Researchers at the Massachusetts Institute of Technology (MIT) have proposed a new theory to explain how oxygen concentration might have built up in the Earth's atmosphere, according to a press release.
Billions of years before the first humans were born, the Earth's atmosphere lacked the oxygen we need to survive. Some microbial organisms were using photosynthesis to generate some oxygen, however, the amounts produced were not sufficient to support many lifeforms. Around 2.3 billion years ago, though, the oxygen levels began to build up in the atmosphere, but the reasons for the same and still unknown.
The Great Oxygenation Event (GOE)
Early in Earth's history, oxygen producers and consumers on the planet maintained equilibrium in a way that left little oxygen in the atmosphere. However, there were two events in the Paleoproterozoic period and the Neoproterozoic period, which saw oxygen levels go from low levels to much higher levels that the Earth has today.
Gregory Fournier, Associate Professor of Geobiology at the Department of Earth, Atmospheric, and Planetary Sciences (EAPS) at MIT, and his colleagues believe that these jumps in oxygen levels weren't the result of gradual change. Instead, there was a positive feedback loop that was activated in the oceans.
According to Fournier and his team, organic carbon, whose breakdown consumes oxygen (also called oxidation) in usual circumstances, was likely unavailable to its consumers during these intervals that led to oxygen build-ups. Since life existed in the oceans, the researchers looked at marine microbes and minerals in ocean sediments to determine if such as situation could arise.
They hypothesized that if microbes in these environments were able to oxidize organic matter partially, the partially oxidized organic matter (POOM) would bind to the minerals in a way that would prevent their further oxidation. The oxygen unused in the process would end up in the atmosphere.
The microorganism we need to thank
To verify their hypothesis, the researchers scanned through the scientific literature to identify microorganisms that could create POOM and found a bacterial group called SAR202 that can achieve the feat using an enzyme called Baeyer-Villiger monooxygenase, or simply BVMO.
Tracing back the genetic origins of this enzyme, the researchers found that the bacteria's ancestors were indeed present prior to the GOE. Interestingly, the gene was acquired by multiple bacterial species during the Paleoproterozoic as well as Neoproterozoic, times when oxygen levels have been known to spike.
While these correlations provide support to the new theory, the researchers need to do extensive work to find the necessary to prove it.
The mystery of the GOE may have just begun to unravel.
Details of the theory can be found in the journal Nature Communications.
The burial of organic carbon, which prevents its remineralization via oxygen-consuming processes, is considered one of the causes of Earth’s oxygenation. Yet, higher levels of oxygen are thought to inhibit burial. Here we propose a resolution of this conundrum, wherein Earth’s initial oxygenation is favored by oxidative metabolisms generating partially oxidized organic matter (POOM), increasing burial via interaction with minerals in sediments. First, we introduce the POOM hypothesis via a mathematical argument. Second, we reconstruct the evolutionary history of one key enzyme family, flavin-dependent Baeyer–Villiger monooxygenases, that generates POOM, and show the temporal consistency of its diversification with the Proterozoic and Phanerozoic atmospheric oxygenation. Finally, we propose that the expansion of oxidative metabolisms instigated a positive feedback, which was amplified by the chemical changes to minerals on Earth’s surface. Collectively, these results suggest that Earth’s oxygenation is an autocatalytic transition induced by a combination of biological innovations and geological changes.