Cooperative microbial interactions mediate community biogeochemical responses to saltwater intrusion in wetland soils

In freshwater wetlands, competitive and cooperative interactions between respiratory, fermentative and methanogenic microbes mediate the decomposition of organic matter. These interactions may be disrupted by saltwater intrusion disturbances that enhance the activity of sulfate-reducing bacteria (SRB), intensifying their competition with syntrophic bacteria and methanogens for electron donors. We simulated saltwater intrusion into wetland soil microcosms and examined biogeochemical and microbial responses, employing metabolic inhibitors to isolate the activity of various microbial functional groups. Sulfate additions increased total carbon dioxide production but decreased methane production. Butyrate degradation assays showed continued (but lower) levels of syntrophic metabolism despite strong demand by SRB for this key intermediate decomposition product and a shift in the methanogen community toward acetoclastic members. One month after removing SRB competition, total methane production recovered by only ∼50%. Similarly, butyrate assays showed an altered accumulation of products (including less methane), although overall rates of syntrophic butyrate breakdown largely recovered. These effects illustrate that changes in carbon mineralization following saltwater intrusion are driven by more than the oft-cited competition between SRB and methanogens for shared electron donors. Thus, the impacts of disturbances on wetland biogeochemistry are likely to persist until cooperative and competitive microbial metabolic interactions can recover fully.