Most life on Earth can be broadly divided into oxygen consumers and oxygen producers.
This delicate balance between givers and takers keeps the concentration of oxygen in our planet’s atmosphere around 21%. And yet, this has not always been the case.
During the first billion years of Earth’s existence, oxygen was relatively scarce. Then, seemingly out of nowhere, the diatomic gas suddenly rose.
More oxygen was given than taken in, but how and why did this happen?
Scientists have been pondering these mysteries for years now, and researchers at the Massachusetts Institute of Technology (MIT) have a new hypothesis. Perhaps some microbes distinguished between producers and consumers of oxygen.
Deep ocean microbes are known to use oxygen to break down organic matter. But what if another microbe took a bite of the ocean’s oxygen before other consumers could access it?
Theoretically, if a microbe only partially oxidizes organic matter, there’s a good chance that the remains will chemically bind to minerals in ocean sediments.
This oxygen burial would prevent organic matter from being more completely oxidized while being broken down by more voracious microbes. As such, the oxygen would have a chance to accumulate in the water before flowing out into the atmosphere. Then the ocean can absorb it again, creating a positive feedback loop.
“This led us to wonder if there is a microbial metabolism that produces POOM (partially oxidized organic matter)?” remembers geobiologist Gregory Fourier.
Turns out there was. Digging through the scientific literature, Fourier and his colleagues – Haitao Shang and Daniel Rothman – landed on a bacterial group known as SAR202.
This modern group of bacteria can partially oxidize organic matter in today’s deep oceans. It can do this via an enzyme known as Baeyer-Villiger monooxygenase, or BVMO.
By tracing the genetic lineage of this enzyme, the authors discovered that it existed among microbes that evolved before the great oxidation event.
Moreover, the oxygen peaks of early Earth seem to coincide with the expansion of this gene. In other words, as the ability to partially oxidize organic matter spread among the microbes, there was also an increase in atmospheric oxygen levels.
The timing could be coincidental, or it could imply that microbes with these genes helped trigger the big oxidation event.
As oxygen became more available in the environment, it likely supported the diversification of similar oxidative metabolisms in other microbes.
“This may seem counterintuitive: oxidative metabolic processes, after all, consume O2,” the authors write.
“A potentially important positive feedback, however, is the interaction of oxidized metabolic products with minerals in sedimentary environments.”
Partially oxidized organic matter is bound more tightly to mineral surfaces in ocean sediments. This means that the enzymes of the microbes cannot access it as easily.
Buried oxygen can therefore persist over large geologic timescales, ultimately leading to oxygen accumulation in the Earth’s oceans and atmosphere.
At some point, this positive feedback loop would have balanced out at 21% oxygen in the atmosphere – likely when enough life forms have evolved to start consuming the element.
The scale between consumers and producers of oxygen has since been established.
Another recent study supports this hypothesis, suggesting that the burial of organic matter in a low-oxygen environment played a bigger role than we thought in Earth’s great oxygenation event.
Instead of photosynthesizing bacteria oxygenating the atmosphere and then the ocean, what if the minerals of the ocean oxygenated the atmosphere?
More research is needed to flesh out these ideas, but so far they seem like possible explanations.
“Proposing a new method and showing evidence of its plausibility is the first but important step,” says Fournier. “We have identified this as a theory worthy of study.”
The study was published in Nature Communication.