Prebiotic ammonia from reduction of nitrite by iron (II) on the early Earth.
Theories for the origin of life require the availability of reduced (or 'fixed') nitrogen-containing compounds, in particular ammonia. In reducing atmospheres, such compounds are readily formed by electrical discharges, but geochemical evidence suggests that the early Earth had a non-reducing atmosphere, in which discharges would have instead produced NO. This would have been converted into nitric and nitrous acids and delivered to the early oceans as acid rain. It is known, however, that Fe(II) was present in the early oceans at much higher concentrations than are found today, and thus the oxidation of Fe(II) to Fe(III) provides a possible means for reducing nitrites and nitrates to ammonia. Here we explore this possibility in a series of experiments which mimic a broad range of prebiotic seawater conditions (the actual conditions on the early Earth remain poorly constrained). We find that the reduction by Fe(II) of nitrites and nitrates to ammonia could have been a significant source of reduced nitrogen on the early Earth, provided that the ocean pH exceeded 7.3 and is favoured for temperatures greater than about 25 degrees C.
Summers
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NASA-Ames Research Center, Moffett Field, California 94035, USA.
Chang
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D000641:Ammonia / Q000138:chemical synthesis*; D018534:Earth, Planet; D019419:Evolution, Planetary; D005296:Ferrous Compounds / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008029:Lighting; D009573:Nitrites / Q000737:chemistry*; D000069497:Origin of Life; D010084:Oxidation-Reduction; D012623:Seawater / Q000737:chemistry; D013696:Temperature
D005296:Ferrous Compounds; D009573:Nitrites; D000641:Ammonia