Abstract
In recent years, the externalization of electrons as part of respiratory metabolic processes has been discovered in many different bacteria and some archaea. Microbial extracellular electron transfer (EET) plays an important role in many anoxic natural or engineered ecosystems. In this study, an anaerobic methane-converting microbial community was investigated with regard to its potential to perform EET. At this point, it is not well-known if or how EET confers a competitive advantage to certain species in methane-converting communities. EET was investigated in a two-chamber electrochemical system, sparged with methane and with an applied potential of +400 mV versus standard hydrogen electrode. A biofilm developed on the working electrode and stable low-density current was produced, confirming that EET indeed did occur. The appearance and presence of redox centers at −140 to −160 mV and at −230 mV in the biofilm was confirmed by cyclic voltammetry scans. Metagenomic analysis and fluorescence in situ hybridization of the biofilm showed that the anaerobic methanotroph ‘Candidatus Methanoperedens BLZ2’ was a significant member of the biofilm community, but its relative abundance did not increase compared to the inoculum. On the contrary, the relative abundance of other members of the microbial community significantly increased (up to 720-fold, 7.2% of mapped reads), placing these microorganisms among the dominant species in the bioanode community. This group included Zoogloea sp., Dechloromonas sp., two members of the Bacteroidetes phylum, and the spirochete Leptonema sp. Genes encoding proteins putatively involved in EET were identified in Zoogloea sp., Dechloromonas sp. and one member of the Bacteroidetes phylum. We suggest that instead of methane, alternative carbon sources such as acetate were the substrate for EET. Hence, EET in a methane-driven chemolithoautotrophic microbial community seems a complex process in which interactions within the microbial community are driving extracellular electron transfer to the electrode.
Highlights
In their natural habitats, most microorganisms live in complex communities in which a multitude of interactions with other community members occur
This study investigated whether an anaerobic methane-oxidizing enrichment culture was able to transfer electrons to a carbon cloth electrode poised to a potential of +400 mV vs. standard hydrogen electrode (SHE)
Low-density current was produced with a maximum of 247 μA/cm2 and a final stable current of ~6.5 μA/cm2, which was attributed to several non-methanotrophic bacterial members of the community
Summary
Most microorganisms live in complex communities in which a multitude of interactions with other community members occur. This includes competition for nutrients and may be important to cross-feeding of essential nutrients or removal of toxic intermediates. Anaerobic oxidation of methane (AOM) was first described in con sortia of anaerobic methanotrophic archaea (ANME) and sulfatereducing bacteria (SRB) in marine environments [3,4,5,6,7]. In 2006, a new type of ANME belonging to the ANME-2d subgroup was described, and experimental evidence confirmed that these archaea oxidize methane with concomitant nitrate reduction [8]. In the same culture a bacterial anaerobic methanotroph was identified as ‘Ca
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