Production of 15N‐depleted biomass during cyanobacterial N2‐fixation at high Fe concentrations

Abstract

In this study we examine the effects of varying Fe, Mo, and P concentrations on δ15N 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‰ 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 N2 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.