Distribution, activities, and interactions of methanogens and sulfate-reducing prokaryotes in the Florida everglades

Abstract

To gain insight into the mechanisms controlling methanogenic pathways in the Florida Everglades, the distribution and functional activities of methanogens and sulfate-reducing prokaryotes (SRPs) were investigated in soils (0 to 2 or 0 to 4 cm depth) across the well-documented nutrient gradient in the water conservation areas (WCAs) caused by runoff from the adjacent Everglades Agricultural Area. The methyl coenzymeMreductase gene (mcrA) sequences that were retrieved from WCA-2A, an area with relatively high concentrations of SO42- (≥39 μM), indicated that methanogens inhabiting this area were broadly distributed within the orders Methanomicrobiales, Methanosarcinales, Methanocellales, Methanobacteriales, and Methanomassiliicoccales. In more than 3 years of monitoring, quantitative PCR (qPCR) using newly designed group-specific primers revealed that the hydrogenotrophic Methanomicrobiales were more numerous than the Methanosaetaceae obligatory acetotrophs in SO42-- rich areas of WCA-2A, while the Methanosaetaceae were dominant over the Methanomicrobiales in WCA-3A (with relatively low SO42- concentrations; ≤4 μM). qPCR of dsrB sequences also indicated that SRPs are present at greater numbers than methanogens in the WCAs. In an incubation study with WCA-2A soils, addition of MoO42- (a specific inhibitor of SRP activity) resulted in increased methane production rates, lower apparent fractionation factors [αapp; defined as (amount of δ13CO2 + 1,000)/ (amount of δ13CH4 + 1,000)], and higher Methanosaetaceae mcrA transcript levels compared to those for the controls without MoO42-. These results indicate that SRPs play crucial roles in controlling methanogenic pathways and in shaping the structures of methanogen assemblages as a function of position along the nutrient gradient.