In a study published in Nature Communications, Concordia researchers describe how Porticoccus arcticus survive in the frigid waters of the Beaufort Sea, just north of the Yukon in the Arctic Ocean’s Canada Basin. The team found genomic evidence that the bacterial population uses acetone carboxylation, a process that converts acetone and carbon dioxide into compounds that can enter the cell’s central metabolic pathways.
“This study introduces two new discoveries,” said David Walsh, a professor in the Department of Biology and the paper’s corresponding author. “The first is a group of organisms, P. arcticus, that has a unique metabolism that uses organic carbon while at the same time has the capacity to use light as an energy source. We call this process photoheterotrophy.
“The second discovery is the way the organism consumes acetone to also consume carbon dioxide out of the atmosphere. This acetone-mediated inorganic carbon assimilation combines what animals do—eating food, in this case, acetone—with what plants do: fixing CO2.”
Metabolic Reconstruction from the Genome
Acetone is produced by marine organisms, including microscopic algae, and through chemical reactions involving sunlight and dissolved organic matter. Although scientists have identified other ways marine bacteria consume acetone, this study provides evidence of a different pathway in Arctic waters.
Genetic analyses suggest that P. arcticus has adapted to life near the ocean surface. At some locations, it accounted for up to nine percent of detected bacteria, making it an important member of the Arctic microbial community.
The researchers note that the bacterium’s relatively small genome contains the complete set of genes needed for acetone carboxylation, as well as multiple genes for light-sensitive proteins called proteorhodopsins that help cells capture energy from sunlight.
Analyses showed that genes involved in both acetone processing and light capture were among the organism’s most active. The researchers say these traits may allow the bacterium to use sunlight for energy while supplementing its carbon supply through acetone metabolism and carbon dioxide assimilation.
“The genomic composition of these organisms led us to reconstruct the different types of energy and carbon metabolisms that exist,” Walsh said. “That is how we found this very specific set of genes associated with the use of acetone, which was unexpected. We discovered that the organism’s genes were telling us about something we did not suspect was happening in an ecosystem. This was a surprise, as we were not actively looking for acetone-degrading bacteria that can use light.”
Localized or Widespread?
Walsh does not know what the implications of a warming Arctic Ocean mean for P. arcticus, though changing sea-ice conditions could affect the bacterium’s future distribution by influencing sunlight and acetone availability.
The research team will also be looking for evidence of this kind of behavior in other oceans as well as in freshwater bacterial populations.
