Cable Bacteria in Freshwater Sediments

In marine sediments cathodic oxygen reduction at the sediment surface can be coupled to anodic sulfide oxidation in deeper an-oxic layers through electrical currents mediated by filamentous, multicellular bacteria of theDesulfobulbaceae family, the so-called cable bacteria. Until now, cable bacteria have only been reported frommarine environments. In this study, we demon-strate that cable bacteria also occur in freshwater sediments. In a first step, homogenized sediment collected from the freshwater streamGiber Å, Denmark, was incubated in the laboratory. After 2 weeks, pH signatures and electric fields indicated electron transfer between vertically separated anodic and cathodic half-reactions. Fluorescence in situ hybridization revealed the pres-ence ofDesulfobulbaceae filaments. In addition, in situmeasurements of oxygen, pH, and electric potential distributions in the waterlogged banks of Giber Å demonstrated the presence of distant electric redox coupling in naturally occurring freshwater sediment. At the same site, filamentousDesulfobulbaceaewith cable bacteriummorphology were found to be present. Their 16S rRNA gene sequence placed them as a distinct sister group to the knownmarine cable bacteria, with the genusDesulfobulbus as the closest cultured lineage. The results of the present study indicate that electric currents mediated by cable bacteria could be important for the biogeochemistry in manymore environments than anticipated thus far and suggest a common evolutionary origin of the cable phenotype withinDesulfobulbaceaewith subsequent diversification into a freshwater and a marine lineage. Filamentous Deltaproteobacteria of the Desulfobulbaceaefamily—so-called cable bacteria (1)—mediate an electron current that links spatially separated cathodic and anodic redo