The Identification of Cable Bacteria Attached to the Anode of a Benthic Microbial Fuel Cell: Evidence of Long Distance Extracellular Electron Transport to Electrodes

Abstract

Multicellular, filamentous, sulfur-oxidizing bacteria, known as cable bacteria, were discovered attached to fibers of a carbon brush electrode serving as an anode of a benthic microbial fuel cell (BMFC). The BMFC had been operated in a temperate estuarine environment for over a year before collecting anode samples for scanning electron microscopy and phylogenetic analyses. Individual filaments were attached by single terminus cells with networks of pilus-like nano-filaments radiating out from these cells, across the anode fiber surface, and between adjacent attachment locations. Current harvesting by the BMFC poised the anode at potentials of ~170–250 mV vs. SHE, and these surface potentials appear to have allowed the cable bacteria to use the anode as an electron acceptor in a completely anaerobic environment. A combination of catalyzed reporter deposition fluorescent in situ hybridization (CARD-FISH) and 16S rRNA gene sequence analysis confirmed the phylogeny of the cable bacteria and showed that filaments often occurred in bundles and in close association with members of the genera Desulfuromonas. However, the Desulfobulbaceae Operational Taxonomic Units (OTUs) from the 16S sequencing did not cluster closely with other putative cable bacteria sequences suggesting that the taxonomic delineation of cable bacteria is far from complete.

Inducing the Attachment of Cable Bacteria on Oxidizing Electrodes

“…The electrochemical reactor was anoxic for more than 100 days. The observation of cable bacteria on the anodic carbon fibers confirmed that they can survive under such conditions and were likely using the anode as electron acceptor as suggested previously (Reimers et al, 2017). The closest culturable relative, Desulfobulbus propionicus, can utilize an electrode as an electron acceptor to oxidize S 0 , H2, and organic acids like 275 pyruvate, lactate, and propionate (Holmes et al, 2004).…”

“…To mimic the conditions where cable bacteria were found attached to electrode fibers in a BMFC (Reimers et al, 2017), a bioelectrochemical reactor was assembled from a polycarbonate core tube (15 cm height and 11.5 cm inner diameter, Fig. 2) as a second phase of this research.…”

“…Two-color CARD-FISH was performed on some carbon fiber samples to look for a previously observed co-occurrence 145 of cable bacteria and other electroactive bacteria on electrode surfaces (Reimers et al, 2017). To perform the twocolor CARD-FISH, horseradish peroxidase on the hybridized DSB706 probes were inactivated by 0.15% H2O2.…”

“…1). The IMF and OFS sites are located about 3 km and 7 km, respectively, upstream from the site where the BMFC was deployed in the abovementioned study (Reimers et al, 2017). The top 20 cm of these sediments were sieved through a 0.5 mm mesh 65 size metal screen to remove macrofauna and shell debris.…”

“…In a previous benthic microbial fuel cell (BMFC) experiment (Reimers et al, 2017), we serendipitously observed the attachment of cable bacteria to carbon fibers serving as an anode in an anaerobic environment. This finding suggested that cable bacteria possess the ability to donate electrons to solid electron acceptors.…”