The history of inorganic nitrogen in the biosphere.

When in the primeval atmosphere ammonia approached exhaustion, bacteria resembling clostridia developed mechanisms for nitrogen fixation. The fixation was continued by the photosynthetic bacteria. In the later, oxidizing, atmosphere the combined activities of the nitrificants and the denitrificants could lead to a large-scale cyclic regeneration of free nitrogen. The possibility of a descent of the nitrificants from hypothetical photosynthetic bacteria, which used ammonia as electron donor, is discussed. The anoxygenic atmosphere contained no nitrate, and therefore neither nitrate fermentation nor nitrate respiration were precursors of aerobic respiration. This evolved from photosynthesis. In nitrate fermentation, nitrate serves only as an incidental electron acceptor; this process is merely an evolutionary sideline. Nitrate respiration evolved from aerobic respiration. While in present conditions the reaction of nitrogen with oxygen and water to give nitrate is exergonic and possibly occurs at a low rate, the antagonistic action of the denitrificants maintains the stationary concentrations of nitrogen and oxygen in the air.