Microbial hydrogen production: state of the art and challenges for its industrial exploitation

The biological processes leading to the production of hydrogen can be classified as follows: (i) biophotolysis of water, carried out by microalgae and cyanobacteria; (ii) fermentative hydrogen production (dark fermentation), carried out by chemoheterotrophic microorganisms; (iii) photofermentative hydrogen production (photofermentation), carried out by photoheterotrophic bacteria; (iv) integrated hydrogen producing systems, which combines dark and photofermentation. In this paper the most important issues of biological hydrogen production are reviewed, in particular focusing on the advantages and the drawbacks of the various processes under study. The main challenges for the development of economically and environmentally sustainable processes are also discussed.