In this episode of Matters Microbial, Dr. Mark Martin is joined by Dr. Nicole Geerlings of the University of Vienna to discuss the fascinating world of cable bacteria. These multicellular, filamentous microbes are capable of conducting electricity over centimeter-scale distances, a massive scale for the microbial world. Dr. Geerlings explains how these bacteria perform a spatial "division of labor" by linking two separate environments: some cells, deep in anoxic sediment, oxidize hydrogen sulfide to generate electrons, which are then conducted along the filament's shared periplasm to other cells at the sediment surface that use oxygen to accept those electrons.
Dr. Geerlings details her Ph.D. research, which used advanced techniques like stable isotope probing with NanoSIMS to investigate the metabolism of individual cells within these filaments. Her work revealed a remarkable anabolic division of labor: cells in the deep, sulfide-rich zone are metabolically active and fix carbon, while the cells in the oxic zone are metabolically inactive, seemingly sacrificing themselves to serve as the terminal for the electrical circuit. This arrangement makes cable bacteria "ecosystem engineers" or "microbial beavers," as they profoundly alter the biogeochemistry of their environment by creating distinct pH and redox zones.
The Wild, Weird, and Wonderful World of Cable Bacteria
0:27 The podcast welcomes Dr. Nicole Geerlings from the University of Vienna to discuss her research on cable bacteria, which can conduct electricity over enormous microbial distances.
3:19 The host introduces the fundamental concept that much of life and metabolism is about moving electrons from a high-energy source (donor) to a low-energy destination (acceptor) to generate energy.
4:14 The topic of cable bacteria is introduced. These are multicellular filaments of bacteria, often centimeters long, that have a unique metabolism.
10:09 Cable bacteria exhibit a spatial division of labor. Cells deep in anoxic sediment oxidize hydrogen sulfide, generating electrons. These electrons are transported along the filament to cells at the oxygen-rich surface, which complete the reaction. This bridges two distinct chemical zones.
13:19 The "long snorkel" metaphor is used to describe the scale of this electron transport. For a microbe, moving electrons several centimeters is equivalent to a human breathing through a 20-kilometer-long snorkel.
14:03 The conductive structures within cable bacteria are not the same as the nanowires found in microbes like Geobacter. They are shared protein-based fibers within the periplasm that contain nickel co-factors.
16:45 Cable bacteria are "ecosystem engineers" or "microbial beavers." Their activity creates a distinct biogeochemical fingerprint in the sediment, including a sharp pH gradient, which benefits them by dissolving iron sulfides and releasing more of their food source.
20:01 Researchers performed elegant experiments to prove that the bacteria themselves, not minerals or other substances in the sediment, were responsible for the long-distance electron transport. One key experiment involved replacing a section of sediment with non-conductive glass beads and finding that the cable bacteria grew through them and maintained the electrical connection.
29:42 Dr. Geerlings used a technique called stable isotope probing with NanoSIMS (nanoscale secondary ion mass spectrometry) to determine which cells in the filament were metabolically active. By providing ¹³C-labeled bicarbonate, she could track which cells were fixing carbon.
32:35 A key finding of the research was an anabolic division of labor. Only the cells in the deeper, anoxic zone were actively fixing carbon and growing. The cells at the top (oxic zone) were not growing, suggesting their sole purpose is to get rid of electrons for the good of the entire filament.
35:52 The filament's conductive structure is robust. Electrons can bypass dead or inactive cells, ensuring that a single failed cell does not break the circuit for the entire community below it.
44:49 Experiments demonstrated the rapid adaptability of cable bacteria. When a filament was cut and the previously anoxic section was exposed to oxygen, it recovered its electrical activity within two minutes, suggesting the machinery for electron transfer is always ready.
57:44 Key Takeaway: Cable bacteria are a prime example of multicellular cooperation in the microbial world. They function as a single integrated unit to bridge spatially separated redox zones and, in doing so, have a profound impact on the biogeochemical cycles of their environment.