Tuesday, 16 August 2011

Yet More Models Of Early Life On Earth Using Yeast

Today, oxygen makes up around 21 % of the Earth's atmosphere. However billions of years ago, it was almost non existent. However a team of MIT scientists led by Jacob Waldbauer believe that there was vast quantities of oxygen in the early Earth. However the life giving gas would have existed in a molecular form dissolved in the ocean rather than free in the atmosphere. The gas would have existed in small pockets or 'oasis' that could have harboured life.

Yeast cells under a microscope (Saccharomyces cerevisiae)
The team found that yeast (Saccharomyces cerevisiae) - an organism with the ability to survive with or without oxygen - is able to produce oxygen dependant compounds vital to life with molecular amounts of the gas. About a decade ago, a team of geochemists found a sedimentary rock that contained vast amounts of fossil steroids. The rock and the steroids were dated to 2.7 billion years old, 300 million years older than the Great Oxygenation Event, when oxygen became a prevalent gas in the atmosphere around 2.4 billion years ago.

Sterols, more commonly known as steroids, such as cholesterol, are common components of cell membranes. They also require at least 10 molecules of free oxygen for synthesis. If there was not an available source of oxygen, the creation of such a molecule would have been very unlikely, let alone an entire cell membrane composed of tens of thousands of sterol molecules. This is where the oxygen oases come in. If they existed, they would have given early life enough oxygen to survive, grow and reproduce.

To test the theory, they set up an experiment using yeast. Yeast uses a combination of oxygen and sugars to create ergosterol - its primary sterol. The yeast can reproduce without oxygen provided that a source of ergosterol is provided. To find the lowest level of oxygen which yeast can consume, Waldbauer grew yeast cells in an environment containing no oxygen, ergosterol and glucose containing a mildly radioactive form of carbon - carbon 13 - as a tracer to see whether the yeast were consuming oxygen.

The team found that the yeast happily took up the ergosterol, but left the glucose. However when a truly negligible, molecular amount of oxygen was introduced to the sample, the yeast began to manufacture bio-synthesised ergosterol. The team could distinguish this from the provided sample due to the contained and traceable amount of carbon 13 within the glucose that the yeast cells used in conjunction with the provided trace amount of oxygen to produce ergosterol.

This experiment shows that early life could have lived without the presence of oxygen. However to become complex, they would have required oxygen. These possible oxygen oases might have played a major role in the evolution of complex life, once again changing the story of our planet's greatest marvel.