Production of¹⁵N-depleted biomass during cyanobacterial N ₂-fixation at high Fe concentrations

In this study we examine the effects of varying Fe, Mo, and P concentrations on 6 N fractionation during N₂ fixation in the cyanobacterium Anabaena variabilis. We show that when grown in Fe-enriched media ([Fe] > 50 nM), this organism produces biomass up to 3%o lower in δ¹⁵N than when grown in Fe-limited media ([Fe] < 50 nM). A compilation of our data with previous measurements of δ¹⁵N in N₂-fixing cyanobacteria reveals a general trend toward the production of more ¹⁵N-depleted biomass at higher Fe concentrations. We discuss our results in the context of negative δ¹⁵N values preserved in Archean and some Phanerozoic sediments, generally attributed to the production of marine organic matter with low δ¹⁵N by N ₂ fixation (and potentially NH₄⁺ regeneration) during periods of fluctuating nutrient dynamics. We suggest that enhanced Fe availability during periods of widespread ocean anoxia can further stimulate the production of ¹⁵N-depleted biomass by N₂-fixing organisms, contributing to the isotopic record.