METAL LIMITATION OF NITROGEN-FIXING CYANOBACTERIA THROUGH TIME AND RESULTING ISOTOPIC TRENDS

Reduced nitrogen is produced primarily by N₂ fixing organisms (diazotrophs) that utilize the enzyme nitrogenase, a metallo-enzyme that contains a Fe-Mo cofactor in its most efficient form (although in the absence of Mo some organisms can form alternative nitrogenases containing Fe-V or Fe only). Progressive oxygenation of the Earth's atmosphere has resulted in fluctuating redox conditions in the Earth's oceans through geologic time, which would have dramatically affected the speciation and bioavailability of trace metals, including Mo and Fe. As a result, microbial metabolisms requiring these metals, including N₂ fixation, could have been limited by metal availability through time.

To examine the effects of metal limitation on cyanobacterial growth and N₂ fixation, we grew Anabaena variabilis str. ATCC 29413 in progressively Mo- and Fe-limited media, including metal concentrations corresponding to various oceanic redox states. Experiments replicating Archean ocean metal concentrations resulted in high growth rates due to high [Fe], but low N₂ fixation rates. In experiments replicating Proterozoic metal concentrations, growth was Fe-limited, but N₂ fixation was not Mo-limited. Experiments replicating modern metal concentrations resulted in N₂ fixation and growth rates similar to that of the Proterozoic experiments.

Furthermore, C/N, carbon and nitrogen isotopic analyses of cyanobacterial biomass from these experiments suggests that the organisms switch growth mode at [Fe] below 280nM, leading to increased C/N ratios consistent with those observed in modern oceans. Nitrogen isotopic ratios in Mo-limited experiments reflect an up to 2 per mil fractionation not seen in Fe-limited experiments, suggesting the Fe-Fe nitrogenase imparts an N isotope fractionation while the Fe-Mo nitrogenase does not. These results indicate that fractionation by the Fe-Fe nitrogenase could have contributed to secular trends in δ15N seen in the Archean.