Microbial community structure in a biofilm anode fed with a fermentable substrate: The significance of hydrogen scavengers

We compared the microbial community structures that developed in the biofilm anode of two microbial electrolysis cells fed with ethanol, a fermentable substrate—one where methanogenesis was allowed and another in which it was completely inhibited with 2‐bromoethane sulfonate. We observed a three‐way syntrophy among ethanol fermenters, acetate‐oxidizing anode‐respiring bacteria (ARB), and a H2 scavenger. When methanogenesis was allowed, H2‐oxidizing methanogens were the H2 scavengers, but when methanogenesis was inhibited, homo‐acetogens became a channel for electron flow from H2 to current through acetate. We established the presence of homo‐acetogens by two independent molecular techniques: 16S rRNA gene based pyrosequencing and a clone library from a highly conserved region in the functional gene encoding formyltetrahydrofolate synthetase in homo‐acetogens. Both methods documented the presence of the homo‐acetogenic genus, Acetobacterium, only with methanogenic inhibition. Pyrosequencing also showed a predominance of ethanol‐fermenting bacteria, primarily represented by the genus Pelobacter. The next most abundant group was a diverse community of ARB, and they were followed by H2‐scavenging syntrophic partners that were either H2‐oxidizing methanogens or homo‐acetogens when methanogenesis was suppressed. Thus, the community structure in the biofilm anode and suspension reflected the electron‐flow distribution and H2‐scavenging mechanism. Biotechnol. Bioeng. 2010;105: 69–78. © 2009 Wiley Periodicals, Inc.