Review An evaluative report and challenges for fermentative biohydrogen production Pallavi Sinha, Anjana Pandey* Nanotechnology and Molecular Biology Lab, Centre for Biotechnology, University of Allahabad, Allahabad 211002, Uttar Pradesh, India article info Article history: Received 13 December 2010 Received in revised form 4 March 2011 Accepted 15 March 2011 Available online 20 April 2011 Keywords: Biohydrogen Fermentative hydrogen production Green house gases Metabolic Engineering abstract Hydrogen, the most abundant and lightest element in the universe, has enormous potential as a future energy. High conversion efficiency, recyclability and nonpolluting nature of hydrogen make it the fuel of future. Various microorganisms are explored for producing hydrogen by exploiting variety of biological organic substrates. The target is the genetic improvement of the organism or the biochemical pathway required for bio- hydrogen production and devising path even better in comparison to the other production methods. The present review discusses different methods of biohydrogen production specifically by the fermentative route, physical factors affecting its production and other aspects for enhancement in the yield of hydrogen production. Metabolic engineering strategies for enhancement in hydrogen production to overcome different limitation have been also summarized. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. 1. Introduction Since the beginning of human history, man has sought fuel for his energy needs. Mankind fuels have continually evolved towards better, more efficient, safer and environmentally cleaner fuels, from wood to animal fat, to coal, to petroleum to natural gas and now to hydrogen. A clear tendency was significant throughout to develop a lighter and cleaner fuel. For more than a century, fossil fuel has been exploited to such an extent that it has not only induced serious environment problem but also exhausted the limited fuel reserves, neces- sitating the quest for a clean alternative energy. H 2 is most promising in the succession of fuel evolution with several technical, socio-economic and environmental benefits to its credit [1]. It has highest energy content per unit weight of any known fuel (142 kJ/g). Handling of H 2 gas is safer with comparison to other known natural gases and the most important fact is that H 2 is environment friendly gas as its combustion results only in water vapours and energy. These features make H 2 an ideal candidate to replace the fossil fuels. Now H 2 has been universally accepted as an environmentally safe renewable energy resource and most suitable alternative of fossil fuels that does not harm environment. At present 40% H 2 is produced from natural gases, 30% from heavy oil and naphtha, 18% from coal, 4% from elec- trolysis and about 1% from biomass. But in present scenario biological H 2 production processes are becoming important mainly due to two reasons: Utilization of renewable energy resources and its operation at ambient temperature and atmospheric pressure. * Corresponding author. Tel.: þ91 09235408067. E-mail address: apandey70@rediffmail.com (A. Pandey). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 36 (2011) 7460 e7478 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.03.077