Effects of CO ₂ on the nitrogen isotopic composition of Trichodesmium and Crocosphaera

Biological nitrogen (N ₂) fixation is the main input of fixed nitrogen to ecosystems on Earth. Nitrogen isotope fractionation during this process is a key parameter for understanding the nitrogen cycle, however, relatively little is known about its regulatory mechanisms. Here we examine the effects of varying CO ₂ concentrations on biomass δ ¹⁵N signatures of the cyanobacterial diazotrophs Trichodesmiumerythraeum and Crocosphaera watsonii. We show that these organisms produce biomass up to ~3 ‰ lower in δ ¹⁵N under either decreased (~180 µatm) or elevated (~1400 µatm) CO ₂ concentrations in comparison to modern levels (~380 µatm). Our results pointed towards changes in nitrogenase enzyme efficiency in response to CO ₂ perturbations impacting isotopic fractionation during N ₂ fixation and thus the biomass δ ¹⁵N. This study contributes to an improved interpretation of the observed fluctuations in the δ ¹⁵N records, and thus the past nitrogen cycle on Earth.