This episode of Matters Microbial features Dr. Nicole Geerlings from the University of Vienna, who discusses her PhD research on the fascinating world of cable bacteria. These unique, multicellular microbial filaments can be centimeters long and are capable of conducting electricity over vast distances from a microbial perspective. They thrive in sediment by spatially separating their metabolism: cells deep in the oxygen-free (anoxic) zone oxidize sulfide to generate electrons, which are then conducted through shared, fibrous structures in the periplasm to cells at the sediment surface, where the electrons are transferred to oxygen.
Dr. Geerlings explains the experiments that identified these bacteria as the conductors, their biogeochemical fingerprint on sediment pH, and their ecological role as "ecosystem engineers" or "microbial beavers" that fundamentally alter their environment. A key finding from her research using Stable Isotope Probing (SIP) and NanoSIMS analysis reveals a striking division of labor: the cells in the oxygen-rich (oxic) zone are not actively growing or fixing carbon. Their primary role appears to be the terminal transfer of electrons, effectively acting as a living "snorkel" for the rest of the filament, which performs its growth deep within the sediment.
Unraveling the Mysteries of Cable Bacteria
3:19 The Basis of Metabolism: The discussion begins with the foundational concept that life is fundamentally about moving electrons from a high-energy source (donor) to a lower-energy destination (acceptor) to generate energy.
4:14 Introduction to Cable Bacteria: The podcast introduces cable bacteria, remarkable microbes that can generate electricity and transport it over long distances, changing our understanding of microbial metabolism.
8:35 Spatially Separated Metabolism: Cable bacteria perform a single metabolic process (a redox reaction) that is split between different parts of the filament. The bottom part acts as the electron donor (oxidizing sulfide), and the top part acts as the electron acceptor (reducing oxygen).
10:09 A Multicellular Marvel: Cable bacteria are multicellular filaments, often several centimeters long, made of thousands of individual cells stacked on top of each other. All the cells must work in concert to generate energy.
11:02 Life in the Sediment: These bacteria position themselves vertically in sediment. Cells deep in the anoxic, sulfide-rich zone generate electrons, which travel up the filament to a few cells in the oxic zone at the surface that use oxygen.
12:50 The Conductive Wires: The electrons are transported through a shared periplasm containing electrically conductive protein fibers. This structure is different from the nanowires found in other bacteria like Geobacter.
16:13 A Biogeochemical Fingerprint: The movement of electrons and protons creates a distinct pH profile in the sediment. The pH drops in the deeper, suboxic zone and rises sharply in the oxic zone, a unique signature of cable bacteria activity.
17:57 The "Snorkel" Metaphor: The electrical conduction over centimeters is a massive distance for a microbe, analogous to a human breathing through a snorkel 20 kilometers long.
20:01 Proving the Hypothesis: Early experiments confirmed that the bacteria themselves, not minerals or other substances in the sediment, were responsible for the long-distance electron transport. Researchers showed that cutting the filaments or placing them across non-conductive glass beads still allowed for the electrical and chemical signatures to persist, as long as the filament remained intact.
27:57 Investigating with Stable Isotopes: To understand how the cells grow, researchers used Stable Isotope Probing (SIP) and NanoSIMS. They fed the bacteria labeled carbon (13C) and used high-resolution mass spectrometry to see which cells incorporated it.
32:49 A Stark Division of Labor: The NanoSIMS results revealed a fascinating finding: cells in the oxic zone were not metabolically active and did not incorporate any new carbon. Their sole function appears to be transferring electrons to oxygen, essentially sacrificing their own growth for the good of the entire filament.
35:52 A Robust Electrical Grid: The conductive structure is incredibly resilient. Even if a cell in the middle of the filament dies or becomes inactive, the surrounding fibers ensure the electrical connection remains unbroken, allowing the filament to continue functioning.
40:33 Microbial Beavers: Cable bacteria are described as "ecosystem engineers." By creating a low-pH zone, they dissolve iron sulfide minerals, releasing more sulfide for them to use and creating a thick layer of iron oxides at the surface, which can impact the entire ecosystem.
44:49 Rapid Adaptation: An experiment showed that when the sediment was cut and the lower part of a filament was suddenly exposed to oxygen, it could reverse its function and begin reducing oxygen within two minutes, demonstrating remarkable metabolic flexibility.
Glossary
Anabolism: The set of metabolic pathways that construct molecules from smaller units. These reactions require energy. In the podcast, this refers to the cells building new biomass by fixing carbon.
Autotroph: An organism that can produce its own food, typically using light (photosynthesis) or inorganic chemical reactions (chemosynthesis). Cable bacteria are autotrophs that fix inorganic carbon.
Biogeochemical Cycle: The pathway by which a chemical substance moves through both the biotic (living) and abiotic (non-living) components of Earth. Cable bacteria have a major impact on the sulfur and iron cycles.
Catabolism: The set of metabolic pathways that breaks down molecules into smaller units to release energy. In the podcast, this refers to the oxidation of sulfide to generate electrons.
Ecosystem Engineer: An organism that creates, significantly modifies, maintains, or destroys a habitat. Their activity has a major impact on the availability of resources for other species.
NanoSIMS (Nanoscale Secondary Ion Mass Spectrometry): A high-resolution imaging technique that bombards a sample with a primary ion beam, causing the sample to eject "secondary" ions. A mass spectrometer then analyzes the mass of these secondary ions, allowing researchers to determine the elemental and isotopic composition of a sample at a sub-micrometer scale.
Periplasm (or Periplasmic Space): In gram-negative bacteria, this is the space between the inner cytoplasmic membrane and the outer membrane. In cable bacteria, this space is shared along the filament and contains the conductive fibers.
Redox Reactions: A type of chemical reaction that involves a transfer of electrons between two species. An oxidation-reduction reaction is any chemical reaction in which the oxidation number of a molecule, atom, or ion changes by gaining or losing an electron.
Stable Isotope Probing (SIP): A research method used in microbiology to identify which microbes in a mixed community are consuming a specific substrate. A substrate (like carbon) is "labeled" with a heavy, non-radioactive (stable) isotope. By tracking where the heavy isotope ends up (e.g., in DNA, RNA, or the cell itself), scientists can identify the active organisms.