2.5亿年前的物种大灭绝由细菌引起

Evidence left at the crime scene is abundant and global: Fossil remains¹ show that sometime around 252 million years ago, about 90 percent of all species on Earth were suddenly wiped out -- by far the largest of this planet's five known mass extinctions. But pinpointing³ the culprit（罪犯） has been difficult, and controversial. Now, a team of MIT researchers may have found enough evidence to convict the guilty parties -- but you'll need a microscope to see the killers⁴.

 

The perpetrators, this new work suggests, were not asteroids⁵, volcanoes, or raging coal fires, all of which have been implicated⁶ previously⁷. Rather, they were a form of microbes -- specifically, methane⁸-producing archaea（古生菌） called Methanosarcina -- that suddenly bloomed explosively in the oceans, spewing prodigious⁹（惊人的） amounts of methane into the atmosphere and dramatically changing the climate and the chemistry of the oceans.

 

Volcanoes are not entirely¹⁰ off the hook, according to this new scenario¹¹; they have simply been demoted to accessories to the crime. The reason for the sudden, explosive growth of the microbes, new evidence shows, may have been their novel ability to use a rich source of organic carbon, aided by a sudden influx¹² of a nutrient¹³ required for their growth: the element nickel, emitted by massive volcanism at just that time.

 

The new solution to this mystery is published this week in the Proceedings¹⁴ of the National Academy of Sciences by MIT professor of geophysics Daniel Rothman, postdoc Gregory Fournier, and five other researchers at MIT and in China.

 

The researchers' case builds upon three independent sets of evidence. First, geochemical evidence shows an exponential (or even faster) increase of carbon dioxide in the oceans at the time of the so-called end-Permian extinction². Second, genetic¹⁵ evidence shows a change in Methanosarcina at that time, allowing it to become a major producer of methane from an accumulation of carbon dioxide in the water. Finally, sediments¹⁶ show a sudden increase in the amount of nickel deposited at exactly this time.

 

The carbon deposits show that something caused a significant uptick（提升） in the amount of carbon-containing gases -- carbon dioxide or methane -- produced at the time of the mass extinction. Some researchers have suggested that these gases might have been spewed out by the volcanic¹⁷ eruptions¹⁸ that produced the Siberian traps, a vast formation of volcanic rock produced by the most extensive eruptions in Earth's geological record. But calculations by the MIT team showed that these eruptions were not nearly sufficient to account for the carbon seen in the sediments. Even more significantly, the observed changes in the amount of carbon over time don't fit the volcanic model.

 

"A rapid initial injection of carbon dioxide from a volcano would be followed by a gradual decrease," Fournier says. "Instead, we see the opposite: a rapid, continuing increase."

 

"That suggests a microbial expansion," he adds: The growth of microbial populations is among the few phenomena¹⁹ capable of increasing carbon production exponentially, or even faster.

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