Microorganisms can exchange electrons over distances exceeding 10.000 times their own length through a conductive network. This electric pathway enables a mutually favourable subdivision of metabolism so that those bacteria with access to oxygen perform the oxygen reduction steps for all the connected organisms, while those with access to electron donors perform the oxidation steps for the community. 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 now been no evidence for a coupling between spatially segregated biogeochemical processes in nature.

A new dimension of biogeochemistry

Natural conductors in marine sediment may transmit electrons over centimeter distances, supporting reactions and interactions of distant electron donors, electron acceptors, and microorganisms. The action of the organisms involved in this long range electron transport 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 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 seems to be the major electron acceptor, 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 are the major sources for sulfide in the system.  We expect that electrically coupled biogeochemistry flourishes in marine sediments after transient oxygen depletion, leaving distinct signatures of such events in the geological record.

The microbial players

The conductive network has not been described yet for marine sediments but there are several possibilities.  Through our experiments we can conclude that microorganisms play an important role in long range electron transport.  The identity of these microorganisms and the mode by which they can transport electrons over long distances through the sediment has not been resolved yet.  However, we are very close to a solution of the problem.

• Risgaard-Petersen, N. , Revilc, A , Meister P. and  Nielsen L.P. Sulfur, iron-, and calcium cycling  associated with natural electric currents running through marine sediment. 2012 Geochemical et Cosmochima acta;  e-pub ahead of print: http://dx.doi.org/10.1016/j.gca.2012.05.036
• Nielsen, L.P., Risgaard-Petersen, N., Fossing, H., Christensen, P.B., Sayama, M. 2010, 'Electric currents couple spatially separated biogeochemical processes in marine sediment', Nature, vol. 463 no. 7284, pp. 1071-1074