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

Abstract

Background: Cable bacteria are filamentous sulphide-oxidisers capable of cm-scale electron transport. They are generally considered restricted to the upper few oxic-suboxic cm of marine sediments, where they couple sulphide oxidation to oxygen or nitrate reduction. Despite their influence on redox gradients, trace metal mobility, and nutrient cycling, their presence and activity in deeper anoxic sediment layers remain unknown. The presence and activity of marine cable bacteria (Candidatus Electrothrix) were investigated at four stations in Sweden and Finland, including deep vertical profiles of anoxic sediment layers, to assess their presence and activity under different environmental contexts.

Results: Using metatranscriptomic data for rRNA-based community profiling and gene expression combined with porewater geochemistry, evidence of abundant and active cable bacteria was found, peaking below 20 cm depth in deep anoxic sediment layers of Koljö Fjord on the Swedish West Coast. This zone coincided with elevated gene expressions related to sulphide oxidation (including sqr) and nitrate reduction (napA), as well as an abundant presence of sulphide and a sharp nitrate peak. Phylogenetic analyses revealed a diverse assemblage of Ca. Electrothrix includes several potential novel taxa. The co-occurrence of cable bacteria activity, sulphide availability, and a nitrate peak at depth suggests that these organisms may be supported by local nitrate production under anoxic conditions.

Conclusions: Our findings challenge the prevailing view that cable bacteria are restricted to shallow sediment horizons and demonstrate their activity and diversity in deep, anoxic layers. This expands the known ecological niche of cable bacteria and suggests that locally produced nitrate under anoxic conditions may facilitate their activity at depth. This discovery advances our understanding of ecology in anoxic marine environments, providing new insights into marine cable bacteria, sediment biogeochemistry, and analogues of early Earth microbial ecosystems.