Fungal plant material decomp

Question 1

intro - role of fungi in decomp plan maerials

Answer 1

Fungi are the dominant decomposers of plant biomass in terrestrial ecosystems, critically important for nutrient recycling and carbon turnover. Their ability to degrade lignocellulosic material—composed of cellulose, hemicellulose, and lignin—relies on powerful extracellular enzymes and oxidative processes. Through these mechanisms, fungi mediate the return of carbon dioxide to the atmosphere, sustaining the global carbon cycle and maintaining soil fertility.

Question 2

2nd para - plant biomass composition and decomposition hierarchy

Answer 2

The plant cell wall consists of a complex network: cellulose microfibrils cross-linked by hemicellulose, embedded in a matrix of pectin and fortified with lignin. These components vary in degradability. Sugars and starches are easiest to break down, followed by hemicellulose, then cellulose, while lignin is the most resistant. This structural complexity necessitates the sequential action of fungal species adapted to different substrates.

Question 3

3rd para - fungal succession and ecologoical sepcilisation

Answer 3

Decomposition follows a predictable fungal succession:

Early colonisers such as Aspergillus and Penicillium (Ascomycota) consume simple sugars and hemicellulose.

Brown rot fungi (mostly Basidiomycota) specialise in cellulose degradation, leaving lignin intact. Their action often results in brown, cracked wood rich in lignin residues.

White rot fungi (e.g. Armillaria, Agaricus) are capable of fully degrading lignocellulose, including lignin, which results in the distinctive bleached appearance of wood.

This succession reflects increasing enzymatic and oxidative capabilities among fungi as decomposition advances.

Question 4

4th para - enzymatic breakfown of cellulose and hemicelulose

Answer 4

ungal degradation of cellulose is achieved through the synergistic action of:

Endoglucanases, which randomly cleave internal β-1,4-glycosidic bonds;

Exoglucanases, which remove cellobiose units from chain ends;

β-glucosidases, which hydrolyse cellobiose into glucose.

Hemicellulose, being more branched and compositionally variable, is broken down by a broader suite of enzymes including xylanases, arabinases, and mannanases. Hemicellulose typically decomposes faster than cellulose due to its lower crystallinity and greater accessibility.

Question 5

5th para - lignin degradation by white rot fungi

Answer 5

Lignin, a highly cross-linked aromatic polymer, is recalcitrant to enzymatic hydrolysis. Its degradation is a hallmark of white rot fungi, which produce lignin-modifying enzymes (LMEs) such as:

Laccases, which oxidise phenolic substrates;

Lignin peroxidases (LiPs) and manganese peroxidases (MnPs), which initiate oxidative cleavage of non-phenolic bonds using hydrogen peroxide;

H₂O₂-generating enzymes, which supply reactive oxygen species necessary for radical-based lignin depolymerisation.

Additionally, compounds like veratryl alcohol act as redox mediators, enhancing enzyme efficiency and protecting them from oxidative inactivation.

Question 6

6th para - contribution to carbon and nutrient cycling

Answer 6

By breaking down plant litter, fine roots, and woody material, fungi release carbon as CO₂, closing the loop initiated by photosynthetic carbon fixation. In doing so, they also liberate essential nutrients such as nitrogen, phosphorus, and potassium. Fungi are especially dominant in low-nitrogen environments, where their enzymatic ability to access carbon from recalcitrant lignin gives them a significant ecological edge.