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

Nitrogen isotope ratios from rocks between 3.2 and 2.75 billion years old are most readily explained by biological nitrogen fixation, most probably using the metal molybdenum as a cofactor, showing that nitrogen fixation is at least 3.2 billion years old and suggesting that molybdenum was available to organisms long before the Great Oxidation Event. Early emergence of nitrogen fixation The evolution of the enzyme nitrogenase, through which organisms can fix atmospheric nitrogen, was a clearly major step in the history of life. What is less certain is the timing. Eva Stüeken et al . have determined nitrogen isotope ratios in marine and fluvial sedimentary rocks between 3.2 and 2.75 billion years old. The ratios are most readily explained by biological nitrogen fixation, probably with molybdenum as a cofactor. This suggests that nitrogen fixation is at least 3.2 billion years old, and contradicts previous suggestions that marine molybdenum was scarce before the Great Oxidation Event. Nitrogen is an essential nutrient for all organisms that must have been available since the origin of life. Abiotic processes including hydrothermal reduction 1 , photochemical reactions 2 , or lightning discharge 3 could have converted atmospheric N 2 into assimilable NH 4 + , 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 biosphere 4 . The evolution of the nitrogen-fixing enzyme nitrogenase, which reduces atmospheric N 2 to organic NH 4 + , 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 fractionations 7 . 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.