Why do DIETers like drinking: Metagenomic analysis for methane and energy metabolism during anaerobic digestion with ethanol.

Stimulating Methanothrix-dominant communities with ethanol is recently considered as a promising strategy of improving the efficiency and stability of anaerobic digestion (AD), while the effects on methanogenic pathway and energy metabolism linked to the establishment of direct interspecies electron transfer (DIET) were not investigated yet. The results showed that, Methanothrix species were the dominant and metabolically active methanogens in the methanogenic sludge fed with the ethanol-type fermentation products, and the abundance of genes that encoded the key enzymes involved in the reduction of carbon dioxide was significantly higher than that fed with the other products, such as propionate and butyrate. Conversely, the abundance of genes that encoded the key enzymes involved in acetate decarboxylation among all the methanogenic sludge were nearly same. In the presence of ethanol, the abundance of gene for pilA significantly increased. The gene for pliA was primarily derived from Sphaerochaeta, Sedimentibacter and Pseudomonas species that were specially abundant and metabolically active. Further analysis showed that, the abundance of genes that encoded V/A-type ATPase in the methanogenic digesters fed with the ethanol-type fermentation products was 1.3-1.5 folds higher than that fed with the other products. As a result, the concentration of total ATP in the cells was increased by 1.8-2.3 folds. These results, and the fact that DIET is the only electron donor to support the reduction of carbon dioxide in Methanothrix species for the first time revealed the mechanisms involved in the establishment of DIET-based methanogenic metabolism with ethanol.