title: Nitrogen isotope fractionation by alternative nitrogenases and past ocean anoxia.
abstract: 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(15)N) 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(15)N (-6 to -7‰). An important contribution of alternative nitrogenases to N2 fixation provides a simple explanation for the anomalously low δ(15)N (<-2‰) 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.
authors.affiliation: Departments of Geosciences and Chemistry, Princeton University, Princeton NJ 08544.
authors.forename: Xinning
authors.initials: X
authors.lastname: Zhang
authors.forename: Daniel M
authors.initials: DM
authors.lastname: Sigman
authors.forename: François M M
authors.initials: FM
authors.lastname: Morel
authors.forename: Anne M L
authors.initials: AM
authors.lastname: Kraepiel
mesh_terms: D000693:Anaerobiosis; D001419:Bacteria / Q000201:enzymology* / Q000254:growth & development; D018533:Biomass; D005591:Chemical Fractionation; D019015:Geologic Sediments / Q000737:chemistry; D008670:Metals / Q000378:metabolism; D009586:Nitrogen Fixation; D009587:Nitrogen Isotopes; D009591:Nitrogenase / Q000378:metabolism*; D009792:Oceans and Seas
chemical_list: D008670:Metals; D009587:Nitrogen Isotopes; D009591:Nitrogenase
keywords: biogeochemistry; paleooceanography; stable isotopes; trace metals