Nitrogen isotope fractionation by alternative nitrogenases and past ocean anoxia

Biological nitrogen fixation constitutes the main input of fixed nitrogen to Earth's ecosystems, and its isotope effect is a key parameter in isotope-based interpretations of the N cycle. The nitrogen isotopic composition (delta N-15) of newly fixed N is currently believed to be similar to-1 parts per thousand, based on measurements of organic matter from diazotrophs using molybdenum (Mo)-nitrogenases. We show that the vanadium (V)- and iron (Fe)-only alternative nitrogenases produce fixed N with significantly lower delta N-15 (-6 to -7 parts per thousand). An important contribution of alternative nitrogenases to N-2 fixation provides a simple explanation for the anomalously low delta N-15 (<-2 parts per thousand) in sediments from the Cretaceous Oceanic Anoxic Events and the Archean Eon. A significant role for the alternative nitrogenases over Mo-nitrogenase is also consistent with evidence of Mo scarcity during these geologic periods, suggesting an additional dimension to the coupling between the global cycles of trace elements and nitrogen.