Tidal freshwater wetlands are dynamic ecosystems, subject to multiple stressors derived from the watershed as well as ones determined by oceanic processes. These habitats are highly sensitive to environmental perturbation, and the effects of global climate change are already evident. The intrusion of saltwater into historically freshwater wetlands is of particular concern, and will occur as a result of both changes in precipitation and rising sea levels. Because salinity directly impacts a wide range of abiotic and biotic processes, it is a major driver of ecosystem structure and function, especially with regards to carbon biogeochemistry. Using an integrated approach that includes in situ field manipulations (repeated additions of brackish water to plots in tidal freshwater wetlands), reciprocal transplant experiments of freshwater and oligohaline wetland soils, and long-term studies of wetlands along existing riverine salinity gradients, our research seeks to better understand the effects of salinization on microbial community composition and its potential to alter the balance between carbon accumulation and mineralization. Our work combines community-level molecular analyses of phylogenetic markers and functional genes to link the genetic potential of the microbial community (DNA pool), levels of gene expression (transcribed to RNA), and biogeochemical process rates.
Results/Conclusions
Our results demonstrate that salinity is a strong driver of microbial community composition and can induce changes in both phylogenic community structure and functional gene abundance. We have identified broad-level taxonomic groups that exhibit a preference for fresh or saltwater and documented an ecological coherence of the salinity response within these phylogenetic groups. We repeatedly observe a decrease in the abundance of methanogens and an increase in the abundance of sulfate reducers associated with elevated salinity, a finding that is consistent with the thermodynamics of sulfate reduction vs. methanogenesis. At elevated salinities, there is discordance between the composition of the overall microbial community (assessed using whole-community DNA extracts) and the active fraction (using RNA extracts), suggesting that salinity directs community composition and differentially regulates gene expression. These changes in microbial community structure were tightly correlated with key aspects of carbon biogeochemistry, including methane and carbon dioxide production rates and extracellular enzyme activity. At the ecosystem level, these biogeochemical changes appear to modulate greenhouse gas fluxes and may affect soil carbon storage. Our research has demonstrated that a more holistic consideration of microbial communities could advance our understanding of the impacts of human activities on our global environment.