The end-Permian extinction is associated with a mysterious disruption to Earth’s carbon cycle. Here we identify causal mechanisms via three observations. First, we show that geochemical signals indicate superexponential growth of the marine inorganic carbon reservoir, coincident with the extinction and consistent with the expansion of a new microbial metabolic pathway. Second, we show that the efficient acetoclastic pathway in Methanosarcina emerged at a time statistically indistinguishable from the extinction. Finally, we show that nickel concentrations in South China sediments increased sharply at the extinction, probably as a consequence of massive Siberian volcanism, enabling a methanogenic expansion by removal of nickel limitation. Collectively, these results are consistent with the instigation of Earth’s greatest mass extinction by a specific microbial innovation.

—Rothman et al.

The end-Permian extinction is the most severe in the fossil record; earlier research proposed linking its occurrence to CO₂ levels deriving from massive Siberian volcanism. However, such arguments have been difficult to justify quantitatively, the MIT team notes; quantitative estimates of direct volcanic outgassing are much too small to account for the changes in the carbon cycle. Other proposals suggest secondary effects of the volcanism—such as raging coal fires—as the mechanism.

The MIT proposal that the disruption of the carbon cycle resulted from the emergence of a new microbial metabolic pathway that enabled efficient conversion of marine organic carbon to methane also relies on a secondary effect of the volcanism—the nickel associated with the volcanic event.

They support their hypothesis with an analysis of carbon isotopic changes leading up to the extinction, phylogenetic analysis of methanogenic archaea, and measurements of nickel concentrations in South China sediments. The results, they suggested, highlight the sensitivity of the Earth system to microbial evolution.

The researchers build their case upon three independent sets of evidence:

1. Geochemical evidence shows an exponential (or even faster) increase of carbon dioxide in the oceans at the time of the end-Permian extinction.

2. Genetic evidence shows a change in Methanosarcina at that time, allowing it to become a major producer of methane from an accumulation of organic carbon in the water.

3. Sediments show a sudden increase in the amount of nickel deposited at exactly this time.

A rapid initial injection of carbon dioxide from a volcano would be followed by a gradual decrease. Instead, we see the opposite: a rapid, continuing increase. That suggests a microbial expansion. The growth of microbial populations is among the few phenomena capable of increasing carbon production exponentially, or even faster.

—Gregory Fournier, co-author

Genomic analysis revealed that Methanosarcina had acquired a particularly fast means of making methane via gene transfer from another microbe. The team’s detailed mapping of the organism’s history now shows that this transfer happened at about the time of the end-Permian extinction. (Previous studies had only placed this event sometime in the last 400 million years.)

Given the right conditions, this genetic acquisition set the stage for the microbe to undergo a dramatic growth spurt, rapidly consuming a vast reserve of organic carbon in the ocean sediments.

For this particular microbe, the limiting nutrient is nickel—which, new analysis of sediments in China showed, increased dramatically following the Siberian eruptions (which were already known to have produced some of the world’s largest deposits of nickel). That provided the fuel for Methanosarcina’s explosive growth.

Our principal observations—a superexponential burst in the carbon cycle, the emergence of efficient acetoclastic methano-genesis, and a spike in the availability of nickel—appear straight-forwardly related to several features of end-Permian environmental change: Siberian volcanism, marine anoxia, and ocean acidification. A single horizontal gene transfer instigated biogeochemical change, massive volcanism acted as a catalyst, and the resulting expansion of acetoclastic Methanosarcina acted to perturb CO₂ and O₂ levels. 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.

—Rothman et al.

The research was supported by NASA, the National Science Foundation, the Natural Science Foundation of China, and the National Basic Research Program of China.

Resources

• Daniel H. Rothman, Gregory P. Fournier, Katherine L. French, Eric J. Alm, Edward A. Boyle, Changqun Cao, and Roger E. Summons (2014) “Methanogenic burst in the end-Permian carbon cycle,” PNAS doi: 10.1073/pnas.1318106111