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 reprecipitation. However, at present, it
is unknown whether and how cable bacteria play an active or direct role in
the mineral reprecipitation 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 microscopy and spectroscopic
techniques. Our observations reveal the formation of polyphosphate granules
within the cells and two different types of biomineral formation directly
associated with multicellular filaments of these cable bacteria: (i) the
attachment and incorporation of clay particles in a coating surrounding the
bacteria and (ii) encrustation of the cell envelope by iron minerals. 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. In
particular, 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.</p
