How Photosynthesis Evolved During The Great Oxidation Event

Photosystem I is highly sensitive to oxygen due
to the reactive nature of its iron sulphur clusters

Photosynthesis is one of the most ancient forms of metabolism, dating back close to the dawn of life itself. As such, it has been enriching our atmosphere with oxygen for billions of years.

For hundreds of millions of these, however, oxygen quickly reacted with compounds in the atmosphere and lithosphere to form oxides. It was only during the Great Oxidation Event that the Earth's oxygen sinks were filled and the gas accumulated in the atmosphere.

The effects on the planet were massive, particularly on the biosphere. Oxygen is highly reactive and would have placed extreme environmental stress on all life forms whose metabolic systems had evolved in anaerobic conditions. It may well have caused the first mass extinction in evolutionary history. Adaptation to the Great Oxidation Event is evident in the fossil record.

A new study, however, demonstrates one of the molecular adaptations which occurred during this time. Photosynthesis, ironically, is very sensitive to oxygen due to the very precise and highly controlled nature of the reactions involved. Particularly important are clusters of iron sulphur minerals in the proteins which catalyse the photosynthetic reactions and transfer electrons. Oxidation of these minerals would make the reaction pathways impossible.

Prior to the Great Oxidation Event the molecular machinery involved in photosynthesis required little or no protection and stabilisation, as the oxygen generated was immediately scrubbed from the atmosphere by environmental oxygen sinks. An aerobic atmosphere, however, presented significant problems. The new study, conducted by researchers from a number of US universities, demonstrated that a particular protein, CGL71, evolved to protect photosynthetic machinery from free oxygen during the assembly of photosystem I, one of two broad sets of reactions involved in photosynthesis.

'When we look at this critical assembly protein, CGL71, it's as if we are looking back in time to the era when photosynthetic apparatus had to gradually adjust to the changing atmospheric conditions of our planet,' said Arthur Grossman from the Carnegie Institution. This new finding clearly demonstrates the impact of the Great Oxidation Event on the molecular metabolics of life, an aspect of evolution sometimes overlooked due to the near ubiquitous nature of such systems as well as long term evolutionary stability.