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 (δ ¹⁵N) of newly fixed N is currently believed to be ∼–1‰, 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 δ ¹⁵N (–6 to –7‰). An important contribution of alternative nitrogenases to N ₂ fixation provides a simple explanation for the anomalously low δ ¹⁵N (