Biodegradation of Azo Dye Methyl Red by Methanogenic Microbial Communities Isolated from Volga River Sediments

Azo dyes are soluble xenobiotics stable under oxidizing conditions, which are widely used in human practice; they are present in liquid and solid industrial and household wastes and regularly enter the environment. In this work, we investigated the possibility of degradation of the technical azo dye Methyl Red (MR) by anaerobic microbial communities isolated from the Volga River sediments, and MR effect on community composition and methanogenic activity. This is the first report on ability of such azo dye-unadopted communities to degrade MR with production of stable N,N-dimethyl-p-phenylenediamine and biodegradable 2-aminobenzoic acid. Comparison of methanogenesis rates in communities with and without MR revealed a decrease in biogas production by 43.80% due to the toxic effect of MR (and, possibly, of aromatic intermediates of its decomposition) on microorganisms. Next-generation sequencing of the 16S rRNA gene revealed significant changes in the structural and functional organization of the methanogenic community in the presence of MR and a shift among the dominant groups. In the community with MR the share of bacteria of the family Geobacteriaceae increased almost 5-fold, while that of the family Clostridiaceae decreased 3‑fold, and the genus Proteiniclasticum became dominant. In the presence of MR, representatives of the families Methanobacteriaceae, Methanofastidiosaceae, Methanoregulaceae, Methanosaetaceae, and Methanomassillicoccaceae, which constituted 33.32% of the total number of archaea in the initial community, were not detected. An increase in the proportion of microorganisms of the families Desulfоvibrionaceae, Desulfosarcinaceae, and Gallionellaceae was presumably related to their possible involvement in MR degradation, since they usually act as syntrophs in methanogenic communities. MR decolorization was confirmed to require the presence of living cells, adsorption being only its initial stage, and the effect of chemical reduction of the azo bond was minimal. Our preliminary laboratory model shows that while natural communities are potentially capable of destroying MR, the azo dye also has a significant effect on their structure.