Aarhus University Seal / Aarhus Universitets segl

Electro biogeochemistry

Cable bacteria use internal, electric wires to transport electrons in the sea bottom over distances of up to several centimeters. The electric pathway through these multicellular, filamentous bacteria enables a subdivision of metabolism so that cells in one end with access to oxygen perform the oxygen reduction steps for all cells, while those with access to electron donors in other places perform the oxidation steps for all. Our discovery of this hitherto unknown pathway in mud from the Bay of Aarhus represents an entire new concept that goes beyond the present paradigms in biogeochemistry and microbial ecology.

The classical view

Biogeochemistry is generally dominated by redox reactions by which the electron donors and acceptors react directly or through electron transport chains inside living cells.  An exception is  that some prokaryotes are able to use solid phase electron acceptors such as ferric oxides through extracellular electron transport mediated by outer‐membrane cytochromes and soluble electron shuttles and possibly also by microbial nanowires. Beyond the micrometer scale, however, there has until the present discovery been no evidence for a coupling between spatially segregated biogeochemical processes in nature.

A new dimension of biogeochemistry

The action of cable bacteria has major impacts on element cycling by redox processes, pH balances, mineral dissolution/precipitations, and electro migration of ions in marine sediment: The presence of an electric mechanism that bridges redox half-reactions in distant regions of the sediment leads to formation of measureable electrical fields which modify ion transport.  The local proton producing and proton consuming half reactions induce pH extremes that accelerate dissolution of iron sulfides and calcium carbonates in anoxic layers and promote the formation of Mg-calcite and iron oxides in the oxic zone. Oxygen and nitrate are used as electron acceptors, and more than 40% of the oxygen consumption in sediments can be driven by long distance electron transfer from distant electron donors. The major e-donor is sulfide which is oxidized to sulfate, and iron sulfides can be the major sources for sulfide. Reports on electric currents and cable bacteria from many, different marine habitats are now coming out from field studies and re-interpretation of literature data.

Cable bacteria genomics

Although no pure cultures of cable bacteria currently exist, these bacteria can be enriched and studied in microcosm setups (cable gardens). Single filaments of cable bacteria can be picked from cable gardens, and consequently be used for whole genome amplification and genome reconstruction. This genomic data is currently being used to learn about the metabolic potential and the ecological role of cable bacteria.