Marine Nitrogen fixation - The basics
N2 fixation is the process by which N2 gas is reduced into two molecules of ammonia. The reaction consumes cellular energy, and the overall reaction is:
N2 + 8H+ + 16ATP + 8e− -----> 2NH3 + H2 + 16ADP + 16 Pi
This reaction is catalysed by the nitrogenase enzyme complex, which is extremely oxygen sensitive. The most common form in the oceans is made up of two distinct proteins: dinitrogenase reductase (the Fe protein) and dinitrogenase (the Fe molybdenum, or FeMo protein). It is encoded by three genes (nifH, nifD and nifK); however, the nif operon can in some species contain up to 20 genes that are also involved in the synthesis and regulation of nitrogenase and its cofactors. The nitrogenase proteins are highly similar among diazotrophs, and the well-conserved nifHgene is commonly used for phylogenetic and ecological studies. Several 'alternative' nitrogenases are also known that contain a third subunit in the FeMo protein that is encoded by nifG. The capacity for N2 fixation occurs in bacteria and archaea of diverse physiologies (including anaerobes, facultative aerobes, aerobes and phototrophs), and although nitrogenase is thought to be an ancient enzyme, it is not uniformly distributed and is present in perhaps a few hundred cultivated species.