Tracing long-distance electron transfer and cable bacteria in freshwater sediments by facilitated diffusion gradient columns

Cable bacteria are capable of oxygen-dependent, electrogenic sulfide oxidation by spatially separating redox half reactions over cm-distances, thus overcoming traditional concepts of diffusion limitation and redox stratification in aquatic sediments. However, their occurrence and ecology in freshwater sediments is not yet well understood, same as possible strategies to selectively enrich and investigate their populations in laboratory systems. Here, we introduce a new agar pillar approach for cable bacteria investigation, by using facilitated diffusion agar pillars within sediment gradient columns. Sediment samples from a selection of freshwater systems (stream, lake, aquifer) were incubated in laboratory mesocosms, including an embedded central agar pillar intended as selective compartment with facilitated diffusion. Counter-gradients of electron donors and acceptors were followed by microprofiling during incubation. After 4 weeks, community composition in sediments and in agar pillars was surveyed by PacBio full-length bacterial 16S amplicon sequencing.