Assessing the effect of saltwater intrusion on the structure and function of microbial communities intidal freshwater wetlands
Tidal freshwater wetlands (TFW) are susceptible to increased inundation and saltwater intrusion from rising sea level. While several studies have documented the adverse impact of salinity on TFW vegetation and nutrient dynamics, there are only a few studies that address the microbial community. The goal of this research was to assess the impact of saltwater intrusion on the structure and function of TFW microbial communities following a salinity increase designed to mimic saltwater intrusion. Given recent research that demonstrates the significance of legacy effects on microbial communities and their associated ecosystem functions, we also set out to determine whether prior exposure to saline conditions affected the community’s response to future salinity increases. To address these objectives, an in situ simulation of saltwater disturbance was conducted by transplanting soils from a pristine TFW downstream to an oligohaline marsh. Soils were incubated in mesh bags and destructively sampled after 1, 3, 5, and 7 weeks of incubation. . The functional response of the microbial communities was examined by comparing CO2/CH4 production rates and extracellular enzyme activity associated with several carbon substrates. Changes in community composition were assessed using 16S-tag sequencing and quantitative PCR (qPCR) of key functional genes associated with anaerobic decomposition.
Results from this transplant experiment demonstrate that saltwater exposure dramatically impacts microbial processes in TFW. Methane production was suppressed in freshwater soils transplanted to oligohaline conditions, most likely due to methanogens being outcompeted for carbon substrates by sulfate-reducing bacteria. Following transplant, the abundance of methanogens also decreased. Rates of carbon dioxide production were highest in the saltwater controls and lowest in the transplanted freshwater community. Analysis of extracellular enzyme activity revealed a salinity effect that differed depending on the lability of the substrate. In general, the breakdown of more recalcitrant compounds was suppressed in the transplanted communities. The transplant to more saline conditions also shifted the composition of the freshwater microbial communities to contain more salt-tolerant taxa. The rate of change of community structure and function varied based on prior exposure to saline conditions.
Taken together, these results indicate that salinity dramatically affects anaerobic carbon mineralization in TFWs, which could alter the role of these ecosystems in the global carbon cycle by changing rates of organic matter sequestration and greenhouse gas emissions. The results also suggest that historical exposure to saline conditions may influence microbial community responses, potentially priming the microbial communities for future increases in salinity.