By Deborah Netburn, Los Angeles Times

LOS ANGELES — Tiny microbes on the bottom of the ocean floor may have been responsible for the largest extinction event our planet has ever seen, according to a new study.

These microbes of death were so small, that one billion of them could fit in a thimble-full of ocean sediment, and yet, they were almost responsible for killing off all the life on our planet, the scientists suggest.

The end-Permian extinction was the most catastrophic mass extinction the Earth has ever seen. It started roughly 252 million years ago — long before the dinosaurs — and it continued for 20,000 years. By the time it was over, nearly 90 percent of all life on Earth had been destroyed, the scientists say.

“It was not as dramatic as the impact that probably killed the dinosaurs, but it was worse,” said Gregory Fournier, an evolutionary biologist at MIT. “Things were very close to being over for good.”

Scientists have struggled to understand exactly what caused the long, slow, mass die-off in this dark era of our planet’s history. The geologic record tells us there was a sharp uptick of C02 levels at the time. That would have caused the oceans to acidify and the Earth to heat up, making the environment inhospitable for most forms of life. But what actually caused the C02 levels to rise has remained a mystery.

Some scientists have suggested an asteroid impact could be to blame; others have proposed that volcanic activity or coal fires might be the culprit.

Now, in a paper published this week in PNAS, researchers from the Massachusetts Institute of Technology and the Chinese Academy of Sciences in Nanjing, China, have fingered a new and unlikely suspect — a tiny methane-spewing microbe known as Methanosarcina.

The first clue that microscopic microbes could be involved in the greatest die-off the Earth has ever known came when MIT geophysicist Dan Rothman was looking at how carbon levels grew during this time. What he saw was not a straight line, but rather a rapid upward curve.

“The growth was like what you might see in a real estate bubble, or a financial bubble,” he explained. “If the C02 came from the sudden combustion of a coal field in Siberia it wouldn’t behave this way. It has this special character that is consistent with microbial processes.”

It was the first time anyone had suggested microbes might be involved with the end-Permian extinction, but far from the first time that microbes have been accused of changing the chemistry of our planet. For example, photosynthetic microbes are responsible for creating the first oxygen in the Earth’s atmosphere.

“It is absolutely normal that microbes are mediating the great elemental cycles,” said Rothman. “What they do is crucial. And if they do better or worse, things change.”

Fournier put it this way: “Microbiologists like to say, ‘Microbes rule the Earth, and we just live on it.’”

To figure out which specific microbe might have a hand in this ancient catastrophe, Rothman took his research to Fournier, who had published a paper about Methanosarcina in 2008. The paper showed that sometime in the last 400 million years, Methanosarcina was the recipient of a gene transfer that allowed it to produce methane more efficiently than ever before.

After seeing Rothman’s analysis, Fournier worked to date the time of the gene transfer more specifically and found it most likely occurred about 250 million years ago.

Although the researchers cannot say for certain that the microbe and the vast quantities of methane it produced were responsible for the end-Permian extinction, they do have one more line of evidence to support their hypothesis. In order to turn acetate into methane, Methanosarcina needs nickel.

“Even if they had all the food in the world they would be limited if they were starved for nickel,” Fournier said.

But if there was a lot of nickel around, the microbes would be unhindered. And, wouldn’t you know, much of the volcanic activity at the time occurred around the Siberian traps, which has some of the world’s largest deposits of nickel. Further more, the researchers found an increase in nickel in sediments from that time.

“Our proposal is unusual, but it does bring together many observations, and ties a lot of stuff together,” Rothman said. “That doesn’t make it right, but it is consistent, and that’s what is necessary to move forward and provide further tests.”