Effects of CO₂ on the Nitrogen Isotopic Composition of Marine Diazotrophic Cyanobacteria

Biological N 2 fixation has been crucial for sustaining early life on Earth. Very negative δ 15 N values detected in Archean sediments, which are not observed in present‐day environments, have been attributed to the low efficiency of proto‐nitrogenases. Alternatively, variations in early atmospheric CO 2 may also play a role. Here we examine the effects of CO 2 concentrations on the biomass δ 15 N signatures of the diazotrophs Trichodesmium erythraeum and Crocosphaera watsonii , which utilize Mo‐Fe nitrogenase (the most common form of the enzyme). Our results show that these organisms produce biomass with δ 15 N values up to ∼3‰ lower under both decreased and elevated CO 2 concentrations compared to modern levels (∼380 μatm). These deviations from modern CO 2 levels reduce nitrogenase enzyme efficiency, leading to increased organismal isotopic fractionation during N 2 fixation. This study offers an alternative explanation for the observed fluctuations in geological δ 15 N records and provides new insights into the past nitrogen cycle on Earth.