Did Microbes Cause The Earth's Most Devastating Mass Extinction?

A little over 250 million years ago, our planet experienced a mass extinction the likes of which have never been seen before or since. About 90% of all species were suddenly wiped out. And new study suggests it wasn't caused by an asteroid or super-volcano — but rather methane-spewing microbes.

It's known as the Permian-Triassic Extinction Event. By the time it ended, some 96% of all marine animals were gone, and about 70% of terrestrial vertebrates. It's also the only known mass extinction of insects, in which some 57% of all families and 83% of all genera disappeared.

A number of theories have been put forth to explain the extinction, including asteroid impacts, increased volcanism, coal fires and explosions from the Siberian Traps, and the sudden release of methane clathrate from the sea floor. Whatever it was, it completely wrecked the Earth's carbon cycle.

Super-Exponential Growth and a Methane Burst

But there's another theory out there that's being considered by MIT researchers Daniel Rothman, Gregory Fournier, and several other researchers at MIT and in China, which implicates a type of microbe instead — the methane-producing Methanosarcina. These microbes appeared and blossomed at just the right time to wreak unspeakable havoc around the globe.

Around 252 million years ago, Methanosarcina bloomed explosively in the oceans, spewing massive amounts of methane into the atmosphere, dramatically changing the climate and the chemistry of the oceans.

At first it may be hard to believe that tiny micro-organisms could wreak such massive destruction. But in vast quantities, they can literally reshape the biosphere. In this case, the microbes may have grown at a super-exponential rate. Geochemical evidence shows that this growth coincided with the mass extinction, and the expansion of a new microbial metabolic pathway. Indeed, genetic evidence points to a change in Methanosarcina during this time which enabled it to become a major producer of methane by accumulating carbon dioxide in the water.

As the microbes exploded in population, and as all the resulting methane drifted up into the atmosphere, the conditions began to resemble current models of global climate change — a sudden, extreme rise in global temperatures, combined with acidification of the oceans.

The researchers describe what likely happened next:

The ensuing biogeochemical disruption would likely have been widespread. For example, anaerobic methane oxidation may have increased sulfide levels, possibly resulting in a toxic release of hydrogen sulfide to the atmosphere, causing extinctions on land. Although such implications remain speculative, our work makes clear the exquisite sensitivity of the Earth system to the evolution of microbial life.

Volcanoes Still Part of the Process

Interestingly, volcanoes may still have contributed to the mass extinction. The explosive growth of the microbes may have benefited from their newfound ability to use a rich source of organic carbon — a process aided by a dramatic influx of a nutrient required for growth, namely nickel. And it just so happens that nickel was being churned out by volcanoes around this time.

What's more, the recorded changes in the amount of carbon over time doesn't fit the volcanic model. The researchers say that the growth of microbial populations is among the few phenomenon capable of increasing carbon production exponentially, or even faster. With volcanoes, it's the other way around.

Read the entire study at PNAS: "Methanogenic burst in the end-Permian carbon cycle."

Image: Chris Butler/science Photo