New theory suggests a way ocean microbes and minerals could oxygenate Earth

(ORDO NEWS) — Most living organisms on Earth can be divided into consumers and producers of oxygen. This delicate balance of oxygen givers and takers keeps the oxygen concentration in our planet’s atmosphere at 21 percent. However, this was not always the case.

In the first few billion years of the Earth’s existence, oxygen was relatively scarce. Then, seemingly out of nowhere, the amount of diatomic gas suddenly increased. More oxygen began to flow than taken, but how and why did this happen?

Scientists have been pondering these mysteries for years, and researchers at the Massachusetts Institute of Technology (MIT) have a new hypothesis. Perhaps some microbes cross the line between producers and consumers of oxygen.

Microbes in the deep ocean are known to use oxygen to break down organic matter. But what if another microbe was biting off ocean oxygen before other consumers could get to it?

Theoretically, if a microbe has only partially oxidized organic matter, then there is a good chance that the residues will chemically bond with minerals in ocean sediments.

This oxygen storage will keep the organic material from completely oxidizing while it is being decomposed by more voracious microbes. Thus, oxygen will have a chance to accumulate in the water before it escapes into the atmosphere. The ocean can then reabsorb it, creating a positive feedback loop.

“This led us to wonder: is there a microbial metabolism that produces POOM (partially oxidized organic matter)?” – recalls geobiologist Gregory Fourier.

It turned out that there is. While researching the scientific literature, Fourier and his colleagues Haitao Shang and Daniel Rothman settled on a group of bacteria known as SAR202.

This modern group of bacteria can partially oxidize organic matter in today’s deep oceans. It can do this with the help of an enzyme known as Beyer-Villiger monooxygenase, or BVMO.

By tracing the genetic lineage of this enzyme, the authors found that it existed among microbes that evolved before the great oxidation event.

What’s more, the spikes in oxygen levels on the early Earth appear to coincide with the spread of this gene. In other words, as the ability to partially oxidize organic materials spread among microbes, there was also an increase in the level of oxygen in the atmosphere.

This coincidence may be coincidental, or it may mean that microbes with these genes helped start the great oxidation event.

As oxygen became more available in the environment, this likely contributed to the diversification of similar oxidative metabolisms in other microbes. “This may seem counterintuitive: oxidative metabolic processes ultimately consume O2,” the authors write.

“However, a potentially important positive feedback lies in the interaction of products of oxidative metabolism with minerals in sedimentary rocks.” Partially oxidized organic matter is more strongly associated with mineral surfaces in ocean sediments. This means that microbial enzymes cannot reach it as easily.

Therefore, buried oxygen can persist for long geologic time, eventually spurring oxygen accumulation in the Earth’s oceans and atmosphere.

At some point, this positive feedback loop had to balance out at 21 percent oxygen in the atmosphere—probably when enough life forms had evolved to start consuming the element. Since then, the scales between consumers and producers of oxygen have been established.

Another recent study supports this hypothesis, suggesting that the burial of organic matter in a low-oxygen environment played a larger role in the great Earth oxygenation event than we thought.

Instead of photosynthetic bacteria oxygenating the atmosphere and then the ocean, what if the minerals in the ocean oxygenated the atmosphere?

Further research is needed to develop these ideas, but so far they look like possible explanations.

“Proposing a new method and showing evidence for its plausibility is a first but important step,” says Fournier. “We have determined that this is a theory worthy of study.”

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