The effect of oxygen availability on long-distance electron transport in marine sediments

Cable bacteria are long, multicellular, filamentous bacteria that can conduct electrons over centimeter distances in marine and freshwater sediments. Recent studies indicate that cable bacteria are widely present in many coastal environments, where they exert a major influence on the biogeochemistry of the sediment. Their energy metabolism can be based on the aerobic oxidation of sulfide, and hence to better understand their natural occurrence and distribution, we examined the growth and activity of cable bacteria in relation to bottom water oxygenation. To this end, we conducted laboratory sediment incubations at four different O-2 levels in the overlying water (10%, 20%, 40%, and 100% air saturation). The abundance of cable bacteria was determined by fluorescence in situ hybridization, while their activity was assessed via microsensor profiling and geochemical pore-water analysis. Cable bacteria did not develop in the 10% air saturation O-2 incubation but were present and active at all higher O-2 levels. These data show that microbial long-distance electron transport can occur under a wide range of bottom water O-2 concentrations. However, the growth rate was notably slower at lower oxygen concentrations, suggesting a reduced metabolic activity of the population when the O-2 supply becomes restricted. Finally, in response to lower O-2 levels, cable bacteria filaments appear to partially emerge out of the sediment and extend into the overlying water, thus likely enhancing their oxygen supply.