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Significance Biological nitrogen fixation is the main route by which nitrogen enters the biosphere. This reaction is catalyzed by nitrogenase, a metalloenzyme that exists in forms containing molybdenum, vanadium, or iron only. The contribution of the “alternative” vanadium and iron-only nitrogenases to nitrogen fixation in the present and the past is unknown. Here we show that the nitrogen isotopic composition (15 N to14 N ratio) of biomass generated from nitrogen fixation by alternative nitrogenases is significantly and characteristically lower than biomass produced by molybdenum nitrogenases. In light of these results, nitrogen isotope measurements in ancient sediments imply an important role for iron-only nitrogenases in nitrogen fixation within certain anoxic, molybdenum-limited ancient environments.

Nitrogen isotope fractionation by alternative nitrogenases and past ocean anoxia