Microbes are biological drivers of Earth’s ecological and geochemical evolution. Some microbes have evolved to use extracellular electron donors and acceptors for energy metabolism. This phenomenon is known as extracellular electron transfer (EET). EET enables microbes to breathe through rocks and other materials outside their cell.
Videlicet, microbes establish an electrical connection to the outside world. They use this connection to generate the power they need to grow.
In a new study, scientists from Harvard and the University of Minnesota explored the tree of life searching for EET. They found it is far more widespread than previously thought. They also discovered that it spread through horizontal gene transfer.
mtrCAB is one set of genes that makes EET possible. Most studies of MtrCAB-mediated EET have been conducted in Shewanella oneidensis.
Co-lead author Isabel R. Baker, a Ph.D. candidate in the Department of Organismic and Evolutionary Biology at Harvard, said, “A lot of our understanding of mtrCAB comes from studies in this particular organism. But we don’t know how widespread this type of metabolism is amongst all of life’s branches. Understanding how widespread it is will help us pinpoint where this kind of metabolism is at play in global biogeochemical cycles.“
Scientists, in this study, employed a phylogenetic and comparative genomics approach to identify the MtrCAB system across all domains of life. They found that these genes existed in far more organisms than previously assumed and in a wide variety of environments worldwide.
Baker said, “We found these genes in microbes all over the planet from virtually every kind of environment, including the deep sea, salt flats, oil refinery sites, the human gut, and even wastewater contaminated by the Manhattan project. Further analysis revealed that the set of genes was horizontally transferred extensively throughout the history of life.“
Co-senior author Professor Jeffrey A. Gralnick said, “The acquisition of genes is analogous to installing an app on your phone to give it new functionality. Horizontal gene transfer is often associated with antibiotic resistance, but here we see a metabolic capability, EET, moving in and out of bacterial genomes.“
Scientists noted, “Whenever the genes landed in different species, the genes involved in EET would change over time to better suit the new organism’s physiology and the environment it lives in.“
Girguis said, “It’s sort of a foregone conclusion that microbes shape our planet, and EET had always been viewed as a niche ability. But we looked at all of the genomic information from animals, Archaea, bacteria, and all other forms of life and found it’s far more widespread than previously assumed. All of the organisms we identified are capable of plugging directly into the substrates in their environment and changing what’s available there.“
Baker said, “The availability of these different substrates change over time as the Earth continues to evolve, either naturally or from human impact. Understanding how these proteins may have coevolved with the history of oxygen on Earth is very important. It could help us understand if this metabolism, or a metabolism like this, helped play a role in one of the massive transformations of our planet’s surface that gave rise to the modern world as we know it.“
- Isabel R. Baker et al. Evidence for Horizontal and Vertical Transmission of Mtr-Mediated Extracellular Electron Transfer among the Bacteria. DOI: 10.1128/mbio.02904-21