Sunday, 11 October 2015

Pushing Back The Origin Of Cyanobacteria

Cyanobacteria were responsible form the Great Oxidation Event
Around 2.5 billion years ago the planet was changed forever: oxygen began to accumulate in the atmosphere. Clouds of methane, which had encircled the Earth for billions of years were oxidised. Temperatures plummeted, triggering the first ice age in the history of the planet. Life on Earth at the time was entirely anaerobic in nature. The new gas was a toxin present on a global scale and may well have caused the first mass extinction in the history of the planet. At the very least, the Earth's geochemistry was radically altered; for all intents and purposes it was a brand new planet.

The Great Oxidation Event, as it is known, is immensely important in the history of the planet. The accumulation of oxygen in the atmosphere required some kind of photosynthetic microorganism and so the geochemical signature of oxygen is taken as evidence of cyanobacteria living at least 2.5 billion years ago. Of course the Great Oxidation Event did not occur overnight. It took tens and possibly hundreds of millions of years to transform the atmosphere, suggesting that cyanobacteria were even older still.

Whiffs of oxygen from 2.9 billion years ago suggested that cyanobacteria may have had an evolutionary history extending back at least as far. A recent discovery, however, has pushed that history back again by an incredible 300 million years. Analysis of a 3.23 billion year old sediment core sample from South Africa show a small but definite signature of having been deposited in an oxygenated environment. The core consisted of several different rock types each of which were analysed using mass spectroscopy with the purpose of detecting the ratios of iron isotopes.

The core sample from South Africa
'Iron oxides contained in the fine-grained, deep sediment that formed below the level of wave disturbance formed in the water with very little oxygen,' said Aaron Satkoski from the University of Wisconsin-Madison. The grainier parts of the sample, however, formed from shallow, wave-stirred sediment rusty. The type of iron oxide, namely ferric oxide, requires more oxygen to form. A turbulent environment close to the surface would have provided the oxygenated environment required to produce the chemical signal of the grainier rock.

Confirmation of the iron results came from studies of uranium and its decay products in the samples, Uranium is only soluble in the oxidized form, so the uranium in the sediment had to contain oxygen when the rock solidified,' said Brian Beard from the University of Wisconsin-Madison. Measurements of lead formed from the radioactive decay of uranium showed that the uranium entered the rock sample 3.2 billion years ago. 'This was an independent check that the uranium wasn't added recently. It’s as old as the rock; it’s original material.'

Various forms of photosynthesis exist but only cyanobacteria perform oxygenic photosynthesis. 'There was evolutionary pressure to develop oxygenic photosynthesis,' said Clark Johnson, also from the University of Wisconsin-Madison. 'Once you make cellular machinery that is complicated enough to do that, your energy supply is inexhaustible. You only need sun, water and carbon dioxide to live.' Scientists have long suspected that oxygenic photosynthesis and cyanobacteria pre-dated the Great Oxidation Event. Conclusive fossil evidence of the antiquity of these things is hard to come by, but the geochemical record is much harder to deceive,