Clarifying electron transfer and metagenomic analysis of microbial community in the methane production process with the addition of ferroferric oxide

Abstract Direct interspecies electron transfer (DIET) is a new mechanism responsible for syntrophic methane production. Supplementation of conductive material in syntrophic communities can facilitate electron transfer and enhance methane production. Ferroferric oxide (Fe3O4) was dosed in an anaerobic sequencing batch reactor (ASBR) fed with tryptone-based synthetic wastewater to examine its effect on the anaerobic treatment process. Long-term dosage of Fe3O4 not only enhanced the maximum methane production rate by 78.3% in a reaction cycle but also improved methane production when hydrogen/carbon dioxide or acetate was used as the substrate. The conductivity of anaerobic sludge, activity of the electron transport chain, and extracellular electron transfer ability were enhanced with the addition of Fe3O4, inducing enhanced system performance for methane production. Proteiniclasticum and Prolixibacter were the enriched acidogens responsible for hydrolysis and acidification. With the addition of Fe3O4, Methanosarcina was the dominant methanogen, and metagenomic analysis further revealed the genes involved in the hydrogenotrophic pathway of methanogenesis. Methanosarcina might be involved in DIET, and Fe3O4 was responsible for its stimulation.