Thin air is no problem for the hardy yeast. Researchers have found that the tiny, oxygen-dependent fungi can churn out some of their most basic building blocks using only trace amounts of the gas. The finding may hint at how organisms living billions of years ago eked out an existence before the planet became flush with oxygen.
In Earth's early eons, few if any oxygen molecules floated free in the atmosphere. Then about 2.4 billion years ago, woosh, the gas spilled into the air as bacteria-like photosynthesizers took over the oceans. Before this "Great Oxygenation Event," oxygen-dependent organisms, such as the one-celled ancestors of yeast and animals, would have found the planet an inhospitable place.
But many may have still managed to grab a toehold. About a decade ago, researchers unearthed the fossilized remnants of steroid molecules, which require oxygen to make, dating back more than 300 million years before the ancient gas-a-thon. For scientists, these molecules, key ingredients in the fatty membranes that protect a number of cells, represented a "conundrum," says Ariel Anbar, a geochemist at Arizona State University in Tempe, who was not involved in this study. At the time, oxygen would have needed to be slim enough in the air to not leave a trace in the geologic record but abundant enough in the oceans to sustain early life forms.
To find those just-right levels, Jacob Waldbauer, a biogeochemist at the California Institute of Technology (Caltech) in Pasadena, and colleagues at the Massachusetts Institute of Technology in Cambridge landed on an unlikely Goldilocks: yeast. Yeast cells probably don't resemble their earliest relatives much (the briny oceans hardly crawled with Belgian microbrews). But the basics of steroid assembly also don't seem to have changed a lot over 2.4 billion years, Waldbauer says. Like many other modern microbes, yeast start with the carbon from several molecules of glucose, then tack on nine or more oxygen molecules to make a single steroid.
The team bathed yeast in liquid bubbled with tiny puffs of oxygen, discovering that the cells can patch together steroids at oxygen levels that would make even the stingiest animals choke . That is, about one molecule of oxygen gas for every 30,000 normally floating around in water today, Waldbauer and colleagues report online today in the Proceedings of the National Academy of Sciences. He estimates that pre-Great Oxygenation Event oceans may have held more than four times that amount.
That's because although the oxygen boom marked the beginning of the photosynthesizers' dominance, early oxygen-producers such as cyanobacteria, an ancient line of bacteria, could have swum about long before. Other gases in that early atmosphere, however, may have combined with much of the oxygen that those microbes spat out, Waldbauer says. Even so, some oxygen likely remained in the oceans, only to get quickly gobbled down by yeast's ancient relatives: "That fast cycle could have been involved in making those steroids," he says.
Waldbauer's study gives "the best indirect evidence" so far that cyanobacteria were pumping out oxygen, whereas oxygen-dependent microbes sucked it down long before the gas became abundant, says Donald Canfield, a geobiologist at the University of Southern Denmark in Odense: "I think it's terrific."
But Joseph Kirschvink, a geobiologist who is also at Caltech, is a skeptic. Over 2 billion years of evolution separates yeast from organisms living during what scientists call the Archean, he says: "We are on very thin ice when you apply the present to the Archean." These early microbes could have produced membrane molecules all on their own, he adds, using long-lost tools that didn't require even a sniff of oxygen.