Inducing the Attachment of Cable Bacteria on Oxidizing Electrodes

Abstract

The scope of the present study is to introduce electrochemical reactors as a tool for investigating the growth of novel filamentous cable bacteria and their unique extracellular electron transfer ability. New evidence that cable bacteria are widely distributed in sediments throughout an estuarine system connected to the NE Pacific Ocean is also presented. Cable bacteria found within Yaquina Bay, Oregon, USA, appear to cluster with the genus, Candidatus Electrothrix. Results of a 135-day bioelectrochemical reactor experiment confirm a previous observation that cable 10 bacteria can grow on oxidatively poised electrodes suspended in anaerobic seawater above reducing sediments.However, several diverse morphologies of Desulfobulbaceae filaments, cells, and colonies were observed on the carbon fibers of the suspended electrodes including encrusted chains of cells. These observations provide new information to suggest what conditions will induce cable bacteria to perform electron donation to an electrode surface, further informing future experiments to culture cable bacteria apart from a sediment matrix. 15 IntroductionLong distance electron transfer (LDET) is a mechanism used by certain microorganisms to generate energy through the transfer of electrons over distances greater than a cell-length. These microorganisms may pass electrons across dissolved redox shuttles, nanofiber-like cell appendages, outer-membrane cytochromes, and/or mineral nanoparticles to connect extracellular electron donors and acceptors Lovley, 2016). Recently, a novel type of LDET 20 exhibited by filamentous bacteria in the family of Desulfobulbaceae was discovered in the uppermost centimeters of various aquatic, but mainly marine, sediments (Malkin et al., 2014;Pfeffer et al., 2012). These filamentous bacteria, also known as "cable bacteria", electrically connect two spatially separated redox half-reactions and generate electrical current over distances that can extend to centimeters, which is an order of magnitude longer than previously recognized LDET distances (Meysman, 2017). 25The unique ability of cable bacteria to perform LDET creates a spatial separation of oxygen reduction in oxic surface layers of sediment from sulfide oxidation in subsurface layers (Meysman, 2017). The spatial separation of these two half-reactions also creates localized porewater pH extremes in oxic and sulfidic layers, which induces a series of secondary reactions that stimulate the geochemical cycling of elements such as iron, manganese, calcium, phosphorus, and nitrogen (Kessler et al., 2018;Rao et al., 2016; Seitaj et al., 2015;Sulu-Gambari et al., 2016b, 2016a. In addition 30 to altering established perceptions of sedimentary biogeochemical cycling and microbial ecology (Meysman, 2017;Nielsen and Risgaard-Petersen, 2015), cable bacteria also possess intriguing structural features that may inspire new engineering applications in areas of bioenergy harvesting and biomaterial design (Lovley, 2016). Pfeffer et al., 2012). Therefore to initiate this enquiry, sedimen...