Nitrogen fixation as evidence for the reducing nature of the early biosphere

Probably the first nitrogen fixers were anaerobic, non-photosynthetic, bacteria, i.e. fermenters. During the evolution of N₂ fixation they still needed nitrogen on the oxidation level of ammonia. Because of the complexities in structure and function of nitrogenase this evolution must have required a long time. The photosynthetic and later the respiring bacteria inherited the capacity for N₂ fixation from the fermenters, but the process did not change a great deal when it was taken over.Because of the long need for NH₃, which is unstable in a redoxneutral atmosphere, a long-persisting reducing atmosphere was needed. The transition to a redoxneutral atmosphere, dominated by CO₂, H₂O and N₂, cannot have been rapid, and the NH₃ in it was recycled. Probably the atmosphere contained for a long time, as was suggested by Urey but is often denied now, a great deal of methane as a reductant. The recycling occurred with participation of intermediates like cyanide, through energy input as UV radiation or as electric discharges. A stationary state was set up.The hypothesis is recalled that coloured, photosynthetic, NH₃ bacteria, analogous to coloured sulphur bacteria, may have existed, or may still exist, in reducing conditions. A few remarks are made about the origin of nitrification in the later, oxidizing atmosphere.