Zuozhu Wen, Ruotong Jiang, Tianli He, Thomas J. Browning, Haizheng Hong, Shuh-Ji Kao, Jin-Yu Terence Yang, Dalin Shi*

Geophysical Research Letters

https://doi.org/10.1029/2024GL110599

Published: 03 October 2024

Abstract

Biological N₂ fixation has been crucial for sustaining early life on Earth. Very negative δ¹⁵N 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 CO₂ may also play a role. Here we examine the effects of CO₂ concentrations on the biomass δ¹⁵N 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 δ¹⁵N values up to~3‰ lower under both decreased and elevated CO₂ concentrations compared to modern levels (~380 μatm). These deviations from modern CO₂ levels reduce nitrogenase enzyme efficiency, leading to increased organismal isotopic fractionation during N₂ fixation. This study offers an alternative explanation for the observed fluctuations in geological δ¹⁵N records and provides new insights into the past nitrogen cycle on Earth.