Some bacteria are capable of extracellular electron transfer, thereby enabling them to use electron acceptors and donors without direct cell contact. Beyond the micrometre scale, however, no firm evidence has previously existed that spatially segregated biogeochemical processes can be coupled by electric currents in nature. Here we provide evidence that electric currents running through defaunated sediment couple oxygen consumption at the sediment surface to oxidation of hydrogen sulphide and organic carbon deep within the sediment. Altering the oxygen concentration in the sea water overlying the sediment resulted in a rapid (<1-h) change in the hydrogen sulphide concentration within the sediment more than 12 mm below the oxic zone, a change explicable by transmission of electrons but not by diffusion of molecules. Mass balances indicated that more than 40% of total oxygen consumption in the sediment was driven by electrons conducted from the anoxic zone. A distinct pH peak in the oxic zone could be explained by electrochemical oxygen reduction, but not by any conventional sets of aerobic sediment processes. We suggest that the electric current was conducted by bacterial nanowires combined with pyrite, soluble electron shuttles and outer-membrane cytochromes. Electrical communication between distant chemical and biological processes in nature adds a new dimension to our understanding of biogeochemistry and microbial ecology.

Abstract. A procedure which includes the Total Reduced Inorganic Sulfur (TRIS) in a single distillation step is described for the radiotracer measurement of sulfate reduction in sediments.The TRIS includes both Acid Volatile Sulfide (AVS: H,S + FeS) and the remaining Chromium Reducible Sulfur (CRS: So, FeS,). The single-step distillation was simpler and faster than the consecutive distillations of AVS and CRS. It also resulted in higher (4-50%) sulfate reduction rates than those obtained from the sum of "S in AVS and CRS. The difference was largest when the sediment had been dried after AVS but before CRS distillation. Relative to the 3SS-AVS distillation alone, the "S-TRIS single-step distallation yielded 8-87% higher reduction rates. The separation and recovery of FeS, So and FeS, was studied under three distillation conditions: 1) cold acid, 2) cold acid with C 9 + , and 3) hot acid with Cr2+. The FeS was recovered by cold acid alone while pyrite was recovered by cold acid with C?+ . A smaller So fraction, presumably of the finer crystal sizes, was recovered also in the cold acid with C?+ while most of the So required hot acid with C?+ for reduction to H,S.

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