Abstract
Hydrogen, the most abundant and lightest element in the universe, has much potential as a future energy source. Hydrogenases catalyse one of the simplest chemical reactions, 2H(+) + 2e(-) ↔ H(2), yet their structure is very complex. Biologically, hydrogen can be produced via photosynthetic or fermentative routes. This review provides an overview of microbial production of hydrogen by fermentation (currently the more favourable route) and focuses on biochemical pathways, theoretical hydrogen yields and hydrogenase structure. In addition, several examples of metabolic engineering to enhance fermentative hydrogen production are presented along with some examples of expression of heterologous hydrogenases for enhanced hydrogen production.