Evidence that molybdenum‐dependent nitrogen fixation is not limited by high sulfate concentrations in marine environments

Abstract

Nitrogen availability frequently controls phytoplankton growth in marine environments. It would seem that N2 fixing microorganisms would have a selective advantage in these environments, but N2 fixation appears to be limited in most marine waters. Previous studies have focused on deficiencies of inorganic nutrients such as P, Fe, and especially Mo as constraints on N2 fixation. It was proposed that a high SO42‒: MoO42‒ ratio interferes with cellular MoO42‒ uptake and in turn results in insufficient quantities for incorporation into the N2‐reducing enzyme complex nitrogenase. In the present study, in situ bioassays demonstrate that Mo is not the limiting nutrient for N2 fixation by marine assemblages. Instead, enrichments of glucose or mannitol were found to initiate or increase N2 fixation in these assemblages. We also show that diazotrophic growth rates of an Azotobacter vinelandii strain containing a single Mo‐dependent nitrogenase are not inhibited at SO42‒: MoO42‒ ratios up to 8 times higher than those calculated for seawater. From these results we propose that widespread limitation of N2 fixation in marine environments is due to deficiencies in organic matter and appropriate microhabitats rather than Mo.