Noteworthy
When first introduced to microbes, we quickly learn to divide them into aerobes and anaerobes. Then we qualify those titles with modifiers to yield such classes as facultative anaerobes (can grow without oxygen but can respire it to great benefit), obligate aerobes (those absolutely depending on oxygen respiration) and strict anaerobes (these will die in the presence of oxygen). But then, just as quickly, we learn that there are always exceptions to the rules. When it comes to growth in the presence of oxygen, well, things depend...
(click to enlarge)
Biogeological timeline showing the evolution of microbial metabolisms in parallel with the rise in atmospheric oxygen concentrations. Source
Take, for example, the long-held tenet that Bacteroides fragilis was a strict anaerobe. That idea was debunked over two decades ago, when Baughn and Malamy showed B. fragilis could grow aerobically at sub-micromolar (hundreds of nanomolar) oxygen concentration, actually benefitting from oxygen metabolism. As a new descriptor for such bacteria, the nanaerobe was coined. In the ensuing twenty years the concept of strict anaerobes continued to erode as more and more microbes capable of tolerating and metabolizing oxygen grew as the concentrations of oxygen that were tested got lower. Turns out, the new rule may someday be that there are no strict anaerobes; it's just a matter of how sensitive our methods are in detecting low oxygen concentrations. Currently, the limit of detection is 3 nanomolar. So, there's still a lot of room to test between 3 nanomolar to zero oxygen, if only we develop methods with such sensitivity. A 2022 review on this subject, by Jasmine Berg and colleagues, is very much worth the read to get a sense of "how low can they go" in terms of oxygen concentration.
Oxygen metabolism is, of course, not only of great interest for understanding present day microbial physiology and ecology. Earth's natural history was greatly altered by the accumulation of atmospheric oxygen over half a billion years (from 2.5 to 2.0 billion years ago) in what is known as the Great Oxygenation Event or GOE, the consequence of the evolution of oxygenic photosynthesis. (The length of that "event" gives great perspective on the duration of our own current events!) Which means that for ~1.5 billion years of life on Earth the atmosphere had practically no oxygen. But apparently it must have had at least some. A recent paper, based on molecular clocks, machine learning and phylogenetic reconciliation, concludes that aerobic microbes (probably nanaerobes) likely emerged before the GOE. Thus, "oxygen tolerance may have been a prerequisite for, rather than a consequence of, the evolution of oxygenic photosynthesis."
("Noteworthy" is the new format for STC's Thursday posts. Please read our Jan 20, 2025 post outlining this and other changes in our blog.)
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