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Everyone thank a yeast cell today!

Appearance of oxygen in the Earth's atmosphere

One of the events during the early days of Life on Earth was the “Great Oxygenation Event” or “Oxygen Catastrophe”. At this point in our planet’s history, significant amounts of free oxygen gas began to appear in the atmosphere, due to the action of photosynthetic microorganisms. This happened approximately 2.4 billion years ago, and prior to this time, the atmosphere was considered a reducing environment with no free oxygen at all. Geochemical evidence suggests that photosynthetic organisms began to produce oxygen in measurable amounts at this time point, however levels remained relatively low for over 1 billion years, due to the action of organic and inorganic environmental sources reacting with it. The geologic record clearly shows the appearance of oxides of iron (essentially rust) appearing at this point. The action of photosynthetic organisms did produce oxygen prior to this time (indicated by the question mark in the Wikipedia graphic shown,) however it did not accumulate to any measurable amount due to the immediate capture of oxygen by compounds such as iron. The reason that this point in our planet’s history is also called the “Oxygen Catastrophe” is because the appearance of measurable oxygen at the 2.4 billion year mark also marked the single most significant extinction event in the history of life on earth.

Oxygen is a compound that paradoxically is essential for life, and at the same time is deadly for living cells. Early cellular life was exclusively anaerobic, and cells carried out all metabolic processes in the absence of oxygen. After the appearance of free oxygen, cells began to take advantage of it as a terminal electron acceptor for energy metabolism, and a variety of oxygen dependencies began to appear in living systems. These included obligate aerobes (dependent on the presence oxygen for growth; animals, many fungi, and some bacteria are examples), facultative aerobes (can utilize oxygen, but can also grow in the absence of oxygen; many bacteria and fungi are examples), and obligate anaerobes (can only grow in the absence of oxygen; all life on Earth initially was an example). Aerobic organisms would actually be killed by oxygen just like anaerobic organisms, except that they have acquired enzymes such as catalase which enable them to grow in the presence of oxygen.

So what does this have to do with the yeast cells from the title? A summary from ScienceNow presents recent findings published in the Proceedings of the National Academy of Sciences (PNAS). Researchers at MIT studied the single celled eukaryote Saccharomyces cerevisiae. They were prompted to pursue these experiments based on two previous observations by geochemists: first, photosynthesis and the production of free oxygen preceded the appearance of free oxygen in the atmosphere by hundreds of millions of years, and second, steroid molecules (components of many membranes) can be detected in fossilized remains of microorganisms in geologic strata predating the 2.4 billion year mark. The presence of steroids is an interesting conundrum, as they require the presence of oxygen to synthesize. Sterols in membranes are good indicators of eukaryotic life; bacterial cytoplasmic membranes do not typically contain sterols, however the enzymatic machinery to synthesize these molecules is highly conserved among those cells that do make them.

The work done by these researchers was to determine whether yeast cells are able to synthesize sterols at extremely low oxygen concentrations, mimicking those that could have existed at the period of time prior to the rapid rise in global oxygen concentrations 2.4 billion years ago. They did so by growing yeast in a minimal medium lacking sterols, but including radiolabeled glucose. If the cells are able to synthesize sterols, the radioactivity in the glucose would then be incorporated into newly synthesized sterols as the cells grow. Their experiment then is relatively simple: identify what sterols containing radioactivity can be isolated at different concentrations of oxygen. Under anaerobic conditions (defined as an O2 concentration of less than 7 nM), radioactive incorporation into steroids was undetectable, but was detectable at all higher levels. The conclusion from this observation is that the biological activity of steroid biosynthesis could have acted as a living “sink” for the molecular oxygen produced by early photosynthetic microorganisms, holding off the Oxygen Catastrophe for many hundreds of millions of years. And remember, those single-celled anaerobes were our forebears, prevented from being killed potentially by the actions of our friends, the yeasts!

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