Passage: N2 + 16ATP + 8e- + 8H+ -> 2NH3 + H2 + 16ADP + 16PiThe process is coupled to the hydrolysis of 16 equivalents of ATP and is accompanied by the co-formation of one equivalent of H2. The conversion of N2 into ammonia occurs at a metal cluster called FeMoco, an abbreviation for the iron-molybdenum cofactor.  The mechanism proceeds via a series of protonation and reduction steps wherein the FeMoco active site hydrogenates the N2 substrate. In free-living diazotrophs, the nitrogenase-generated ammonia is assimilated into glutamate through the glutamine synthetase/glutamate synthase pathway. The microbial nif genes required for nitrogen fixation are widely distributed in diverse environments.Nitrogenases are rapidly degraded by oxygen. For this reason, many bacteria cease production of the enzyme in the presence of oxygen. Many nitrogen-fixing organisms exist only in anaerobic conditions, respiring to draw down oxygen levels, or binding the oxygen with a protein such as leghemoglobin.

Question: Where do most nitrogen-fixing organisms exist?
anaerobic conditions