Oxygen may have been made on Earth hundreds of millions of years before it breathed new life into the atmosphere, researchers at the Massachusetts Institute of Technology, or MIT, discovered.
The MIT researchers suggest that oxygen may have been laying low in what they call "oxygen oases" in the oceans, long before the Great Oxidation Event, or GOE. Researchers have found evidence that small aerobic organisms may have evolved to survive on extremely low levels of oxygen in undersea oases.
Former MIT graduate student Jacob Waldbauer performed laboratory experiments with the help of Professor of Geobiology Roger Summons and Dianne Newman, on yeast, which can survive with or without oxygen. They found that yeast is able to produce key oxygen-dependent compounds, even with only little puffs of the gas.
This, they say proposes that early ancestors of yeast could have worked with whatever small amounts of oxygen that may have been circulating in the oceans before it was able to be detected in the atmosphere.
The finding is published 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 in a press release. "The fact that you could have oxygen-dependent biosynthesis very early on in the Earth's history has significant implications."
The MIT researchers' finding of potential early oxygen isn't the first, and it may have helped to settle a debate within the earth sciences community.
A MIT press release states that about 10 years ago, geochemists came across sedimentary rocks that contained fossil steroids, which are an essential make up of some organisms' cell membranes.
Making a single molecule of a sterol, such as cholesterol, from scratch requires at least 10 molecules of oxygen. And 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, according to the press release.
However, other evidence regarding the presence of oxygen in rocks of similar age is inconclusive, and so many geologists have questioned whether the fossilized steroids are indeed proof of early oxygen, the release also noted.
Waldbauer and his colleagues suggest that perhaps oxygen was in fact present on Earth 300 million years before it spiked in the atmosphere, but at concentrations so that that the gas wouldn't have left much of a trace in the rock record.
The MIT researchers reasoned that, even at such low levels, this oxygen may have been adequate enough to feed aerobic, sterol-producing organisms.
So the researchers tested their theory by looking to modern yeast as a model. Yeast naturally uses oxygen, in combination with sugars, to synthesize ergosterol, its primary sterol, and the organism can grow without the gas, provided there's a source of ergosterol.
The team then set up an experiment to identify the point at which yeast switches from anaerobic to aerobic activity. This was done in order to find the lowest level of oxygen yeast can consume.
They found that, without oxygen present, yeast took up sterol from the medium but made none from scratch.
When Waldbauer added tiny amounts of oxygen, the yeast began using oxygen in combination with glucose to produce its own sterols. Eventually the scientists found that yeast are able to make steroids using vanishingly small concentrations of oxygen, 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, MIT says.
"We know all kinds of biology happen without any O2 at all," Waldbauer says. "But it's quite possible there was a vigorous cycle of O2 happening in some places, and other places it might have been completely absent."