Inducing the attachment of cable bacteria on oxidizing electrodes

Abstract

Abstract. Cable bacteria (CB) are multicellular, filamentous bacteria within the family of Desulfobulbaceae that transfer electrons longitudinally from cell to cell to couple sulfide oxidation and oxygen reduction in surficial aquatic sediments. In the present study, electrochemical reactors that contain natural sediments are introduced as a tool for investigating the growth of CB on electrodes poised at an oxidizing potential. Our experiments utilized sediments from Yaquina Bay, Oregon, USA, and we include new phylogenetic analyses of separated filaments to confirm that CB from this marine location cluster with the genus “Candidatus Electrothrix”. These CB may belong to a distinctive lineage, however, because their filaments contain smaller cells and a lower number of longitudinal ridges compared to cables described from other locales. The results of a 135 d bioelectrochemical reactor experiment confirmed that these CB can migrate out of reducing sediments and grow on oxidatively poised electrodes suspended in anaerobic seawater. CB filaments and several other morphologies of Desulfobulbaceae cells were observed by scanning electron microscopy and fluorescence in situ hybridization on electrode surfaces, albeit in low densities and often obscured by mineral precipitation. These findings provide new information to suggest what kinds of conditions will induce CB to perform electron donation to an electrode surface, further informing future experiments to culture CB outside of a sediment matrix.

Using Oxidative Electrodes to Enrich Novel Members in the Desulfobulbaceae Family from Intertidal Sediments

“…The two beakers, Reactor A (RA), in which the 3 electrodes were supported above the sediment-water interface, was built by packing 8 cm of sediment inside the reactor. This is the same experiment described in Li et al (2020) [39]. Reactor B (RB), in which the 3 electrodes were set 2 cm below the sediment-water interface, was built by packing 11 cm of sediment inside the reactor.…”

“…Sediment sub-core samples were frozen at −50 • C for storage and later subjected to DNA extraction. Filamentous biomass was extracted from sediment sub-cores as described in Li et al (2020) [39].…”

“…An agarose solution (0.2%, w:v) was used to mount all types of sample. The treatments of mounted samples and visualization using confocal laser scanning microscopy (CLSM) were performed as described by Li et al (2020) [39]. By these methods, Alexa Fluro 488 (ThermoFisher, Waltham, MA, United States) was deposited along with the hybridized DSB706-HRP probe, and 4 ,6-diamidino-2-phenylindole (DAPI) was applied to all samples as a counterstain.…”

“…Anodic current collection of RA steadily increased to about 30.5 ± 2.5 µA by day 86 and then rose to about 75 ± 8 µA on day 101. After the peak, anodic current collection of RA reduced to about 30 ± 5 µA (Li et al, 2020 [39] and Figure S2). On day 135, one of the duplicate electrodes was removed, and the current collection of the leftover electrode started to increase to about 55 µA (Figure S2b), suggesting the duplicate electrodes were in competition for substrate.…”

“…The anode of a benthic or sediment MFC was found early on to enrich members in the family of Desulfobulbaceae [13]. Additionally, Desulfobulbaceae filaments resembling CB have been observed by microscopy on the carbon fibers serving as anodes in a benthic MFC and in a bioelectrochemical reactor incubating intertidal sediment [39,40]. The environment around the anode of a MFC can exhibit a sustained redox gradient leading to an inexhaustible surface for the energy metabolism of microorganisms that utilize DEET [41], including the Desulfobulbaceae.…”