Assessing how disruption of the methanogenic community and their syntrophic relationships in tidal freshwater marshes via saltwater intrusion may affect CH4 emissions
Tidal freshwater wetlands are predicted to experience moderate increases of salinity due to sea level rise. One consequence is saltwater intrusion is a change in the pathways of anaerobic carbon mineralization and a shift from methanogenesis (MG), which dominates in freshwater systems, to sulfate reduction, which dominates under saline conditions. Some functional guilds of MG (e.g., hydrogenotrophic MG) can survive via syntrophic relationships wherein they consume the by-products of primary and secondary fermentation (e.g., CO2, H2, and low molecular weight fatty acids (e.g., butyrate and acetate)) and, in turn, facilitate the growth of fermenters by limiting accumulation of their waste. Other functional groups of MG (e.g., acetogens) can live independently.
We hypothesized that: (i) saltwater intrusion into tidal freshwater wetlands would disrupt syntrophic degradation of organic matter and select for a distinct MG community, most likely dominated by acetogens, and (ii) syntrophic CH4 production and MG community structure will not be restored when freshwater conditions return. Although MGs are monophyletic group within the Euryarchaeota phylum, not all of the taxa contain the same metabolic and physiological capabilities, so these sorts of shifts in community composition and interspecies interactions could have a long-term effect on the rates and pathways of sediment organic matter decomposition.
To test these hypotheses, we exposed soil microcosms to elevated sulfate concentrations and monitored CH4 and CO2 flux, diversity of MG and bacteria taxa, and expression of key functional genes. To assess syntrophic activity, we used butyrate degradation assays that combined metabolic inhibitors of methanogenesis and sulfate reduction.
Butyrate degradation assays confirmed the significance of syntrophic relationships in the methane production in control (freshwater) microcosms. Following addition of sulfate to the microcosms, CH4 production decreased significantly. Production increased slightly upon the removal of sulfate stress (via complexation with sodium molybdate (Na2MoO4)) but did not return to pre-disturbance levels. Further, the proliferation of sulfate reducers shifted fatty acid degradation away from methanogen-mediated pathways and, as with CH4 production, function was not restored after the removal of sulfate stress. Overall, these results suggest the saltwater intrusion events will disrupt syntrophic degradation of organic matter, select for methanogens that are adapted to different metabolic niches, and influence the microbial community’s response to subsequent salinity increases. Further, these results suggest that changes in the MG community may impact greenhouse gas emissions from these ecosystems.