New evidence has emerged for one of the theories competing to explain the largest mass extinction in the history of the Earth, an event so terrible it is known as The Great Dying. A team from MIT have sheeted the blame home to the methane producing single-celled organisms, Methanosarcina.
Any effort to explain the vanishing of 70% of terrestrial vertebrates during the Permian-Triassic Extinction Event has plenty of competition. Probably the most favored theory is the clathrate gun hypothesis, where a sharp rise in sea temperature or fall in sea level caused melting of methane clathrates that would in turn trigger much greater warming. Other proposals include gasses from a supervolcano, a nearby supernova or an asteroid strike. The last has recently had an usual twist with the proposal that a relatively modestly sized asteroid smashed into shale deposits, releasing such huge amounts of methane that its effect was more devastating than the larger asteroid responsible for the death of the dinosaurs.
Professor Daniel Rothman and colleagues agree that methane was responsible, but they pin the blame on a different source. There are three known metabolic pathways to produce methane. Methanosarcina is the only genus that uses all three: combining carbon dioxide and hydrogen gas, metabolizing acetate or transforming methanol or methylamines it produces most of the biologically-generated methane released today, including from cattle's stomachs.
Although Methanosarcina is today found everywhere from the deep ocean to the human gut, 252 million years ago Rothman believes it bloomed extensively across the oceans, potentially releasing so much methane that the climate broke. It is estimated that ocean surface temperatures reached 40°C (104 °F).
Methanosarcina has been implicated before as a cause for the Permian-Triassic extinction event. Two years ago Rothman noted that volcanic explanations don't fit with the speed with which methane rocketed upwards in the biological record. Methanosarcina utilizes nickel in methane production, and Rothman proposes that volcanic eruptions in the Siberian traps released the nickel into the oceans that allowed the growth to occur.
The nickel release itself would have occurred on the same geological timescales Rothman has criticized as too slow to explain the Great Dying, but in Proceedings of the National Academy of Sciences he and his colleagues say, “We propose 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.”
If nickel had been slowly building up in the oceans for some time, and some clever microbe suddenly worked out how to utilize it, we should not be surprised if it took over the oceans in unprecedented quantities. Too bad about all the other lifeforms who woke up and found the world had changed around them.
As support for the idea, the researchers point to three lines of evidence, “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.”
The second of these has been the biggest stumbling block to earlier versions of the theory. Previous estimates of the point where Methanosarcina gained the capacity to use nickel so efficiently have placed the timing 20 million years later. If Rothman is right, the pieces of the puzzle appear to fit together.
Other research has shown that Methanosarcina acquired the ability to produce methane in this way through the transference of a gene from a type of bacteria that had developed it hundreds of millions of years before, but never put it to such widespread use. By comparing the genomes of 50 species of Methanosarcina and related species and tracking the changes in ribosomal sequences Rothman produces an estimate of 240 million years, with an error margin of 41 million years, easily wide enough to allow for it to have occurred in time to cause the great extinction. The team claims, “The discrepancy with a previous estimate derives from the earlier use of an autocorrelated clock model that is less reliable for the estimation of deep-time phylogenies than the approach used here.”
The sheer scale of the Permian-Triasic extinction event is sobering. While other great extinctions have devastated species on land, this one saw 96% of marine species disappear and even insect diversity crashed, for the only time since their evolution. If a single species of microorganism was responsible it should give pause to those who question whether humans can change the climate. The damage was so widespread that it took as much as 10 million years for biodiversity to recover, far longer than after other great mass extinctions.