Microbes and minerals may have set off Earth's oxygenation
Scientists propose a new mechanism by which oxygen may have first built
up in the atmosphere.

Date:
March 14, 2022
Source:
Massachusetts Institute of Technology
Summary:
Around 2.3 billion years ago, oxygen began building up in the
atmosphere, eventually reaching the life-sustaining levels we
breathe today. A new hypothesis suggests a mechanism for how this
may have happened.



FULL STORY ==========================================================================
For the first 2 billion years of Earth's history, there was barely any
oxygen in the air. While some microbes were photosynthesizing by the
latter part of this period, oxygen had not yet accumulated at levels
that would impact the global biosphere.


==========================================================================
But somewhere around 2.3 billion years ago, this stable, low-oxygen
equilibrium shifted, and oxygen began building up in the atmosphere,
eventually reaching the life-sustaining levels we breathe today. This
rapid infusion is known as the Great Oxygenation Event, or GOE. What
triggered the event and pulled the planet out of its low-oxygen funk is
one of the great mysteries of science.

A new hypothesis, proposed by MIT scientists, suggests that oxygen
finally started accumulating in the atmosphere thanks to interactions
between certain marine microbes and minerals in ocean sediments. These interactions helped prevent oxygen from being consumed, setting off a self-amplifying process where more and more oxygen was made available
to accumulate in the atmosphere.

The scientists have laid out their hypothesis using mathematical and evolutionary analyses, showing that there were indeed microbes that
existed before the GOE and evolved the ability to interact with sediment
in the way that the researchers have proposed.

Their study, appearing in Nature Communications, is the first to connect
the co-evolution of microbes and minerals to Earth's oxygenation.

"Probably the most important biogeochemical change in the history
of the planet was oxygenation of the atmosphere," says study author
Daniel Rothman, professor of geophysics in MIT's Department of Earth, Atmospheric, and Planetary Sciences (EAPS). "We show how the interactions
of microbes, minerals, and the geochemical environment acted in concert
to increase oxygen in the atmosphere." The study's co-authors include
lead author Haitao Shang, a former MIT graduate student, and Gregory
Fournier, associate professor of geobiology in EAPS.



==========================================================================
A step up Today's oxygen levels in the atmosphere are a stable balance
between processes that produce oxygen and those that consume it. Prior
to the GOE, the atmosphere maintained a different kind of equilibrium,
with producers and consumers of oxygen in balance, but in a way that
didn't leave much extra oxygen for the atmosphere.

What could have pushed the planet out of one stable, oxygen-deficient
state to another stable, oxygen-rich state? "If you look at Earth's
history, it appears there were two jumps, where you went from a steady
state of low oxygen to a steady state of much higher oxygen, once in
the Paleoproterozoic, once in the Neoproterozoic," Fournier notes.

"These jumps couldn't have been because of a gradual increase in excess
oxygen.

There had to have been some feedback loop that caused this step-change
in stability." He and his colleagues wondered whether such a positive
feedback loop could have come from a process in the ocean that made some organic carbon unavailable to its consumers. Organic carbon is mainly
consumed through oxidation, usually accompanied by the consumption of
oxygen -- a process by which microbes in the ocean use oxygen to break
down organic matter, such as detritus that has settled in sediment. The
team wondered: Could there have been some process by which the presence
of oxygen stimulated its further accumulation?


========================================================================== Shang and Rothman worked out a mathematical model that made the
following prediction: If microbes possessed the ability to only partially oxidize organic matter, the partially-oxidized matter, or "POOM," would effectively become "sticky," and chemically bind to minerals in sediment
in a way that would protect the material from further oxidation. The
oxygen that would otherwise have been consumed to fully degrade the
material would instead be free to build up in the atmosphere. This
process, they found, could serve as a positive feedback, providing a
natural pump to push the atmosphere into a new, high- oxygen equilibrium.

"That led us to ask, is there a microbial metabolism out there that
produced POOM?" Fourier says.

In the genes To answer this, the team searched through the scientific literature and identified a group of microbes that partially oxidizes
organic matter in the deep ocean today. These microbes belong to the
bacterial group SAR202, and their partial oxidation is carried out
through an enzyme, Baeyer-Villiger monooxygenase, or BVMO.

The team carried out a phylogenetic analysis to see how far back the
microbe, and the gene for the enzyme, could be traced. They found that
the bacteria did indeed have ancestors dating back before the GOE, and
that the gene for the enzyme could be traced across various microbial
species, as far back as pre-GOE times.

What's more, they found that the gene's diversification, or the number
of species that acquired the gene, increased significantly during times
when the atmosphere experienced spikes in oxygenation, including once
during the GOE's Paleoproterozoic, and again in the Neoproterozoic.

"We found some temporal correlations between diversification of
POOM-producing genes, and the oxygen levels in the atmosphere," Shang
says. "That supports our overall theory." To confirm this hypothesis
will require far more follow-up, from experiments in the lab to surveys
in the field, and everything in between. With their new study, the team
has introduced a new suspect in the age-old case of what oxygenated
Earth's atmosphere.

"Proposing a novel method, and showing evidence for its plausibility,
is the first but important step," Fournier says. "We've identified this
as a theory worthy of study." This work was supported in part by the
mTerra Catalyst Fund and the National Science Foundation.


========================================================================== Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by Jennifer
Chu. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Haitao Shang, Daniel H. Rothman, Gregory P. Fournier. Oxidative
metabolisms catalyzed Earth's oxygenation. Nature Communications,
2022; 13 (1) DOI: 10.1038/s41467-022-28996-0 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220314095704.htm

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