MIT News
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About 2.3 billion years ago, oxygen began to accumulate in the atmosphere, eventually reaching the survival rates we breathe today. A new hypothesis put forward by MIT scientists suggests a mechanism for how this might happen. Pictured are examples of Paleoproterozoic organisms.
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Nationalgeographic.co.id—For the first two billion years of Earth’s history, there was almost no oxygen in the air. While some microbes During photosynthesis in the latter part of this period, oxygen had not yet accumulated at a rate that would impact the global biosphere.
But about 2.3 billion years ago, this stable low oxygen balance shifted, and oxygen began to build up in the atmosphere, eventually reaching the levels that sustain the life we breathe today. This rapid infusion is known as Great Oxygenation Event, or GOE. What triggered these events and pulled the planet out of its terrifyingly low-oxygen state is one of science’s great mysteries.
However, a new hypothesis, proposed by MIT scientists, suggests that oxygen eventually begins to accumulate in the atmosphere thanks to interactions between certain marine microbes and minerals in marine sediments. This interaction helps prevent oxygen from being consumed, triggering a self-reinforcing process in which more oxygen is available to accumulate in the atmosphere.
The scientists have worked out their hypothesis using mathematical and evolutionary analysis, suggesting that there were indeed microbes that existed before GOE and developed the ability to interact with sediments in the way the researchers have proposed.
Their study, has been published in the journal Nature Communications on March 14, 2022 entitled Oxidative metabolisms catalyzed Earth’s oxygenationthis is the first to link the co-evolution of microbes and minerals with the oxygenation of the Earth.
“Perhaps the most important biogeochemical change in the planet’s history is atmospheric oxygenation,” the study authors said Daniel Rothman, professor of geophysics in MIT’s Department of Earth, Atmospheric, and Planetary Sciences (EAPS). “We show how microbial, mineral, and environmental geochemical interactions act together to increase oxygen in the atmosphere.”
The early Earth’s atmosphere was very deprived of oxygen. However, this changes after a violent oxidation event. So, what could push the planet out of one stable, oxygen-deficient state to another oxygen-rich stable state?
“If you look at Earth’s history, there seem to be two jumps, where you go from a low oxygen stable state to a much higher oxygen stable state, once in the Paleoproterozoic, then again in the Neoproterozoic,” said the co-authors. Gregory Fournier, professor of geobiology at EAPS, as Tech Explorist reports. “This jump was not possible due to a gradual increase in excess oxygen. There must have been some feedback that caused this step change in stability.”
He and his colleagues wondered if such positive feedback could come from processes in the oceans that make some organic carbon unavailable to consumers. Organic carbon is mainly consumed through oxidation, usually accompanied by oxygen consumption, which is a process by which microbes in the ocean use oxygen to break down organic matter, such as detritus that settles in sediments. The team wondered: Could there be a process by which the presence of oxygen stimulates further accumulation?
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To find the answer, scientists created a mathematical model. Their model predicts: If microbes had only a partial ability to oxidize organic matter, the partially oxidized material, or “POOM,” would effectively become “sticky” and chemically bind to minerals in the sediment in a way that would protect the material from oxidation further. Oxygen that should be consumed to completely degrade the material will instead be free to accumulate in the atmosphere.
“That made us ask, is there any microbial metabolism out there that produces POOM?” said Fourier.
To answer this, the team searched the scientific literature and identified a group of microbes that partially oxidize organic matter in the deep ocean today. This microbe belongs to the bacterial group SAR202, and its partial oxidation is carried out via the enzyme, Baeyer-Villiger monooxygenase, or BVMO.
The team performed phylogenetic analyzes to see how far the microbes, and the genes for enzymes, could be traced. They found that bacteria did have an ancestor before GOE, and the gene for the enzyme could be traced across multiple microbial species, as far back as before GOE.
What’s more, they found that gene diversification, or the number of species acquiring genes, increased significantly during times when the atmosphere experienced a spike in oxygenation, including once during the Paleoproterozoic GOE, and again in the Neoproterozoic.
“We found several temporal correlations between diversification of POOM-producing genes and atmospheric oxygen levels. That supports our overall theory.” main author said Haitao Shanga former MIT graduate student.
Confirming this hypothesis will require more follow-up, from laboratory experiments to field surveys, and everything in between. With their new study, the team has introduced a new suspect in an ancient case of what oxidizes Earth’s atmosphere.
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