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

[1] In this study we examine the effects of varying Fe, Mo, and P concentrations on delta(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 delta(15) N than when grown in Fe-limited media ([Fe]< 50 nM). A compilation of our data with previous measurements of delta(15) N in N-2-fixing cyanobacteria reveals a general trend toward the production of more N-15-depleted biomass at higher Fe concentrations. We discuss our results in the context of negative delta(15) N values preserved in Archean and some Phanerozoic sediments, generally attributed to the production of marine organic matter with low delta(15) N 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 delta N-15-depleted biomass by N-2-fixing organisms, contributing to the isotopic record.