Impact of Anthropogenic N-loading on Sulfate-Reducing Diazotrophic Communities in New England Salt Marshes

‘Biological Nitrogen Fixation (BNF) is an ancient microbial process crucial to oligotrophic ecosystems in which inert N2 atmospheric gas is reduced to bioavailable ammonium (NH4+). New England salt marshes are one such ecosystem that harbors a diverse community of N-fixing microbes that satisfy the high N-requirement of this productive system. Increased sewage outfall and fertilizer deposition fulfills, in part, this N-requirement. This study addresses how BNF responds to human based N-loading in salt marshes with an emphasis on Sulfate-reducing Nfixers. We compared sediments across sites that had high and low NO3- and NH4+ concentrations in order to determine the effect of elevated nutrient concentrations on BNF. We selected sites from the Great Sippewissett Marsh and Plum Island based on where we observe near-pristine conditions and where we see anthropogenic nitrogen loading in the form of sewage or fertilizer. The Acetylene Reduction Technique was used to quantify N-fixation, colorimetry was used to determine environmental N-concentrations and the Polymerase chain reaction allowed us to see whether the N-fixing gene, nifH, was being expressed as mRNA. NH4+ was found to inhibit Nfixation, while NO3- doesn’t reveal any strong trends. MoO42- addition assays were used to inhibit SO42- reduction, allowing us to determine the contribution of sulfate reducing-N-fixation to BNF. The N-fixing community at Plum Island are almost entirely dominated by sulfate reducers and are significantly different from that at the Great Sippewissett Marsh (p<0.001). Cores from Reference marshes were also incubated anaerobically for 16 days under different N-loading regimes to look at short term effects of N-loading on BNF. The anaerobic incubations substantially reduced N-fixation across all treatments but no significant differences were observed between treatments.’ Download paper here

Recommended citation: Miranda, KK, Giblin, AE, and Ruff, E. (2020). MBL SES Program. Impact of Anthropogenic N-loading on Sulfate-Reducing Diazotrophic Communities in New England Salt Marshes.