In a mere blink of time, pond life put the planet on ice

August 5, 2005

It took an algae just a geological instant to create a snowball Earth. Anna Fazackerley explores the first biogenic climate disaster

Cyanobacterium does not sound particularly threatening. At its most troublesome, when polluting a lake or pond, for example, it can cause sickness in humans and has been known to kill the occasional pet dog.

But some 2.3 billion years ago, this seemingly innocuous single-cell organism turned on the Earth and sent life itself plummeting towards oblivion. In what has been described as the first biogenic climate disaster, the evolution of cyanobacteria - commonly known as blue-green algae - unleashed a great "snowball Earth" event that suffocated our planet under a thick blanket of impenetrable ice for up to 70 million years.

The Earth on which cyanobacteria first emerged was an alien environment. Geologists believe that 2.5 billion years ago, at the start of the Palaeoproterozoic era, there was little or no oxygen in the atmosphere while methane existed in concentrations thousands of times higher than it has today. Levels of carbon dioxide were also much greater.

Despite the fact that the young Sun was about 20 per cent less bright, the two powerful greenhouse gases combined to insulate the planet, making it far warmer.

In such a world, cyanobacteria emerged as the first organisms able to use water to photosynthesise, needing only nutrients such as phosphate to harness the energy of the Sun - Jand, as a by-product, pump oxygen into the atmosphere.

Most other organisms around at the time would have been unable to cope with the poisoning resulting from this ruthless pollution. But worse, the rising level of oxygen in the atmosphere would have wrecked the methane greenhouse that had kept the whole planet so warm for so long.

Research by Joseph Kirschvink, professor of geobiology, and Robert Kopp, a research student at the California Institute of Technology in the US, suggest this could have happened much faster than anyone had previously suspected. In a paper published this week in the Proceedings of the National Academy of Sciences , the scientists argue that cyanobacteria evolved about 2.3 billion years ago and had an immediate and dramatic impact on the Earth.

As the oxygen produced by the bacteria destroyed the methane greenhouse, temperatures dropped. Land and oceans swiftly became swathed in ice. In a terrible feedback loop, as the planet became increasingly sheathed in white, more of the Sun's energy was reflected back into space and so temperatures fell even further, plunging to - 50C.

For between 35 million and 70 million years, the Earth was completely shrouded in ice about a mile thick. It was a frozen planet. Kirschvink notes: "This scenario raises the spectre of one mutant organism being able to destroy an entire planetary ecosystem. Perhaps it was the first biogenic climate disaster."

Yet it is the speed of the event suggested in the scientists' paper that they find startling. "The idea that the rise of oxygen triggered a glaciation is itself not controversial," they say. "But we believe this was a much more rapid process than people think. Cyanobacteria evolved and, in a relatively short period of time, caused the Earth to be encased in ice for a very long period of time."

Most scientists investigating the so-called Palaeoproterozoic snowball Earth event agree that it happened 2.3 billion years ago. But there is no consensus as to how long the cyanobacteria had been around before they started to alter the atmosphere.

One line of thought holds that they had already existed for perhaps 400 million years, another that they had evolved very early on, perhaps 1.5 billion years before.

Kopp and Kirschvink are highly sceptical of the ancient organic compounds that some claim as evidence that the cyanobacteria had been around for a long time before the snowball. Besides concern over possible sample contamination, they point out that such biomarkers could indicate the existence of some completely different organism that had become extinct.

The scientists modelled a series of known pre-snowball episodes in Canada and South Africa in which the clash between glaciers and rocks released enough nutrients into the oceans to allow the cyanobacteria to destroy the methane greenhouse in as little as a million years.

But that did not happen. There is no clear record of oxygen in the atmosphere during these milder glacial periods that preceded the snowball, between about 2.9 billion and 2.32 billion years ago. Therefore, Kopp and Kirschvink argue, the bacteria were simply not present then, evolving only shortly before the onset of the snowball.

James Kasting, distinguished professor of geosciences and meteorology at Penn State University in the US, likes the idea, though he still regards it as speculative. "It would resolve the problem of why there was a long delay between the origin of cyanobacteria and the rise of atmospheric oxygen," he says.

Sceptics have dismissed the very concept of a snowball Earth because they say that life could not have recovered from such a catastrophe.

Kirschvink argues that while the Earth was frozen, volcanoes remained active. Volcanic gases gradually built up and, once there was enough carbon dioxide in the atmosphere, the planet began to thaw. The ancestors of all the organisms alive today somehow managed to survive beneath the ice until conditions improved.

But Kopp points out: "Because the record of life in the Palaeoproterozoic is fairly poor, we cannot say how many more branches of the tree of life fell extinct in the disaster."

The shadow of global warming that hangs over us today is alarming. But the Earth has endured far worse. That possibly the most extreme climate disaster could have been triggered by the activities of such an apparently insignificant organism as cyanobacteria is especially disquieting.

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