Mapping electron sources and sinks in a marine biogeobattery

Filamentous, multicellular bacteria of the Desulfobulbaceae family form a biogeobattery in marine sediments by mediating an electric coupling between sulfide oxidation in deeper anoxic layers and oxygen reduction at the sediment surface. The electric fields generated in such biogeobatteries were determined with microelectrodes and correlated well with geochemical indicators of microbial electrogenic activity in the top 2 cm of the sediment. The electric fields collapsed within less than a minute when oxygen was removed and reestablished when oxygen was reintroduced, demonstrating a direct coupling between aerobic microbial activity and presence of electric fields. Modeling of the electric potential distribution was used to quantify and localize electron sources and sinks. Most anodic oxidation was confined to the lower part of the anoxic zone with nondetectable free sulfide, suggesting that iron sulfide was the main sulfide source and had already been depleted in the upper part of the anoxic zone. The flow of electrons from anoxic sediment layers fuelled up to 80% of the aerobic activity of the sediment. Responses of the electric field to the presence and absence of nitrate, nitrite, or nitrous oxide in the overlying water readily demonstrated potentials for cathodic reduction of nitrite or nitric oxide and possibly nitrate but not nitrous oxide.