Wetland ecosystems play a critical role in the global carbon cycle, contributing ca. 25% of global emissions of methane (CH4), a potent greenhouse gas. Many wetland ecosystems are currently experiencing sea level rise, and microorganisms play a key role in mediating ecosystem-level response to a rising sea. In this project, we address two questions: i) How does microbial community structure change across the natural salinity and sulfate gradient along a tidal river? and ii) How does seawater influence microbial community structure and greenhouse gas (CO2, CH4) production? To address these questions, we collected 5 soil cores from each of 5 positions along a tidal river with salinities ranging from freshwater to saline. These cores were homogenized and the V4 region of the 16S gene was sequenced following the Earth Microbiome Project protocols. Microbial interaction networks were generated for each position using the mutual information criterion. A subsample of replicate cores per position were combined to generate a single inoculum per position. We monitored changes in microbial community structure and production of CO2 and CH4 of incubations under one of three seawater treatments: fresh, brackish, salt.
Results/Conclusions
Natural gradient: Microbial taxonomic composition was markedly different along the salinity gradient, with Shannon’s diversity decreasing with salinity. The percent of OTU with significant interactions was lowest on the fresh end of the gradient, in part due to the higher diversity there, but was highest at intermediate salinities. Microbial communities at the fresh end of the gradient had significantly weaker interactions overall, suggesting that significant correlations were more likely to reflect similar changes in habitat preferences rather than direct interaction among microbial taxa.
Incubation experiment: Application of the salt treatment decreased diversity for all non-saline inocula, with the most pronounced effect on the freshwater inoculum. Application of the fresh treatment resulted in weaker interactions among taxa for inocula from the intermediate salinity positions, but had little effect on inocula from the most saline and most fresh position, suggesting that release from salinity stress was greatest for brackish inocula and resulted in decreased reliance on other taxa. Minimal methane production was observed prior to consumption of available sulfate from the seawater treatments. Methane production under the salt treatment remained low after sulfate consumption was completed. Thus, under natural conditions areas with seawater intrusion can be expected to have lowered methane production.