Isotopic evidence for biological nitrogen fixation by molybdenum-nitrogenase from 3.2 Gyr

Nitrogen is an essential nutrient for all organisms that must have been available since the origin of life. Abiotic processes including hydrothermal reduction¹, photochemical reactions², or lightning discharge³ could have converted atmospheric N₂ into assimilable NH₄⁺, HCN, or NO_x species, collectively termed fixed nitrogen. But these sources may have been small on the early Earth, severely limiting the size of the primordial biosphere⁴. 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^5,6. 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 fractionations⁷. 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.