As metals and carbon cycle through the environment, they change between forms that organisms can metabolize or incorporate into their cells and ones that the cells can’t use at all. Superoxide and other reactive oxygen species play a key role in driving these cycles. Scientists previously thought these important oxidants had only light-dependent sources.
Until the last decade, researchers believed that reactive oxygen species in the environment were produced through photochemical reactions. Recently, they’ve found that a number of eukaryotic and cyanobacteria species produce superoxide outside their cell bodies.
In the new work, a group led by postdoctoral researcher Julia M. Diaz and scientist Colleen M. Hansel of Woods Hole Oceanographic Institution identified 27 species of bacteria that produce superoxide. The microbes come from variety of aquatic and land environments, such as lakes, hydrothermal vents, and soils.
The bacteria appear to produce superoxide at a steady rate, but the source is probably not within the cells, the scientists say. The reactive oxygen species is toxic and its production is tightly controlled inside bacteria. The cells normally wouldn’t contain enough of the species to explain the extracellular concentrations the team observed.
Instead, the researchers propose that an oxidoreductase enzyme loosely attached to the outside of cell walls converts O2 to O2-. The necessary electrons for the reaction probably come from a source inside the cell.
The finding is particularly notable because the bacterial species derive energy by digesting organic compounds rather than through photosynthesis, says Andrew Rose, a chemistry professor at Australia’s Southern Cross University. As a result, he says, “extracellular superoxide is likely to exist at environmentally-relevant concentrations in vast areas of the Earth’s surface that do not receive direct sunlight, such as the deep oceans and terrestrial soils.” Studying these bacteria could help researchers understand the geochemistry in those environments.