![]() ![]() This setup established an observation zone where cable bacteria stretched out from the sediment to the oxic–anoxic interface, which was clearly delineated by a microaerophilic veil of motile, aerobic bacteria (Fig. Electronema aureum GS 16 were observed under semi-natural conditions on a microscope slide (a so-called trench slide), where oxygen diffused in from the edge of the coverslip, while organic matter, sulfide, and other nutrients were provided from sediment in a central compartment (trench) 17, 18. ![]() Here we use a combination of microscopic observations, metagenome sequencing, laser microdissection, and Raman microscopy to demonstrate the dynamics and intimacy of this association, tentatively identify the bacteria involved, and propose a likely mechanism for electron transfer between associated bacterial flocks and cable bacterial filaments.īacteria flock around cable bacteria and appear metabolically stimulatedĬable bacteria from an enrichment of the freshwater strain Ca. These apparent associations have led to speculations that bacteria in anoxic sediment somehow may use cable bacteria as an electron conduit to oxygen 14, 15. In freshwater sediments, they can cause a 4.5-fold stimulation of sulfate reduction and drastically lower methane emission 10, 11.Ĭable bacteria activity has also been linked to enhanced carbon assimilation of autotrophic sulfide oxidizers 12 and correlated with the distribution of iron-cycling bacteria in marine sediments 13. Via pH gradients and electric fields induced by their relocation of electrons, they also indirectly influence iron, calcium, cobalt, and arsenic cycling and all ion fluxes in their habitats 6, 7, 8, 9. They occur globally in marine and freshwater sediments and aquifers 3, 4, 5 and interfere directly with sulfur, oxygen, carbon, and nitrogen cycling 6. The association and close interaction with such diverse partners might explain how oxygen via cable bacteria can affect microbial communities and processes far into anoxic environments.Ĭable bacteria are long filamentous bacteria that can transmit electrons over centimeter distances and thereby couple the oxidation of sulfide to the remote reduction of oxygen or nitrate 1, 2. Metagenomic analysis indicates that most of the flocking bacteria appear to be aerobes, including organotrophs, sulfide oxidizers, and possibly iron oxidizers, which might transfer electrons to cable bacteria for respiration. Raman microscopy shows that flocking bacteria are more oxidized when closer to the cable bacteria, but physical contact seems to be rare and brief, which suggests potential transfer of electrons via unidentified soluble intermediates. Here, we report that diverse bacteria swim in a tight flock around the anoxic part of oxygen-respiring cable bacteria and disperse immediately when the connection to oxygen is disrupted (by cutting the cable bacteria with a laser). ![]() This activity induces geochemical changes in the sediment, and other bacterial groups appear to benefit from the electrical connection to oxygen. Cable bacteria are centimeter-long filamentous bacteria that conduct electrons via internal wires, thus coupling sulfide oxidation in deeper, anoxic sediment with oxygen reduction in surface sediment. ![]()
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