July 24. The end of coal

One possible explanation for the decline in carbon dioxide in the atmosphere toward the end of the Pennsylvanian, which we talked about yesterday, is that so much of the carbon was being tied up in coal and therefore not available to the atmosphere. Plants take CO2 in – and if they died and became coal rather than decomposing more completely, the carbon would be sequestered in the earth, in coal beds.

Some recent research by scientists at Clark University and the US Department of Energy, and elsewhere, published in 2012, suggests that one reason for all the plants and all the coal in the Pennsylvanian was a sort of arms race between plants and the enemies that could destroy them – specifically, fungi.

When plants evolved a biopolymer called lignin, it allowed them to better transfer fluids around their bodies. Lignin gives wood both strength and functionality for plants, and a vast amount of the world’s carbon is contained in lignin and related chemicals in plants. It may have been the evolution of lignin that allowed plants to grow to large sizes, beyond the tiny stems of Cooksonia back in the Silurian.

Having lignin may have given plants another advantage of sorts – protection against attack by some other organisms. The recent work suggests that the abundance of coal in the Pennsylvanian is because there was nothing around to attack dead woody tissue, because nothing had evolved the necessary chemicals to break it down. Dead wood simply accumulated and altered under physical weathering conditions, together with the acids formed by the plants themselves. Nothing was eating the wood, so it became coal.

Toward the end of the Carboniferous, a fungus evolved that had the chemicals needed to decompose lignin. Was this enough to mean that following periods yielded less coal because more wood was broken down, with carbon returning to the atmosphere more than to the earth? The authors of the recent work think so.

White-rot fungus photo by Sten Porse
via Wikipedia under Creative Commons license.

This has some significant consequences for earth history as well as for present-day energy studies. The Department of Energy was involved because they were studying ways of degrading wood to make better biofuels. But geologically, this might help account for the low point in atmospheric carbon dioxide at the end of the Pennsylvanian and early Permian Period. Later, the additional input due to fungal decomposition of wood could have brought CO2 levels up again. Never discount the power of plants over time – remember that the oxygenation of earth’s atmosphere was probably due to the action of algae and cyanobacteria back in the Precambrian.

So, there is apparently a coincidence in timing between the evolution of genes in these fungi that could degrade lignin and the end of abundant coal, toward the end of the Pennsylvanian. But there are probably other factors in the decline of rainforests, such as we discussed yesterday, and while a coincidence in timing is interesting, it isn’t proof. And the work was based on genome studies – there are not many fungi in the fossil record. Probably more telling is the fact that there is hardly anything other than the enzymes from funguses that can really attack lignin and break it down. So the evolution of those chemicals must have indeed been an important event in the history of plant life, and in the evolution of the carbon cycle on earth. It’s definitely an intriguing idea.

—Richard I. Gibson

White-rot fungus photo by Sten Porse via Wikipedia under Creative Commons license.   

Wood-decay fungus 

Links (all to reports on the 2012 study):

http://science.energy.gov/ber/highlights/2012/ber-2012-06-a/

http://www.sciencemag.org/content/336/6089/1715.short

http://www.sciencedaily.com/releases/2012/06/120628181721.htm