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Electrogenic sulfur oxidation (e-SOx) is a newly discovered pathway of microbial sulfide oxidation, mediated by so-called cable bacteria. The reduction of oxygen near the sediment-water interface is coupled by long-distance electron transport to the oxidation of sulfide in deeper sediment, and in this way, electrical currents are induced within the seafloor that range over centimeter scale distances. Previously, electrogenic sulfur oxidation has been shown to generate extreme pH excursions in the pore water, and as a result, the process strongly amplifies the cycling of various pH-sensitive minerals, such sulfide minerals and carbonates. Here we show that e-SOx also strongly influence the early diagenesis of trace metals in coastal sediments. For this, field observations at a shallow subtidal site in the North Sea were combined with dedicated laboratory incubations of repacked sediments. High resolution microsensor profiling (pH, H 2 S and O 2 ) confirmed the typical geochemical signature of e-SOx both in situ as in the laboratory experiments. Pore water analysis revealed a strong mobilization of both arsenic and cobalt within the electro-active sediment zone. The dissolution of iron sulfides, resulting from the acidification of the pore water by e-SOx, appears to be the main driver for the observed release of cobalt and arsenic. The current generated by e-SOx affects charged species. In this manuscript, we have proposed a new estimation method for the associated current density, based on the rate of sulfide oxidation. Overall, electrogenic sulfur oxidation has a major impact on the cycling of arsenic and cobalt in coastal sediments, and may substantially increase the effluxes of these trace metals to the coastal ocean. [ABSTRACT FROM AUTHOR]

The East Anglian salt marsh system (UK) has recently generated intriguing data with respect to sediment biogeochemistry. Neighbouring ponds in these salt marshes show two distinct regimes of redox cycling: the sediments are either iron-rich and bioturbated, or they are sulphide-rich and unbioturbated. No conclusive explanation has yet been given for this remarkable spatial co-occurrence. Here, we quantify the geochemical cycling in both pond types, using pore-water analyses and solid-phase speciation. Our results demonstrate that differences in solid-phase carbon and iron inputs are likely small between pond types, and so these cannot act as the direct driver of the observed redox dichotomy. Instead, our results suggest that the presence of bioturbation plays a key role in the transition from sulphur-dominated to iron-dominated sediments. The presence of burrowing fauna in marine sediments stimulates the mineralisation of organic matter, increases the iron cycling and limits the build-up of free sulphide. Overall, we propose that the observed dichotomy in pond geochemistry is due to alternative stable states, which result from non-linear interactions in the sedimentary iron and sulphur cycles that are amplified by bioturbation. This way, small differences in solid phase input can result in very different regimes of redox cycling due to positive feedbacks. This non-linearity in the iron and sulphur cycling could be an inherent feature of marine sediments, and hence, alternative stable states could be present in other systems. [ABSTRACT FROM AUTHOR]