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Anaerobic conversion of organic wastes and biomass to methane is an important bioenergy strategy, which depends on poorly understood mechanisms of interspecies electron transfer to methanogenic microorganisms. Metatranscriptomic analysis of methanogenic aggregates from a brewery wastewater digester, coupled with fluorescence in situ hybridization with specific 16S rRNA probes, revealed that Methanosaeta species were the most abundant and metabolically active methanogens. Methanogens known to reduce carbon dioxide with H2 or formate as the electron donor were rare. Although Methanosaeta have previously been thought to be restricted to acetate as a substrate for methane production, Methanosaeta in the aggregates had a complete complement of genes for the enzymes necessary for the reduction of carbon to methane, and transcript abundance for these genes was high. Furthermore …