White Rot Fungus and the Carbon Cycle . The end of Coal

A second important role is that they are very stable and so provide defence against fungal and bacterial attack. The other main constituent of the cell wall is cellulose made from linear chains of glucose units. This is much more easily attacked and digested for food by bacteria and fungi if then can get to it, lignin forms a barrier. Lignin is a macromolecule formed from the combination of many phenolic aromatic groups via oxidative coupling . Because of its high stability, lignin is incapable of being broken down through simple decomposition.

White-rot fungi are characterised by their ability to break down the lignin and cellulose of wood. As a result of this ability, white-rot fungi are considered a vital component of the current carbon cycle because of their ability to access carbon pools that would otherwise remain inaccessible. The name “white rot” derives from the white colour and rotting texture of the remaining crystalline cellulose from wood degraded by these fungi. However this group of fungi only developed the enzymes needed to attack lignin 300 million years ago. And this had a profound effect. Prior to that the carbon remained there on the forest floor and eventually fossilised under pressure of sediments above and turned into coal! White-rot fungi employ these high redox potential enzymes that the fungi now encoded for in its DNA that break lignin down into smaller aromatic rings which other enzymes can break up easily.

The biological degradation of lignin by white rot fungi is described as an enzymatic combustion. Today it involves 3 groups of peroxidase enzymes. The so-called manganese peroxidases (MnPs), lignin peroxidases (LiPs) and versatile peroxidases (VPs) together with auxiliary enzymes to degrade the initial breakdown products. MnPs oxidize the often minor phenolic moiety of lignin via Mn3+ chelates or directly at the heme cofactor. In contrast, LiPs attack nonphenolic lignin directly using a solvent-exposed catalytic tryptophan forming a reactive radical, as shown using model compounds. Finally, VPs combine the structural and catalytic properties of MnPs and LiPs, including the important oxidising surface tryptophan. The first peroxidases were not able to degrade lignin directly and used diffusible metal cations to attack its phenolic moiety. Phylogenetic analysis of the peroxidases suggests that later in evolution these enzymes would have acquired the ability to degrade nonphenolic lignin using the tryptophanyl radical interacting with the bulky polymer at the surface of the enzymeas in LiPs and VPs. Also, ligninolytic peroxidases progressively increased the redox potential of their reactive species making the reaction possible and faster. Finally, their stability at acidic pH, where lignin decay takes place in the Carboniferous swamps, increased, and their catalytic tryptophan environment became more negative to stabilise lignin cation radicals.

Interestingly this evolution only occurred once, but brown rot fungi, a more specialised group then lost this ability whilst evolving other less energy intensive enzymes resulting in only partial breakdown of the lignin but importantly still providing access to the cellulose they need as a food source.

Collapsing cells 

So the new fungal ability to digest lignin brought to an end the 60 million year long period of coal deposition known as the Carboniferous period!

Carboniferous forest reconstruction