Phototrophic Fe(II)-oxidizing bacteria, or “photoferrotrophs”, were probably a key driver of primary productivity and Fe(III) mineral formation in the Earth’s oceans prior to the rise of oxygen. As O2 began to rise and oxygenate the photic zone, these anaerobic microbes would have seen their habitable space decrease. Furthermore, the transformation from an anoxic to an oxic world would have facilitated an increase in the concentration of oxidized nitrogen species such as nitrate in the oceans. Nitrate-dependent oxidation of Fe(II) is widespread in modern anoxic environments and could, in theory, have competed with photoferrotrophs for Fe(II) in regions where nitrate, light and low-O2 conditions still co-existed. However, the competition dynamics between these different types of anaerobic Fe(II) oxidation are not well constrained.
Utilizing various co-culture experiments, we have shown that Fe(II) oxidation by photoferrotrophs is not only out-competed but completely inhibited by the activity of nitrate-dependent Fe(II)-oxidizing bacteria. We primarily attribute this inhibition to the observed accumulation of nitric oxide (NO) during denitrification, which is highly toxic at low nanomolar concentrations. These results suggest that the evolution of the nitrogen cycle over the Great Oxidation Event could represent a yet unreported control on the extent of Fe(III) mineral formation by photoferrotrophs in the ancient oceans.
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