Wen, Zuozhu, Jiang, Ruotong, He, Tianli, Browning, Thomas J. , Hong, Haizheng, Kao, Shuh‐Ji, Terence Yang, Jin‐Yu and Shi, Dalin (2024) Effects of CO 2 on the Nitrogen Isotopic Composition of Marine Diazotrophic Cyanobacteria. Geophysical Research Letters, 51 (19). e2024GL110599. DOI 10.1029/2024GL110599.

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Abstract

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.

Plain Language Summary

The isotope effect of biological N 2 fixation is crucial for understanding the nitrogen cycle, but its regulation under different atmospheric CO 2 levels as appeared in Earth's history is not well understood. Our research shows that CO 2 levels significantly influence the nitrogen isotope composition in the biomass of the diazotrophic cyanobacteria Trichodesmium and Crocosphaera by affecting the nitrogenase efficiency and thus the growth rate. This study sheds light on the geological changes in the δ 15 N records and provides new insights into the historical nitrogen cycle on Earth.

Key Points

The effects of CO 2 on the biomass δ 15 N signatures of the diazotrophs Trichodesmium erythraeum and Crocosphaera watsonii are examined
Both species produce biomass with δ 15 N values lower under both decreased and elevated CO 2 concentrations compared to modern CO 2 levels
CO 2 ‐controlled nitrogenase efficiency significantly influences organismal isotopic fractionation during N 2 fixation

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