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MIT: Oxygen's watery past
Date:8/16/2011

ochemists 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 O2; 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 O2 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 O2 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 O2, in combination with sugars, to synthesize ergosterol, its primary sterol. Yeast can also grow without O2, so long as a source of ergosterol is provided. To find the lowest level of O2 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 O2 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 O2, sup
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Contact: Caroline McCall
cmccall5@mit.edu
Massachusetts Institute of Technology
Source:Eurekalert

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