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Friday, August 26, 2011

Early oxygen usage

Aerobic organisms may have thrived on tiny amounts of oxygen in Earth’s oceans before there was any atmospheric oxygen (O2).  Jacob Waldbauer and his team from MIT have shown that yeast can utilize O2 in even nanomolar amounts (billionths of a percent).

Although O2 is an abundant and critical part of Earth’s atmosphere today, this wasn’t always the case.  About 2.3 billion years ago, the ‘Great Oxidation Event’ (GOE) forever changed the composition of our air, and led to the evolution of all complex life on Earth today*.  However, there is some indication in the fossil record that organisms were using O2 as long as 2.6 billion years ago, before there was any noticeable amount available. Could such tiny amounts of O2 trigger a change from an anaerobic to an aerobic lifestyle? To answer that question, the MIT team grew modern yeast cells under varying conditions.

Without oxygen, yeast cells require ergosterol in their growth medium, but if O2 is present they will switch to metabolizing glucose.  The team provided yeast cells with an anaerobic, or oxygen-free, environment in which to live.  The growth medium contained both ergosterol and radioactively labeled glucose.  They then provided tiny amounts of oxygen. They yeast began metabolizing the carbon13 labeled glucose when O2 levels were barely detectable. 

This suggests that aerobic metabolisms may have evolved on Earth long before the GOE. Even negligible levels of O2 in isolated pockets of the Earth’s early oceans may have been utilized by opportunistic organisms.

*A single example of an anaerobic multicelluar organism has been discovered.