Extinction Event Brought Closer to Resolution

 
 
An ancient detective story has picked up a new lead. How ancient? Around 252 million years ago, the largest extinction event known to science occurred. This is not the more glamorous event that snuffed out the dinosaurs (the K-Pg event), but a much older and more sinister catastrophe that wiped out more than 90% of all species extant at the time.

While the K-Pg extinction is understood today to have been largely the result of an asteroid impact in the Yucatán Peninsula, the Permian-Triassic (P-Tr) extinction, otherwise known as the “Great Dying,” has been shrouded in some mystery. For one, the P-Tr crisis lasted 200,000 years. Given the rapidity of the dinosaur extinction 65 million years ago, many scientists contend this lengthy interval is inconsistent with an impact event. This doesn’t rule out the impact hypothesis, but it does suggest that the correct explanation will take on multiple layers; a debris impact alone is not likely to be the culprit.

All current evidence appears to point to the greenhouse effect as bearing the brunt of the responsibility, but which mixture of climate-changing gases and the timings of the specific geologic events that supplied the infusion are less certain. To date, evidence for environmental havoc convergent with the boundary suggests an increase in CO2 levels by 2,000 ppm and a 8-10 °C (14.4 -18 °F) rise in temperature. (Contrast this with today’s CO2 atmospheric concentration of 405 ppm.)

Compounding matters, the Siberian Traps of Russia may have belched out an estimated 1,200 billion tons of methane and 4,000 billion tons of sulfur dioxide over a stretch of 900,000 years, cutting right across this geologic boundary. The chemical reactions resulting from these toxic levels would also have destroyed the ozone layer, letting in severe doses of UV radiation from the sun. Fitting these various pieces together has been a challenge.

The Not-So-Humble Archaea

New lines of evidence may help resolve the global crime scene fossilized in the rocks below. The original idea was tossed into the ring back in 2012: microbes. Not just any microbe, but a single, popcorn-shaped archaea specie known as Methanosarcina. Now, the same team from MIT have published a new paper this month in the Proceedings of the National Academy of Sciences clearing away some of the uncertainties surrounding the initial paper.

Methanosarcina barkeri fusaro

Key to the story is horizontal gene transfer, a mechanism of gene acquisition common in bacteria, archaea and other prokaryotes. Through molecular sequencing the team determined that the methanogen picked up a gene from a neighbor microbe close to the time of the extinction event. (Previous estimates gave too statistically unimpressive a range to be considered plausible.) This gene helped expedite the production of methane. Methane is a greenhouse gas, whose effect on climate is much larger during its atmospheric lifetime of 8.4 years relative to CO2’s, which has a smaller effect but a much longer duration exceeding 100 years. With the microbe now spewing out massive amounts of methane, the planet experienced an acclivitous spike in global temperatures.

The gene alone was not enough to accomplish the greatest serial killing spree in history, however. Methanosarcina also needed an influx of its primary energy source: nickel. This precursor, the researchers believe, was supplied by the Siberian traps, along with the smaller-scale Emeishan Traps in China, long known to have been the most prodigious source of this element. The heightened and unprecedented volcanic activity coincided with the P-Tr boundary and was just the fuel the microbe needed to wage its terrestrial devastation. Methanosarcina grew much larger and devoured the ocean’s stores of carbonate ions, on which many marine organisms such as bivalves rely to form their calcium carbonate shells.

The aftermath of this interlocking succession of events became the most severe extinction event in history. Some 96% of all marine life and 70% of terrestrial vertebrates blinked out of existence. It is also the only known mass extinction of insects. In taxonomic terms, some 57% of all families and 83% of all genera along the tree of life went extinct, as did over 90% of all species sea, land and air. Because so much was lost, the recovery of life on Earth took a great amount of time. Estimates range from 10 million years for some types of life to as much as 30 million years for four-limbed animals, essentially resetting evolution.

The ultimate cold case is far from settled, but with the more precise calculations by the team at MIT we now have good reason to rule out volcanism as the only thread on which the explanation hangs and instead roll its coincident effects into a more holistic theory in which prolonged bursts of volcanic activity facilitated a dramatic turn in microbial evolution. Daniel Rothman, geophysics professor and author of the latest study, concedes that a lot of this hypothesis rides on the accuracy of the isotope analysis. Future studies may alter the timeline, rendering Methanosarcina either innocent of its alleged crimes or a lesser accessory to a larger scheme of destruction.

Reference: Methanogenic burst in the end-Permian carbon cycle

Source article: Ancient whodunit may be solved: Methane-producing microbes did it!