The source material was collected from Chesapeake Bay (38.55505 N, 76.42794 W) (mesohaline), water column depth 26 meters, sediment horizon 0-10 centimeter below sea floor.

Incubation conditions: 16 degrees Celsius, S=15.5, dark, aerated.

Incubation Set-up: Sediments were homogenized and packed into polycarbonate core liners, sealed with a stopper at the bottom, and open to aerated aquarium water at the top. In a subset of cores, a polycarbonate filter (pore size 0.2 microns) was secured at 0.5 cm depth, to prevent downward growth of cable bacteria in these treatments. Sediments were incubated for up to 46 days. At 6 time points, microsensor profiles were measured, followed by destructive sampling.

Porewater extraction: Sediments were sectioned at 0.5 cm depth increments in an anaerobic glove bag under nitrogen atmosphere. Porewaters were separated by centrifugation (3500 rpm for 10 minutes), and filtered (0.2 micron) and aliquoted in the anaerobic glove bag. Samples for ferrous iron measurements were preserved with trace-metal grade nitric acid (final pH < 2).

Geochemical Measurements: Porewater sulfate and chloride were analyzed by suppressed ion chromatography following 100-fold dilutions (Dionex Integrion IC). Porewater ferrous iron (Fe2+) was measured colorimetrically using the Ferrozine assay. Ammonium was measured colorimetrically using the phenol hypochlorite method.

Cell Enumeration: Samples for microbial cell enumeration were preserved in 96% molecular grade ethanol (1:1 v/v), gently mixed with sterile autoclaved toothpicks, and stored at -20°C. Sediment-associated cells were separated from sediment grains using an acetate buffer to dissolve carbonates, followed by washing with NaCl solution, and then gentle vortexing with a detergent mixed with methanol (Lunau et al. 2005, Kallmeyer et al. 2008). Cells were recovered using density centrifugation with Nycodenz (50% w/v). Cable bacteria filaments were enumerated using fluorescence in situ hybridization (FISH) with the DSB706 oligoprobe (Malkin et al. 2022), and total cell counts were enumerated following staining with SyBR Green I.

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