Mineral formation induced by cable bacteria performing long-distance electron transport in marine sediments.

Cable bacteria are multicellular, filamentous microorganisms that are capable of transporting electrons over centimeter-scale distances. Although recently discovered, these bacteria appear to be widely present in the seafloor, and when active, they exert a strong imprint on the local geochemistry. In particular, their electrogenic metabolism induces unusually strong pH excursions in aquatic sediments, which induces considerable mineral dissolution, and subsequent mineral re-precipitation. However at present, it is unknown whether and how cable bacteria play an active or direct role in the mineral re-precipitation process. To this end we present an explorative study of the formation of sedimentary minerals in and near filamentous cable bacteria using a combined approach of electron microscopic and spectroscopic techniques. Our observations reveal three different types of biomineral formation directly associated with cable bacteria: (1) the formation of intracellular polyphosphate granules, which are associated with a localized accumulation of calcium and magnesium, (2) the attachment and incorporation of clay particles in a sheath surrounding the surface of the cable bacterium filaments, and (3) the encrustation of cable bacteria filaments by newly formed solid phases containing high amounts of iron. These findings suggest a complex interaction between cable bacteria and the surrounding sediment matrix, and a substantial imprint of the electrogenic metabolism on mineral diagenesis and sedimentary biogeochemical cycling. Particularly the encrustation process leaves many open questions for further research. For example, we hypothesize that the complete encrustation of filaments might create a diffusion barrier and negatively impact the metabolism of the cable bacteria. [ABSTRACT FROM AUTHOR]