Evidence for cable bacteria inhabiting deep in anoxic sediment reveals a novel ecological niche

Background Cable bacteria are filamentous sulfide oxidizers capable of electron transport over cm-scale distances. Traditionally, they are thought to inhabit only the upper few cm of sediment, where they couple sulfide oxidation to oxygen or nitrate reduction. Despite their influence on redox gradients, trace metal mobility, and nutrient cycling, their presence and activity in deep anoxic sediments remain undocumented. We investigated the presence and activity of marine cable bacteria (CandidatusElectrothrix) at four stations in Sweden and Finland, including deep vertical profiles of anoxic sediments. Results Using metatranscriptomic data for both rRNA-based community profiling and gene expression analysis, along with porewater geochemistry data from four stations in Sweden and Finland, we detected metabolically active Ca. Electrothrix in both regions. In Koljö Fjord (Sweden West Coast), Ca. Electrothrix was unexpectedly abundant deep in anoxic layers, with peak abundance below 20 cm. Phylogenetic analyses revealed a diverse assemblage spanning multiple Ca. Electrothrix clades, suggesting that novel lineages adapted to these conditions. Genes for nitrate respiration (napA), sulfide oxidation (sqr), and nickel uptake were highly expressed, indicating in-situ activity. Gene expression patterns were aligned with a sulfide-rich zone and a sharp nitrate peak below 20 cm. This nitrate peak likely results from sulfammox (i.e., anaerobic oxidation of ammonium by sulfate), driven by associated sulfammox bacteria such as Bacillus benzoevorans, Ca. Anammoxoglobus, and Bacillus cereus. Conclusions Our findings reveal a previously unrecognized niche for cable bacteria deep in anoxic sediment layers, where local nitrate production via sulfammox and sulfide availability may sustain their activity, independent of electron acceptors near the surface. This discovery challenges existing models of cable bacteria ecology and suggests alternative physiological modes. Furthermore, the results expand the ecological scope of marine cable bacteria, highlighting potential syntrophic relationships deep in anoxic sediment layers, offering insights into both modern biogeochemical processes and analogs of early Earth microbial ecosystems.