Syntrophic interactions among anode respiring bacteria (ARB) and Non‐ARB in a biofilm anode: electron balances

Abstract

We demonstrate that the coulombic efficiency (CE) of a microbial electrolytic cell (MEC) fueled with a fermentable substrate, ethanol, depended on the interactions among anode respiring bacteria (ARB) and other groups of micro-organisms, particularly fermenters and methanogens. When we allowed methanogenesis, we obtained a CE of 60%, and 26% of the electrons were lost as methane. The only methanogenic genus detected by quantitative real-time PCR was the hydrogenotrophic genus, Methanobacteriales, which presumably consumed all the hydrogen produced during ethanol fermentation ( approximately 30% of total electrons). We did not detect acetoclastic methanogenic genera, indicating that acetate-oxidizing ARB out-competed acetoclastic methanogens. Current production and methane formation increased in parallel, suggesting a syntrophic interaction between methanogens and acetate-consuming ARB. When we inhibited methanogenesis with 50 mM 2-bromoethane sulfonic acid (BES), the CE increased to 84%, and methane was not produced. With no methanogenesis, the electrons from hydrogen were converted to electrical current, either directly by the ARB or channeled to acetate through homo-acetogenesis. This illustrates the key role of competition among the various H(2) scavengers and that, when the hydrogen-consuming methanogens were present, they out-competed the other groups. These findings also demonstrate the importance of a three-way syntrophic relationship among fermenters, acetate-consuming ARB, and a H(2) consumer during the utilization of a fermentable substrate. To obtain high coulombic efficiencies with fermentable substrates in a mixed population, methanogens must be suppressed to promote new interactions at the anode that ultimately channel the electrons from hydrogen to current.

Influence of External Resistance on Electrogenesis, Methanogenesis, and Anode Prokaryotic Communities in Microbial Fuel Cells

“…Another study showed that a flowthrough system fed with acetate (5.4 kg chemical oxygen demand [COD]/m 3 /day) had increasing methane production as the anode potential decreased in the anode potential range of Ϫ300 mV Ͻ E an Ͻ Ϫ100 mV versus SHE (55). In an ethanol-fed MFC, hydrogenotrophic Methanomicrobiales was the only detected methanogen (38). In MEC systems, hydrogenotrophic methanogenesis has been proposed as the main methanogenic pathway due to hydrogen flux to the anode (7,26).…”

Spatiotemporal development of the bacterial community in a tubular longitudinal microbial fuel cell

“…Previous studies suggest the existence of competition between methanogens and electricity producing bacteria for H 2 (Freguia et al, 2008) and acetate (Virdis et al, 2009). Methane production has been observed in MFCs fed with fermentable substrates (glucose and ethanol) (Freguia et al, 2007;Kim et al, 2007;Lee et al, 2008;Parameswaran et al, 2009), and in acetate fed MFCs at the early stages of enrichment when using digester sludge inoculum (Kim et al, 2005). However the serial transfer of enriched electrodes under exo-electrogenic conditions diminishes methane production in acetate fed MFCs, suggesting that acetate-oxidizing exoelectrogens are capable of out-competing acetoclastic methanogens as it would be thermodynamically favourable (Lovley & Phillips, 1988).…”

“…Using short-term kinetic tests with H 2 , Freguia et al (2008) showed that part of the anodic H 2 produced by glucose fermentation was used by hydrogen-oxidizing methanogens. By electron balance and finding that the hydrogenotrophic methanogenic genus Methanobacteriales was the only methanogen present in the anodic biofilm (~4% of the total microbial community), Parameswaran et al (2009) demonstrated that the electrons available in H 2 from ethanol fermentation were routed to CH 4 . A hydrogenotrophic methanogen, Methanobacterium bryantii, dominated the archaeal community in a cellulose-fed MFC (Ishii et al, 2008), whereas acetoclastic methanogens accounted for ~19% and hydrogenotrophic methanogens accounted only for ~5% of the total microbial community in a glucose-fed MFC (Chung & Okabe, 2009).…”

“…However, other bacteria able to oxidize H 2 (Pantoea, Klebsiella, Desulfovibrio and Geobacter) were found in increased relative proportions at the end of the process and could also have been competing for H 2 with methanogens. Parameswaran et al (2009;2010) showed the abundance of homo-acetogens and electron flow from H 2 to current via acetate in ethanol-fed MFC only when methanogenesis was inhibited. Therefore they suggested promoting of growth of homo-acetogens in anodic biofilm based on their higher tolerance to oxygen and ability to out-compete hydrogenotrophic methanogens at low temperature and pH in order to prevent electron flow to CH 4 (Parameswaran et al, 2010).…”

Alamethicin Suppresses Methanogenesis and Promotes Acetogenesis in Bioelectrochemical Systems

“…Both of these inhibitors suppress methanogenesis only temporarily, which can resume once the chemicals are removed from solution. They are not considered a viable strategy for long-term and large-scale MES systems because the amount added (up to 50 mM BES) can be toxic to other microorganisms, and the use of these chemicals at high concentrations can be uneconomical (9). Like BES and methyl fluoride, alamethicin is an inhibitor of methanogenesis, as demonstrated by a previous study on different peptide antibiotics (10).…”