a functional [Fe]-hydrogenase gene, the cells of Scenedesmus obliquus produce no significant amounts of H-2 under S-depleted conditions. Biochemical analyses indicate that S. obliquus decreases almost the complete metabolic activities while maintaining a low level of respiratory activity. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.