Conductive materials change the composition and activity of butyrate-degrading enrichment cultures with hydrogen as the main electron shuttle

Conductive materials (CM) can improve methane production (MP) efficiency in many methanogenic systems. However, several types of CM exist, and there are uncertainties regarding whether they all improve MP efficiency to the same extent and modulate microbial communities in a similar way. To investigate that, different microbial enrichments with and without activated carbon (AC), magnetite (Mag), and zeolites (Zeo) (at 0.5 g/L) were developed. MP profiles and microbial composition changes were compared among enrichments. The behavior of all enrichments was different, although the initial inoculum sludge was the same. Lag phase duration was lower in AC enrichment, while the complete conversion of butyrate to methane was faster in Mag enrichment. Syntrophomonas was the most abundant bacterial genus in all enrichments, but changes in the methanogenic community were evident. Acetoclastic methanogens were more diverse in Mag enrichment, with microorganisms assigned to Methanosarcina and Methanothrix genera, but Methanothrix was the only acetoclastic methanogen in the other enrichments. On the other hand, different species of hydrogenotrophic methanogens prevailed in distinct enrichments. The metatranscriptomics results revealed that the dominant mechanism of interspecies electron transfer in the AC enrichment utilized hydrogen as the electron carrier, and no evidences of direct interspecies electron transfer (DIET) could be found. These results showed how different CM modulate microbial communities and affect MP efficiency through mechanisms that do not necessarily involve DIET or mediation via CM.