One of the most profound effects of climate change on freshwater wetlands is salinity increase due to the intrusion of saltwater as sea level rises. Studies of how microbial community structure will change following this sort of disturbance show discordant responses, but the general expectation based on thermodynamic constraints is that salinity will suppress methanogen abundance and increase the abundance of sulfate-reducing bacteria. Salinity also is expected to affect nitrate reduction, but the anticipated response varies based on the functional guild that is considered. As a space-for-time proxy to understand the long-term effects of saltwater intrusion, we evaluated wetland microbial community structure and potential function across an existing salinity gradient in the Pamunkey River (Virginia). Five sites were selected based on salinity (0.25 to 10 PSU) and sampled in May of 2015. Ancillary physicochemical data included pH, salinity, organic matter content, C:N ratio, and water-extractable ions (concentrations of Na+, K+, Mg2+, Ca2+, Cl-, SO42-, NO3-, NO2- and NH4+ via ion chromatography). Abundance estimates of total bacteria (16s rDNA) and specific quantification of nitrate-reducers (nrfA, nirS, and nirK), methanogens (mrcA), and sulfate-reducing bacteria (dsrA) were accomplished using quantitative PCR (qPCR). Community structure was examined using metagenomic analysis (16S amplicons and whole-genome sequencing analyzed in MG-RAST).
Results/Conclusions
Microbial diversity measures at the freshwater site were higher than sites affected by salt. Metagenomic analyses indicated that microbial communities at freshwater (<2 PSU) and oligohaline sites (>2 PSU) were distinct. No significant difference was observed in the abundance of nitrate reducers, but the abundances of sulfate reducers (higher where salinity > 2 PSU) and methanogens (higher where salinity < 2 PSU) appeared to be driven by salinity and not by soil organic matter availability (15-30%), C:N ratio (9-15), porewater nutrients, or pH (5.9-6.5). We found evidence that microbial community structure varies across the wetland salinity gradient and an indication that future salinization of freshwater wetlands will result in loss of microbial richness. Changes in functional group abundance suggest significant shifts in carbon biogeochemistry, which could lead to altered rates of greenhouse gas emissions (methane, carbon dioxide, and nitrous oxide) and changes in the carbon storage capacity of these wetlands. In this way, the microbial community response to increased salinity may feedback with global climate change, and community-level analyses such as ours are key to understanding the mechanisms of this effect.