Monday, 19 July 2010


Living on a planet with an oxygen-rich atmosphere we tend to forget that our planet is an anomaly.

About 4.5 billion years ago, when the solar system accreted out of a disk of gas and dust, the Earth was thoroughly reduced. Over the course of the first 1-2 billion years our planet became slowly, but inextricably ever more oxidized. Vast amounts of iron rich sediments precipitated out of the oceans, known as "Banded Iron Formations" or BIFs, indicating that reduced ferrous iron, Fe2+, converted into ferric iron, Fe3+. This required a large, sustained supply of oxidizing power.

In his 1984 book, "The Chemical Evolution of the Atmosphere and Oceans," H. D. Holland estimated that, over the 2+ billion years during which the BIFs precipitated, at least 1012 gram oxygen had to be injected into the Earth's oceans every year. Sometime around 2.4 to 2.3 billion years ago, the global oxidation accelerated. During this remarkable period, known as the "Great Oxidation Event," free O2 appeared in Earth's atmosphere and soon increased to the over 20% O2, which we now enjoy.

The Great Oxidation Event is attributed to the invention of photo synthesis: the capacity of living organisms, using sunlight, to split H2O and CO2 into O2 plus reduced H and C, which in turn combine to produce organic compounds. New forms of life appeared that could harness the newly available chemical energy: first microbial and later multi-cellular organisms prospered in the oceans and eventually conquered the land.

If the Great Oxidation Event can be linked to oxygenic photo synthesis, the question remains what process might have driven the earlier slow oxidation of Earth.

One school of thought has promoted the idea that some form of oxygenic photo synthesis was invented very early on, soon after the origin of life. Maybe colonies of photosynthetic bacteria, similar to today's cyanobacteria, were building stromatolites in shallow waters along the coasts of early continents, pumping out enough oxygen to precipitate the BIFs and prepare the way for the stupendous rise of free O2 in Earth's atmosphere during the Great Oxidation Event.

The "invention" of oxygenic photosynthesis so early in Earth's history poses serious problems. Oxygen is one of the most reactive elements in nature, and is toxic to life adapted to reducing environments. Before pumping out oxygen as part of their metabolism, the microorganisms must have learned how to handle this dangerous by-product of their cellular biochemistry and how to extract the energy. They must have learned how to detoxify those Reactive Oxygen Species, commonly referred to in microbiologists' circles as ROS, which are the scourge of all forms of life.

One possible solution to this dilemma is that, long before the Great Oxidation Event, the Earth might have been slowly oxidized by some non-biological process. Such a process would have given the microorganisms time to adapt to the changing environment or, as Dr. Lynn Rothschild of the NASA Ames Research Center said, "It would have provided a training ground for early life to learn how to handle oxygen."

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