Recent advances in hydrogen production by photosynthetic bacteria

The photosynthetic bacteria have a very versatile metabolic repertoire and have been known for decades to produce hydrogen during photofermentative growth. Here, recent advances in hydrogen production by these organisms are reviewed and future directions highlighted. Often used as a second stage in two stage hydrogen production processes; first stage fermentative sugar to hydrogen and organic acids; second stage, organic acids to hydrogen, recent studies have highlighted their ability to directly convert sugars to hydrogen. Several studies have attempted to optimize a single stage batch process and these, and a study with continuous cultures have shown that yields approaching 9 mol H2/mol glucose can be obtained. One of the drawbacks of this system is the dependency on light, necessitating the use of photobioreactors, thus potentially greatly adding to the cost of such a system. In another approach which avoids the use of light energy, microaerobic fermentation of organic acids to hydrogen, driven by limited oxidative phosphorylation has been demonstrated in principle. Further advances will probably require the use of metabolic engineering and more sophisticated process controls in order to achieve higher stoichiometries, approaches that might be applied to other, light dependent, hydrogen production process by these organisms.