Improving biohydrogen productivity by microbial dark- and photo-fermentations: Novel data and future approaches

Hydrogen (H2) is an effective, environmentally friendly and renewable source of fuel that can be produced during dark- and photo-fermentation by different facultative and obligate anaerobic and purple bacteria and microalgae. This product is known as biohydrogen. It has the advantage of variable yield at low temperature (for mesophiles growing best at moderate temperature) and relatively low production cost, if compared with thermochemical methods. To develop fermentative H2 production biotechnology using cheap carbonaceous by-products and utilization of organic wastes, the selection or construction of effective bacterial strains and optimization of technology process conditions are required. Here we review recent new data that have been obtained with Escherichia coli, Clostridium beijerinskii, Rhodobacter sphaeroides and other bacteria. Activities of [Ni-Fe]-hydrogenases of dark-fermentative bacteria and [Mo]-nitrogenase and [Ni-Fe]-hydrogenase of photo-fermentative species have been examined after growth with different carbon sources, using pure cultures, as well as co-culture and mixed-cultures technologies. Importantly, H2 production from cheap and readily available substrates like crude glycerol or different industrial, agricultural and other carbon-based wastes by bacteria is a sustainable technology. Consequently further approaches and strain-improvement could increase H2 production in a cost-effective way, and they will lead to both small- and large-scale H2 production. Moreover, they will provide significant economic and environmental benefits for renewable and sustainable energy supply in the near future.