SALINIZATION IMPACTS ON HISTORICALLY FRESHWATER BACTERIAL COMMUNITIES

Abstract

Microorganisms regulate the movement of energy and nutrients through ecosystems. When environmental changes influence the composition of microbial communities, associated biogeochemical cycles can change in potentially unpredictable ways. Salinization is a widespread environmental concern for both inland and coastal wetlands. Sea level rise is a long-term problem, with salinization occurring gradually. Storm surge, drought, and geomorphology alterations can also cause salinization events. This saltwater intrusion is known to reduce freshwater wetland ecosystem functions such as decreased inorganic nitrogen removal and carbon storage. Though both fresh and salt water microbial communities have been studied, it is unclear how historically freshwater wetland microbial communities will respond to increased salinization and to the influx of saltwater microbial communities. Salinization is a strong environmental filter and with increased salinity, microbial communities will shift due to the inability of freshwater species to quickly adapt to saline conditions and compete with saltwater microbes. I hypothesized that bacterial communities will decrease in taxonomic and phylogenetic diversity as salinity is increased, and microbial communities will differ in composition according to salinity and colonization of saline communities. Salinity-induced shifts in microbial community composition will directly influence rates of decomposition and CO2 respiration. I used a combination of an experimental mesocosm and bioinformatics approaches to examine how salinity and dispersal of aquatic communities impact bacterioplankton community structure and function. Findings indicate that salinity, but not dispersal, reduced bacterial taxonomic (i.e., based on species identify and abundance) and phylogenetic (based on relatedness of species) diversity and community composition changed along the experimental salinity gradient. The compositional shifts were accompanied by changes in carbon mineralization rates. A unique group of bacterial taxa, many of which were unable to be classified, represented each salinity environment. Together, changes in bacterial community composition were associated with changes in carbon cycling functions. Possible impacts to carbon cycling and storage, and biogeochemical cycles in general, include reduced storage potential and slowed cycling rates. By considering microbial community responses, it may be possible to manage freshwater wetland communities to promote carbon storage for climate change mitigation.