The fixation of gaseous molecular nitrogen, N2, to biologically utilizable nitrogen is performed as an essential part of the global nitrogen cycle by bacteria that may be either free living or exist in symbiotic associations with plants, bivalves, or marine diatoms.21 The central enzymic reduction of nitrogen to ammonia is that catalyzed by nitrogenase:

N2 + 8H+ + 8e — ^ 2NH3 + H2

Molecular hydrogen is an obligatory product of the overall reaction — which, how­ever, is so energy-expensive that most nitrogen fixers rarely evolve H2, employing another hydrogenase (“uptake hydrogenase”) to recycle the H2 via oxidation by O2 and conservation of part of the potentially released energy to support nitrogenase
action. For this reason, nitrogenase-containing microbes are not viewed as likely sources of H2 for biofuels.