Hydrogen photo-evolution upon S deprivation stepwise: An illustration of microalgal photosynthetic and metabolic flexibility and a step stone for future biotechnological methods of renewable H2 production

The metabolic flexibility of some photosynthetic microalgae enables them to survive periods of anaerobiosis in the light by developing a particular photofermentative metabolism. The latter entails compounds of the photosynthetic electron transfer chain and an oxygen-sensitive hydrogenase in order to reoxidise reducing equivalents and to generate ATP for maintaining basal metabolic function. This pathway results in the photo-evolution of hydrogen gas by the algae. A decade ago Melis and coworkers managed to reproduce such a condition in a laboratory context by depletion of sulfur in the algal culture media, making the photo-evolution by the algae sustainable for several days (Melis et al. 2000). This observation boosted research in algal H2 evolution. A feature, which due to its transient nature was long time considered as a curiosity of algal photosynthesis suddenly became a phenomenon with biotechnological potential. Although the Melis procedure has not been developed into a biotechnological process of renewable H2 generation so far, it has been a useful tool for studying microalgal metabolic and photosynthetic flexibility and a possible step stone for future H2 production procedures. Ten years later most of the critical steps and limitations of H2 production by this protocol have been studied from different angles particularly with the model organism C. reinhardtii, by introducing various changes in culture conditions and making use of mutants issued from different screens or by reverse genomic approaches. A synthesis of these observations with the most important conclusions driven from recent studies will be presented in this review.