For a long time, the first photosynthetic cyanobacteria were hardly able to change the oxygen content of the atmosphere – it was only around 2.3 billion years ago that the elixir of life found its way into the world. Researchers are now presenting a new explanation for this mysterious process: An interaction between special marine microbes and minerals in the ocean sediments could have triggered the oxygen enrichment of the earth.
With every breath, humans and animals supply themselves with a portion of the gas they need for their metabolism: Our air contains around 21 percent oxygen and dissolved O₂ is also available to aquatic animals. The oxidizing agent to release energy thus forms a foundation of life as we know it. However, our world did not always have this wealth of available O₂ to offer, as is known from geological studies: In the first two billion years of the earth’s history, the atmosphere contained hardly any oxygen. This was the case long after photosynthetic microbes that release oxygen had evolved. Still, enough oxygen could not accumulate to affect the global biosphere. Apparently he was rebound to the same extent as he was previously released.
But then the so-called Great Oxygenation Event finally came: around 2.3 billion years ago, the stable, low-oxygen balance shifted – the breathing gas began to accumulate in the atmosphere and finally reached the levels that still make life possible for humans and animals today. It was one of the most momentous processes in the history of the earth. So far, however, what got our planet out of its oxygen-deficient state has remained a scientific mystery. Although there are already explanatory approaches and hypotheses, a complete picture has not yet emerged.
On the trail of the gaseous treasure
The latest contribution to clarifying the Great Oxygenation Event now comes from a team of scientists led by Gregory Fournier from the Massachusetts Institute of Technology in Cambridge. The basis of their study was a look at the fundamentals of the earth’s oxygen system: the current content in the atmosphere is the result of a stable balance between processes that produce oxygen and those that consume it. Apparently, however, a different system existed before the Great Oxygenation Event, where the ratio of oxygen producers and consumers did not leave much additional oxygen for the atmosphere.
“If you look at Earth’s history, there appear to have been two jumps where the system went from a stable state with little oxygen to a stable state with much more oxygen — one in the Paleoproterozoic and one in the Neoproterozoic,” says Fournier. “These jumps cannot therefore have been caused by a gradual slight increase in excess oxygen.” So the team turned to the question of which process could have caused the jumps. The focus was on the organically bound carbon: It is mainly broken down by oxidation: Microbes in the ocean consume oxygen in order to utilize the organic material, such as detritus that has settled in the sediment. The scientists developed computer models to obtain information about which developments in this system could have played a role in the enrichment of oxygen.
Interactions between microbes and minerals
As they report, the system then finally spat out a plausible, theoretical mechanism: If certain microbes had started to only partially oxidize organic substances at the time, a particularly effective substance could have arisen: “partially oxidized organic matter” (POOM), which due to their chemical properties would have been “sticky”. This is how POOM would have bound to minerals in the sediment, preventing further oxidation, the scientists explain. The oxygen that would otherwise have been used to completely degrade the material could then have accumulated in the atmosphere. As their models suggested, this process may have brought the atmosphere into a new, oxygen-rich equilibrium.
The researchers were then able to further substantiate this hypothesis, which until then had seemed “thin”. “We looked into the question of whether there is a microbial metabolism out there that produces POOM,” says Fourier. The team searched the scientific literature and finally identified a group of microbes that still partially oxidize organic material in the deep sea. These microbes belong to the SAR202 bacterial group and their partial oxidation is accomplished by an enzyme called Baeyer-Villiger monooxygenase (BVMO). To get clues as to when these microbes or their abilities arose, the researchers carried out a phylogenetic analysis and investigations using the concept of the molecular clock.
As they report, it became apparent that the bacteria actually have ancestors that apparently existed before the Great Oxygenation Event. In addition, the gene for the enzyme in various types of microbes could also be traced back to the time before oxygen enrichment. In addition, the researchers found genetic evidence that hereditary diversification—the number of species that acquired the gene—had increased at precisely the times when atmospheric oxygen levels were skyrocketing. “We found some temporal correlations between the diversification of POOM-producing genes and atmospheric oxygen levels,” says Shang. “So that supports our general theory.”
In conclusion, however, the researchers emphasize that further investigations are now required to confirm their POOM theory – from experiments in the laboratory to surveys in the field. “Proposing a new explanation and proving its plausibility is the first important step. We’ve only completed it once,” said Fournier.
Source: Massachusetts Institute of Technology, professional article: Nature Communications, doi: 10.1038/s41467-022-28996-0