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 δ 15 N fractionation during N 2 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 δ 15 N than when grown in Fe‐limited media ([Fe] < 50 nM). A compilation of our data with previous measurements of δ 15 N in N 2 ‐fixing cyanobacteria reveals a general trend toward the production of more 15 N‐depleted biomass at higher Fe concentrations. We discuss our results in the context of negative δ 15 N values preserved in Archean and some Phanerozoic sediments, generally attributed to the production of marine organic matter with low δ 15 N by N 2 fixation (and potentially NH 4 + 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 15 N‐depleted biomass by N 2 ‐fixing organisms, contributing to the isotopic record.