Exocellular electron transfer in anaerobic microbial communities

Abstract

SummaryExocellular electron transfer plays an important role in anaerobic microbial communities that degrade organic matter. Interspecies hydrogen transfer between microorganisms is the driving force for complete biodegradation in methanogenic environments. Many organic compounds are degraded by obligatory syntrophic consortia of proton-reducing acetogenic bacteria and hydrogen-consuming methanogenic archaea. Anaerobic microorganisms that use insoluble electron acceptors for growth, such as iron-and manganese-oxide as well as inert graphite electrodes in microbial fuel cells, also transfer electrons exocellularly. Soluble compounds, like humic substances, quinones, phenazines and riboflavin, can function as exocellular electron mediators enhancing this type of anaerobic respiration. However, direct electron transfer by cell-cell contact is important as well. This review addresses the mechanisms of exocellular electron transfer in anaerobic microbial communities. There are fundamental differences but also similarities between electron transfer to another microorganism or to an insoluble electron acceptor. The physical separation of the electron donor and electron acceptor metabolism allows energy conservation in compounds as methane and hydrogen or as electricity. Furthermore, this separation is essential in the donation or acceptance of electrons in some environmental technological processes, e.g. soil remediation, wastewater purification and corrosion.

Role of “Dehalococcoides” spp. in the Anaerobic Transformation of Hexachlorobenzene in European Rivers

“…For example, as nonfermentative bacteria, Dehalococcoides spp. depend in the environment on the H 2 supplied by other microorganisms (38). Conversely, our results also showed that river water and sediment microorganisms could support the growth and dechlorination activity of Dehalococcoides spp.…”

Extracellular Quinones Affecting Methane Production and Methanogenic Community in Paddy Soil

“…The genus Geobacter has been surprisingly discovered to be capable of forming highly conductive network of pilis that facilitate long-range electron transfer [34,38,39], which is becoming an important feature of Geobacter species in anaerobic environments [17,26,27]. There is increasing evidence that Geobacter species are important syntrophic microorganisms capable of establishing syntrophic association with methanogens [19,29,45]. Recent study demonstrated the possibility of direct interspecies electron transfer (DIET) between Geobacter species and Methanosaeta concilii within upflow anaerobic sludge blanket reactor methanogenic aggregates, which were electrically conductive largely contributed by Geobacter species [32].…”

“…The quinone structure allows humics to mediate extracellular electron transfer from a bacterium to an insoluble electron acceptor or to another bacterium [7,45], which was generally demonstrated by the use of AQDS. Under standard condition, AQDS is considered to be incapable of functioning as an electron carrier in methanogenic consortia because the redox potential of AQDS/AH 2 QDS (E 0 ′=−184 mV) is too high to drive CO 2 reduction to methane (E 0 ′ of CO 2 /CH 4 =−240 mV).…”

Biotechnological aspects of sulfate reduction with methane as electron donor

“…It has been suggested that electrons are transferred from ANME to SRB via extracellular redox shuttles (Widdel and Rabus 2001;Wegener et al 2008), via membrane bound redox shuttles or so called ''nanowires'' (Reguera et al 2005;Stams et al 2006;Thauer and Shima 2008;Wegener et al 2008). If this is indeed the case, the methane oxidizers could selectively be grown on a methane-fed anode and the involved sulfate reducers on a sulfate-fed cathode of a microbial fuel cell.…”