Methanogens blamed for the worst extinction event in history

The end-Permian extinction, known as the Great Dying, was the greatest extinction event in the history of life on Earth. Beginning around 252.28 million years ago, it occurred within 20,000 years and has shown a greater rate of taxonomic loss than any of the other ‘Big 5’ extinction events (or number 6, the current ‘holocene extinction’). It is known to coincide with a huge disruption to the Earth’s carbon cycle, which has previously been credited to massive volcanic activity in what is now Siberia. However, recent quantitative estimates of volcanic output are too small to be responsible for the dramatic disruption to the carbon cycle. Researchers have been searching for a second contributor to global carbon cycle change, and the authors of this paper from February have pointed the finger at methanogens.

Their accusation is based upon three observations that provide their evidence for implicating methanogens in the extinction of 70% of terrestrial vertebrates and 96% of all marine species (which as a marine biologist, I’m against).

Firstly, the inorganic carbon reservoir of the oceans expanded massively, showing what the authors call ‘superexponential growth’, coinciding with the extinction event. This means there was a huge decrease in organic carbonate carbon, and huge increases in inorganic methane and carbon dioxide. The dramatic increase in methane here being the smoking gun.

Secondly, a methanogenic pathway that converts acetate to methane is thought to have become widespread towards the end of the Permian. This pathway involves the conversion of acetate to acetyl coenzyme A, a step that occurs via two alternative pathways in modern methanogenic taxa. One of these pathways, dubbed the ‘AckA/Pta pathway’ is now prevalent in the genus Methanosarcina. Phylogenetic analysis of the order Methanosarcinales suggests the emergence of Methanosarcina at the end of the Permian, coinciding with the mass extinction. Rapid methanogenic expansion is predicted due to depletions in dissolved oxygen and sulphate concentrations.

Thirdly, nickel concentrations have been shown to increase sharply at the time of the extinction, thought to be the result of volcanic activity in Siberia, which still today has the world's largest deposits of nickel. This would have removed nickel limitation, as all methanogens require nickel for the active site of their methyl-coenzyme M reductase (MCR) enzyme. All methanogens would have prospered, but the authors believe that the acetoclastic  Methanosarcina would have been especially favoured due to the low oxygen, low sulphate, substrate-rich environment of the end-Permian.

Their three observations – a massive increase oceanic methane and carbon dioxide, the emergence of acetoclastic methanogens and an increase in nickel – correlate well with known features of the end-Permian: Siberian volcanic activity, marine anoxia and ocean acidification. They conclude that the emergence of a single metabolic pathway initiated biogeochemical change and that, catalysed by massive volcanic activity that provided the nickel, the expansion of methangens and in particular Methanosarcina was responsible for the dramatic disturbances to the carbon cycle that led to the Great Dying.

The intended implication of this study is surely to fill in the missing link in the story of the end-Permian extinction, to provide a novel explanation for the Great Dying given that the previous theory that held volcanism as solely responsible had been found lacking. However, anyone who chose the Microbes, Methane and Climate Change study topic in first year may remember the current fears about the climate change threat methanogens pose in thawing arctic permafrost. This study highlights the ability of methanogens and microbes in general to alter the environment on a global scale, with potentially devastating consequences.

Rothman, D., Fournier, G., French, K., Alm, E., Boyle, E., Cao, C.
& Summons, R. (2014). Methanogenic burst in the end-Permian carbon cycle. PNAS.1318106111