Effects of salinity and nutrients on microbial assemblages in Louisiana wetland sediments

Abstract

To evaluate the effects of saltwater intrusion and nutrient enrichment on wetland microbial communities, we measured changes in sediment microbial structure and function in response to increased salinity and nutrients. Sediments were collected from a cypress-tupelo swamp near Lake Pontchartrain, Louisiana, USA, and maintained in microcosms treated with elevated salinity, nitrogen (N), or phosphorus (P). Impacts on bacterial community diversity and composition were determined via molecular techniques, while effects on function were assessed through measurement of extracellular enzyme activity. Salinity increased bacterial diversity, P had no effect, while N reduced diversity. Deltaproteobacteria dominated all treatments, although their representation, along with that of the Alphaproteobacteria and Planctomycetes, was reduced following N addition. P addition reduced the proportion of Alphaproteobacteria, while salinity increased the proportion of Betaproteobacteria. Exposure to elevated salinity also decreased phosphatase and N-acetlyglucosaminidase activity by almost 20%, with less effect on β-glucosidase. P addition had no impact on extracellular enzyme activity. Overall, exposure to elevated salinity depresses microbial function and changes the sediment microbial assemblage. These wetlands are likely N-limited, and while N additions may regenerate plant communities, they also change the structure of the sediment microbial community, decreasing diversity and impacting the mineralization of other nutrients.