Novel Biological Wiring System Detected in a Methane-Consuming Microbial Symbiosis
Every year, large amounts of methane (CH4) are produced in coastal wetlands and deep ocean sediments through the decay of organic material or seepage from geological reservoirs. Fortunately, microbes consume the majority of this potent greenhouse gas before it reaches Earth’s atmosphere. Although these subsurface environments are typically depleted of oxygen, methane can still be oxidized by symbiotic partnerships between methane-consuming archaea and sulfate-reducing bacteria that collaboratively transfer electrons from methane to sulfate (rather than O2) to generate useful energy. Observed near sites of environmental CH4 production, consortia of cells performing anaerobic oxidation of methane (AOM) form mixed balls composed of tens to hundreds of cells, but the exact mechanism by which they consume CH4 and share energy is not fully understood. In a new study, scientists at the California Institute of Technology used high-resolution microscopy paired with mass spectrometry (NanoSIMS) to examine the relationship between spatial distribution of microbes and metabolic processes in AOM consortia. To their surprise, the researchers found that metabolically active partner microbes did not need to be closely associated with each other, even though each organism performs only half of the critical methane-consuming reaction. Using data from these studies, the team constructed a computational model of consortial metabolism that predicted an extracellular conduit allowing direct transfer of electrons between the organisms. By re-examining the genomes of both microbes, the team identified a previously overlooked set of genes in the archaeal partner encoding an electron transfer system similar to those observed in known electroconductive bacteria. Histological staining was then used to detect this system in active AOM consortia, revealing components arrayed across the extracellular space between the microbes. These results indicate the presence of a biological wiring system within AOM consortia that allows the two partners to more efficiently consume methane, share resulting energy, and form larger consortial structures than would otherwise be possible. These findings reveal another new aspect of the diverse metabolic capacities present in the microbial world and considerably advance our understanding of a key microscale mechanism driving a carbon cycle process of global significance.
McGlynn, S. E., G. L. Chadwick, C. P. Kempes, and V. J. Orphan. 2015. “Single Cell Activity Reveals Direct Electron Transfer in Methanotrophic Consortia,” Nature, DOI: 10.1038/nature15512.