Biogeosciences, 17, 597–607, 2020 https://doi.org/10.5194/bg-17-597-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Inducing the attachment of cable bacteria on oxidizing electrodes Cheng Li, Clare E. Reimers, and Yvan Alleau College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA Correspondence: Cheng Li ([email protected]) Received: 22 August 2019 – Discussion started: 2 September 2019 Revised: 17 December 2019 – Accepted: 14 January 2020 – Published: 6 February 2020 Abstract. Cable bacteria (CB) are multicellular, filamentous membrane cytochromes, and/or mineral nanoparticles to bacteria within the family of Desulfobulbaceae that transfer connect extracellular electron donors and acceptors (Li et al., electrons longitudinally from cell to cell to couple sulfide ox- 2017; Lovley, 2016). Recently, a novel type of LDET ex- idation and oxygen reduction in surficial aquatic sediments. hibited by filamentous bacteria in the family of Desulfobul- In the present study, electrochemical reactors that contain baceae was discovered in the uppermost centimeters of var- natural sediments are introduced as a tool for investigating ious aquatic, but mainly marine, sediments (Malkin et al., the growth of CB on electrodes poised at an oxidizing poten- 2014; Trojan et al., 2016). These filamentous bacteria, also tial. Our experiments utilized sediments from Yaquina Bay, known as “cable bacteria” (CB), electrically connect two spa- Oregon, USA, and we include new phylogenetic analyses of tially separated redox half reactions and generate electrical separated filaments to confirm that CB from this marine loca- current over distances that can extend to centimeters, which tion cluster with the genus “Candidatus Electrothrix”. These is an order of magnitude longer than previously recognized CB may belong to a distinctive lineage, however, because LDET distances (Meysman, 2017). their filaments contain smaller cells and a lower number of The unique ability of CB to perform LDET creates a spa- longitudinal ridges compared to cables described from other tial separation of oxygen reduction in oxic surface layers locales. The results of a 135 d bioelectrochemical reactor ex- of organic-rich sediment from sulfide oxidation in subsur- periment confirmed that these CB can migrate out of reduc- face layers (Meysman, 2017). The spatial separation of these ing sediments and grow on oxidatively poised electrodes sus- two half reactions also creates localized porewater pH ex- pended in anaerobic seawater. CB filaments and several other tremes in oxic and sulfidic layers, which induces a series of morphologies of Desulfobulbaceae cells were observed by secondary reactions that stimulate the geochemical cycling scanning electron microscopy and fluorescence in situ hy- of elements such as iron, manganese, calcium, phosphorus, bridization on electrode surfaces, albeit in low densities and and nitrogen (Kessler et al., 2018; Rao et al., 2016; Seitaj often obscured by mineral precipitation. These findings pro- et al., 2015; Sulu-Gambari et al., 2016a, b). In addition to vide new information to suggest what kinds of conditions altering established perceptions of sedimentary biogeochem- will induce CB to perform electron donation to an electrode ical cycling and microbial ecology (Meysman, 2017; Nielsen surface, further informing future experiments to culture CB and Risgaard-Petersen, 2015), CB also possess intriguing outside of a sediment matrix. structural features that may inspire new engineering applica- tions in areas of bioenergy harvesting and biomaterial design (Lovley, 2016; Meysman et al., 2019). Much is still unknown about the basic mechanism(s) 1 Introduction that CB use to perform LDET. It has been suggested that when long filaments form, a chain of cells at the sulfidic Long-distance electron transfer (LDET) is a mechanism terminal catalyzes anodic half reactions (e.g., 0:5 H2S C used by certain microorganisms to generate energy through ! 2− C − C C 2H2O 0:5 SO4 4e 5H ), while a cathodic half re- the transfer of electrons over distances greater than a cell- − C action (O2 C 4e C 4H ! 2H2O) is completed by cells at length. These microorganisms may pass electrons across dis- the oxic terminal. Electron transfer then occurs along the lon- solved redox shuttles, nanofiber-like cell appendages, outer- Published by Copernicus Publications on behalf of the European Geosciences Union. 598 C. Li et al.: Inducing the attachment of cable bacteria on oxidizing electrodes gitudinal ridges of CB filaments via electron hopping pro- 2.2 Sediment incubation moted by extracellular cytochromes positioned within a re- dox gradient or via conductive electronic structures such as To cultivate CB of Yaquina Bay, IMF sediment was initially pili (Bjerg et al., 2018; Cornelissen et al., 2018; Kjeldsen et incubated for 60 d. These first incubations were started 2 d al., 2019; Meysman et al., 2019; Pfeffer et al., 2012). These after collection and performed after homogenizing the sieved hypotheses await further verification, and CB remain uncul- sediments under a flow of N2 and then packing the sediment tured and difficult to grow outside of sediment. This difficulty into triplicate polycarbonate tubes (15 cm height and 9.5 cm complicates efforts to study them using different techniques, inner diameter). These cores were submerged in an aquar- such as electrochemical assays and metatranscriptomics. ium containing aerated seawater collected from Yaquina Bay In a previous benthic microbial fuel cell (BMFC) experi- and held at 15 ◦C, a temperature that is about average for the ment in a marine estuary (Reimers et al., 2017), we serendip- mudflats of Yaquina Bay (Johnson, 1980). Once a distinc- itously observed the attachment of CB to carbon fibers serv- tive suboxic layer was evident from color changes in the top ing as an anode in an anaerobic environment above sedi- centimeters of the cores, profiles of porewater pH, O2, and ments. This finding suggested that CB possess the ability to H2S were measured to 2–3 cm depth with commercial mi- donate electrons to solid electron acceptors, and it indicated croelectrodes (Unisense A.S., Aarhus, Denmark) to confirm a range of cathodic potentials favorable for electron trans- geochemical evidence of CB activity (see below). Multiple fer (Reimers et al., 2017). However further investigations are small sub-cores (0.5 cm diameter, 3 cm in length) were then still needed to study the conditions that allow the attachment taken out from each incubated core using cut-off syringes. of CB to a poised electrode and to document electron transfer Some of these sediment plugs were washed gently to reduce mechanisms at their cathodic terminus. In the present study, the volume of fine particles, and CB biomass was further sep- we first clarify the phylogenetic placement of CB found in arated out from the sediment matrix by using custom-made sediments from Yaquina Bay, Oregon, where the BMFC was tiny glass hooks following Malkin et al. (2014). Sediment previously deployed. Then, we describe the design of a bio- plugs and separated filamentous biomass were frozen or fixed electrochemical reactor configured to mimic the environment for subsequent phylogenetic and microscopic characteriza- in the anodic chamber of a BMFC and verify conditions that tions. can induce CB attachment on electrodes. Results assert that when oxygen is not available, CB can glide through sedi- 2.3 Reactor configuration and operation ments and seawater to an electrode poised at oxidative poten- tials. Thus, the present study provides new information about To mimic the conditions where CB were found attached to the chemotaxis of CB in environments other than sediments, electrode fibers in a BMFC (Reimers et al., 2017), a bio- revealing key conditions for their attachment to surfaces and electrochemical reactor was assembled from a polycarbonate growth in both natural and engineered environments. core tube (15 cm height and 11.5 cm inner diameter, Fig. 1) as a second phase of this research. A lid, a center rod to locate and support the electrodes, and a perforated bottom partition 2 Materials and methods were made from polyvinyl chloride (PVC, McMaster-Carr, Elmhurst, IL). Three carbon brush electrodes, which would 2.1 Study site and sediment collection serve as two anodes and a control electrode (Mill-Rose, Men- Several studies suggest that CB may be found widely in tor, OH, 2 cm in diameter and 8.9 cm total length), were in- serted through septa within holes in the core lining to meet coastal sediments possessing high rates of sulfide genera- ◦ tion coupled with organic matter mineralization (Larsen et the center rod and were spaced radially at 120 angles from al., 2015; Malkin et al., 2014; Pfeffer et al., 2012). There- each other. fore, to initiate this enquiry, sediment with these two char- To initiate the experiment, the reactor was placed inside an acteristics was collected from Yaquina Bay, Oregon, USA, 8 L plastic beaker (with perforated walls) containing 3 cm of using a hand shovel at a site on an intertidal mud flat (IMF, IMF sediments at the bottom. Enough additional IMF sed- 44◦37030 N, 124◦00026 W). The IMF site is located about iment was then placed inside the reactor to form an 8 cm 3 km upstream from the site where the BMFC was deployed thick layer after settling and compacting. In this configura- in the abovementioned study (Reimers et al., 2017). The top tion, the sediment–water interface was approximately 1 cm 20 cm of these sediments were sieved through a 0.5 mm mesh away from the lower extent of the carbon brush electrodes. size metal screen to remove macrofauna and shell debris. The beaker was then gently lowered inside an aquarium filled Then the sieved sediments were allowed to settle and stored with Yaquina Bay seawater until fully submerged, and the re- in sealed buckets in a cold room at 5 ◦C.