The greatest mass extinction may have been the doing of microbes

The worst time to be alive in Earth’s history is unarguably the end-Permian, about 250 million years ago. It is the period when the greatest-ever extinction event recorded took place, killing 97% of all species, an event so severe it has been called The Great Dying.

This event has generally been blamed on massive volcanic eruptions that took place at the same time. But now, in a new analysis, researchers at the Massachusetts Institute of Technology (MIT) argue that the mass extinction event may have been instigated by microbes. These microbes led to a perturbation of the carbon cycle that caused environmental shocks, such as global warming and ocean acidification. The shocks wiped out species in great numbers over a period of tens of thousands of years – a blip on geological scales.

Felt like the end of time

The end-Permian extinction, which took place about 250 million years ago, is the most severe of five known mass extinction events. It killed off the last of the trilobites – a hardy marine species that had survived two previous mass extinction. While land plants survived, almost all forests disappeared. Worse of all, it is the only known extinction event where even insects weren’t spared.

For an event of this size to take place, a lot of things would have had to go wrong. At the time the world was made up of a single supercontinent called Pangea. This large landmass, by altering the dynamics of how carbon is cycled with subducting plates, may have pushed global temperatures to the highest they had ever been.

Then, over the course of about a million years, huge eruptions in Siberia created basalts that cover an area that was about seven times the size of France. This may have pushed the environment past a tipping point by sending even more carbon dioxide into the atmosphere. That would have caused the oceans to acidify, killing more marine life, and heat up, releasing frozen methane. The upshot of all this would have been a “runaway” climate that kept heating up and removing more oxygen from the environment.

The mighty microbe

But Daniel Rothman of MIT thinks that the numbers don’t add up. “The changes in the carbon cycle globally are difficult to reconcile with only volcanic activity in Siberia,” he said.

His calculations, just published in the Proceedings of the National Academy of Sciences, were hinting that something else must have caused the runaway event. One hypothesis was that microbial life may have been responsible for that.

“This hypothesis is not as outrageous as it seems. After all, about 2.4 billion years ago, it was microbes in the form of cyanobacteria that gave our atmosphere all of its oxygen,” Rothman added. That period, called the Great Oxygenation Event, also killed most organisms that were adapted to the lack of oxygen and began one of the longest cold periods in Earth’s history. So microbes can certainly have global impact.

With colleagues at MIT, Rothman looked at the evolutionary history of Earth and spotted the rise of a particular type of microbe that occurred around the time of the Great Dying. That microbe, called Methanosarcina, had the ability to digest organic matter to produce methane. (Molecular biologists at MIT have shown that Methanosarcina evolved this ability thanks to the transfer of a single gene from the Clostridia class of bacteria.)

Rothman knew that the chemical process involved in creating the methane relied on the metal nickel. He went looking for evidence that Methanosarcina was thriving at the time in the sedimentary layer of the Meishan region of China. If the environment at that time had any more nickel than normal, then the sediments would hold the record of it.

Rothman chose the Meishan region to look for nickel because it is a particularly well-studied region. Its sedimentary layers have been used to mark and standardise different periods of Earth’s geological history, and they span the period of the Great Dying.

The search was successful. There was indeed a higher amount of nickel in the sediments deposited during that period. Methanosarcina would not have just been effective at creating methane – they would have flourished.

The nickel, Rothman suggests, would have been added to the oceans, where Methanosarcina lived and grew, by the continuous volcanic activity occurring in Siberia. The growing amount of nickel, transported by ocean currents, would have allowed more Methanosarcina to convert organic matter into methane, which would be converted to carbon dioxide through reactions with oxygen. This would have meant increased global temperatures and acidification of the oceans. The latter would have combined with the loss of oxygen (used up in creating the carbon dioxide) to accelerate the extinction in the oceans. And the dead organisms would have provided Methanosarcina with more organic matter to digest.

In short, a microbial innovation may have tipped over the balance to cause the Great Dying.

Marc Reichow at the University of Leicester remains sceptical of these results. He argues that there is no evidence that the increased nickel came from Siberian volcanoes. Rothman agrees that current data cannot identify the source of the nickel.

“This is an interesting hypothesis, but I think that Great Dying was the doing of many ‘kill mechanisms’ rather than just a single mechanism suggested here,” Reichow said.

There is also doubt over the exact period in which Methanosarcina actually evolved. Current techniques for estimating its origins based on DNA sequence differences have a huge error margin, which means it could have been well before or after the Great Dying.

Rothman concedes that there are limitations. “We believe that volcanism alone could not have caused this extinction event. Instead, what we have done is broadened the conversation by suggesting that it is possible that microbes may have caused it to happen.”

“The implications for today are that there many ways in which natural fluctuations can happen in Earth’s carbon cycle. When studying the changes happening to the carbon cycle now, we should try to take into consideration as many of those as possible to make future predictions.”The Conversation

This article was originally published on The Conversation. Image credit tjt195 (CC-BY-NC).


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