Today,
oxygen takes up a hefty portion of Earth’s atmosphere: Life-sustaining dioxygen
molecules make up 21% of the air we breathe. However, very early in Earth’s
history, dioxygen was a rare—if not completely absent—player in the turbulent
mix of primordial gases. It wasn’t until the “Great Oxidation Event”
(GOE), nearly 2.3 billion years ago, when oxygen made any measurable dent in
the atmosphere, stimulating the evolution of air-breathing organisms and,
ultimately, complex life as we know it today.
Now,
new research from the Massachusetts Institute of Technology (MIT) suggests dioxygen
may have been made on Earth hundreds of millions of years before its debut in
the atmosphere, keeping a low profile in “oxygen oases” in the
oceans. The MIT researchers found evidence that tiny aerobic organisms may have
evolved to survive on extremely low levels of the gas in these undersea oases.
In
laboratory experiments, former MIT graduate student Jacob Waldbauer, working
with Professor of Geobiology Roger Summons and Dianne Newman, formerly of MIT’s
Department of Biology and now at the California Institute of Technology, found
that yeast—an organism that can survive with or without oxygen—is able to
produce key oxygen-dependent compounds, even with only miniscule puffs of the
gas.
The
findings suggest that early ancestors of yeast could have been similarly
resourceful, working with whatever small amounts of dioxygen may have been
circulating in the oceans before the gas was detectable in the atmosphere. The
team published its findings in the Proceedings of the National Academy of
Sciences.
“The
time at which oxygen became an integral factor in cellular metabolism was a
pivotal point in Earth history,” Summons says. “The fact that you
could have oxygen-dependent biosynthesis very early on in the Earth’s history has
significant implications.”
The
group’s results may help reconcile a debate within the earth sciences
community: About a decade ago, geochemists encountered sedimentary rocks
containing fossil steroids, an essential component of some organisms’ cell
membranes. Making a single molecule of a sterol, such as cholesterol, from
scratch requires at least 10 molecules of dioxygen; since the molecular fossils
date back to 300 million years before the GOE, some have interpreted them as
the earliest evidence of oxygen’s presence on Earth. But because other evidence
for the presence of oxygen in rocks of similar age is inconclusive, many
geologists have questioned whether the fossilized steroids are indeed proof of
early oxygen.
Waldbauer
and colleagues suggest that perhaps dioxygen was in fact present on Earth 300
million years before it spiked in the atmosphere—just at extremely low
concentrations that wouldn’t have left much of a trace in the rock record. They
reasoned that, even at such low levels, this dioxygen may have been sufficient
to feed aerobic, sterol-producing organisms.
To
test their theory, they looked to modern yeast as a model. Yeast naturally uses
dioxygen, in combination with sugars, to synthesize ergosterol, its primary
sterol. Yeast can also grow without dioxygen, so long as a source of ergosterol
is provided. To find the lowest level of dioxygen yeast can consume, the team
set up an experiment to identify the point at which yeast switches from anaerobic
to aerobic activity.
Waldbauer
grew yeast cells with a mixture of essential ingredients, including ergosterol
as well as glucose labeled with carbon-13. They found that, without oxygen
present, yeast happily took up sterol from the medium but made none from
scratch. When Waldbauer pumped in tiny amounts of oxygen, a switch occurred,
and yeast began using dioxygen in combination with glucose to produce its own
sterols. The presence of carbon-13 differentiates the biosynthesized sterol
from that acquired from the growth medium.
The
scientists found that yeast are able to make steroids using vanishingly small,
nanomolar concentrations of dioxygen, supporting the theory that oxygen —and
its producers and consumers—may have indeed been around long before the gas
made an appearance in the atmosphere.
Waldbauer
and Summons surmise that oxygen production and consumption may have occurred in
the oceans for hundreds of millions of years before the atmosphere saw even a
trace of the gas. They say that in all likelihood, cyanobacteria, blue-green
algae living at the ocean surface, evolved the ability to produce dioxygen via
sunlight in a process known as oxygenic photosynthesis. But instead of building
up in the oceans and then seeping into the atmosphere, dioxygen may have been
rapidly consumed by early aerobic organisms. Large oceanic and atmospheric
sinks, such as iron and sulfide spewing out of subsea volcanoes, likely
consumed whatever dioxygen was left over.
“We
know all kinds of biology happens without any dioxygen at all,” says
Waldbauer, now a postdoc at Caltech. “But it’s quite possible there was a
vigorous cycle of dioxygen happening in some places, and other places it might
have been completely absent.”