Methanogenic pathway, 13C isotope fractionation, and archaeal community composition in the sediment of two clear-water lakes of Amazonia

We studied the methanogenic pathway and archaeal community composition in the sediment of two clear-water lakes, Lake Batata and Lake Mussura, in Amazonia. We measured CH4 production and delta C-13 of CO2, CH4, and acetate-methyl in the presence and absence of CH3F, an inhibitor of acetotrophic methanogenesis. The fractionation factor of methanogenesis from CO2 was rather high in both lake sediments, which was consistent with the low concentrations of H-2 and the small negative Gibbs free energy of hydrogenotrophic methanogenesis. The delta C-13 of acetate-methyl was relatively low compared to that of organic matter and decreased further upon inhibition of acetate consumption by CH3F. Collectively, the data possibly suggest involvement of syntrophic acetate oxidation besides acetotrophic methanogenesis. The isotopic data were used to calculate the percent contribution of CO2 reduction to total methanogenesis, which was rather high (approximately 53-63%). Copy numbers of bacterial and archaeal 16S ribosomal ribonucleic acid (rRNA) genes were about 10-fold higher in Lake Mussura than in Lake Batata, indicating that microbial numbers were not a limiting factor for production rates of CH4, which were similar in both lake sediments. The composition of the archaeal community was analyzed by cloning and sequencing of the genes coding for 16S rRNA and methyl coenzymeMreductase (mcrA), demonstrating the presence of acetotrophic Methanosaetaceae and different hydrogenotrophic methanogenic orders (Methanomicrobiales, Methanobacteriales, Methanocellales) in both lake sediments. Although methanogenic communities and pathways were principally comparable to those found in lake sediments of the midlatitudes, there were several particularities, e.g., the possible involvement of syntrophic acetate oxidation.