The biological production of hydrogen has been known since the early 1900s and the enzymes involved were discovered in the 1930s. Hydrogen production has been found in many prokaryotes, green microalgae, and a few eukaryotes as shown in Table 5.5 (Das and Veziroglu, 2001; Happe et al., 2002).

The production of hydrogen is due to the presence of two enzymes either nitro — genase or hydrogenase in the organism. Nitrogenase has the ability to use ATP and electrons to reduce substrates including protons to hydrogen gas and has been found in photoheterotrophic bacteria such as Rhodobacter sp.:

Hydrogenases have been found in a large number of green microalgae such as Chlamy- domonas reinhardtii and Chlorococcum littorale, anaerobic bacteria such as Clost­ridium sp. and Cyanobacteria sp. Hydrogenases can be either uptake or reversible hydrogenases and can be divided into three classes based on their metal composition. These classes are Ni/Fe, Fe and metal-free where the Fe hydrogenase has a unique active centre giving the enzyme a 100-fold higher activity:

hydrogenase

H2 « 2H+ + 2e

These hydrogenases and nitrogenase are responsible for hydrogen production by a number of microorganisms.