Microbial paths to renewable hydrogen production

Hydrogen technologies are under intense research and development at present, to enable their future use as a fuel, in particular for the transportation sector. Whilst the engineering of hydrogen storage and use is relatively well developed, with prototype cars already on the road, a viable, sustainable means of renewable hydrogen production is lacking. A number of options for the biological production of hydrogen from abundantly available substrates - water, waste streams and, potentially, lignocellulosic materials - are available. Thus, solar energy can either be directly converted to hydrogen in some scenarios, or fixed carbon compounds generated through conventional photosynthesis can subsequently be converted to hydrogen in others. Much is already known about the underlying biological principles, but each microbial path to hydrogen production presents technical barriers to realization on a practical scale. Here, the key limiting factors in each approach are highlighted and future perspectives for biological hydrogen production are discussed.