title: Isotopic evidence for biological nitrogen fixation by molybdenum-nitrogenase from 3.2 Gyr.
abstract: Nitrogen is an essential nutrient for all organisms that must have been available since the origin of life. Abiotic processes including hydrothermal reduction, photochemical reactions, or lightning discharge could have converted atmospheric N2 into assimilable NH4(+), HCN, or NOx species, collectively termed fixed nitrogen. But these sources may have been small on the early Earth, severely limiting the size of the primordial biosphere. The evolution of the nitrogen-fixing enzyme nitrogenase, which reduces atmospheric N2 to organic NH4(+), thus represented a major breakthrough in the radiation of life, but its timing is uncertain. Here we present nitrogen isotope ratios with a mean of 0.0 ± 1.2‰ from marine and fluvial sedimentary rocks of prehnite-pumpellyite to greenschist metamorphic grade between 3.2 and 2.75 billion years ago. These data cannot readily be explained by abiotic processes and therefore suggest biological nitrogen fixation, most probably using molybdenum-based nitrogenase as opposed to other variants that impart significant negative fractionations. Our data place a minimum age constraint of 3.2 billion years on the origin of biological nitrogen fixation and suggest that molybdenum was bioavailable in the mid-Archaean ocean long before the Great Oxidation Event.
authors: Department of Earth &Space Sciences and Astrobiology Program, University of Washington, Seattle, Washington 98195-1310, USA.
authors: Eva E
authors: EE
authors: Stüeken
authors: Department of Earth &Space Sciences and Astrobiology Program, University of Washington, Seattle, Washington 98195-1310, USA.
authors: Roger
authors: R
authors: Buick
authors: Department of Geology, University of Johannesburg, Auckland Park 2006, South Africa.
authors: Bradley M
authors: BM
authors: Guy
authors: Department of Earth &Space Sciences and Astrobiology Program, University of Washington, Seattle, Washington 98195-1310, USA.
authors: Matthew C
authors: MC
authors: Koehler
mesh_terms: D005075:Biological Evolution; D019143:Evolution, Molecular; D019015:Geologic Sediments / Q000737:chemistry; D049690:History, Ancient; D008982:Molybdenum / Q000378:metabolism*; D009586:Nitrogen Fixation; D009587:Nitrogen Isotopes / Q000032:analysis*; D009591:Nitrogenase / Q000378:metabolism*; D009792:Oceans and Seas; D010084:Oxidation-Reduction; D013997:Time Factors
chemical_list: D009587:Nitrogen Isotopes; D008982:Molybdenum; D009591:Nitrogenase