Assumed audience
General adult who has completed Fungal Biology Fundamentals. No chemistry background required.
Fungi are heterotrophs
Plants make their own food from sunlight, water, and carbon dioxide through photosynthesis. Fungi cannot do this — they have no chloroplasts and no photosynthetic machinery. Fungi are heterotrophs: they must obtain organic carbon and energy from external sources. This dependence on other organisms (living or dead) for food is the metabolic fact that shapes every aspect of fungal ecology.
But fungi do not eat the way animals do. Animals take food inside their bodies and digest it internally. Fungi do the opposite.
Extracellular digestion
Fungi feed by extracellular digestion: they secrete digestive enzymes into the surrounding substrate, break down complex molecules outside their bodies, and absorb the resulting small molecules through their hyphal walls. The fungus eats by growing through its food. Its digestive field extends beyond its body into the material around it.
This strategy means the fungus’s body and its food are physically interpenetrated. There is no gut, no mouth, no clear line between inside and outside. The mycelium ramifies through the substrate, secreting enzymes ahead of the advancing hyphal tips and absorbing products behind them.
What fungi break down
The main structural polymers in dead plant matter are:
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Cellulose: the most abundant organic polymer on Earth — long chains of glucose that form the structural fibers of plant cell walls. Fungi break it down using cellulase enzymes, releasing glucose for energy.
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Hemicellulose: branched polysaccharides that fill the matrix between cellulose fibers. Broken down by hemicellulases.
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Lignin: a complex, irregular polymer that gives wood its hardness and resistance to decay. Lignin is the most difficult of the three to degrade. Only fungi — specifically white-rot Basidiomycota and some Ascomycota — can fully dismantle it, using oxidative enzymes (lignin peroxidase, manganese peroxidase, laccase) that generate free radicals to cleave lignin’s chemical bonds.
Together, these three polymers form lignocellulose — the composite material of wood, straw, leaf litter, and most dead plant tissue. Lignocellulose is the largest reservoir of organic carbon on land, and fungal decomposition is the primary way it re-enters biological circulation.
White rot and brown rot
Not all wood-decay fungi use the same strategy:
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White-rot fungi degrade all three components — lignin, cellulose, and hemicellulose — leaving behind pale, soft, fibrous residue. They attack lignin first, exposing the cellulose for enzymatic breakdown. Example: Trametes versicolor (turkey tail).
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Brown-rot fungi attack cellulose and hemicellulose but leave lignin largely intact. They use Fenton chemistry — generating hydroxyl radicals from hydrogen peroxide and iron — to disrupt the cellulose matrix. The residue is brown, crumbly, and lignin-rich. Example: Postia placenta.
These two strategies produce different soil conditions. White rot creates well-decomposed, nutrient-rich humus. Brown rot creates lignin-rich soil with different water-holding and nutrient properties. The type of decomposer shapes the soil that future organisms will inhabit — a form of niche construction.
Beyond wood: other substrates
Not all fungi eat wood. Saprotrophic fungi colonize every kind of dead organic matter — leaf litter, animal dung, insect carcasses, keratin (hair, hooves), even petroleum products and plastics. Each substrate requires a different enzymatic toolkit. Keratinolytic fungi (like dermatophytes) produce keratinases that break down the tough protein in skin and nails. Coprophilous fungi (dung fungi like Pilobolus) specialize in the nutrient-rich but microbially competitive substrate of herbivore dung.
Mycorrhizal fungi obtain carbon not from dead matter but from living plant partners, in exchange for delivering mineral nutrients. They still use extracellular enzymes — proteases and phosphatases — to access organic nitrogen and phosphorus from soil, but their primary carbon source is plant-derived sugars and lipids transferred through arbuscules or the Hartig net.
The Carboniferous connection
Before fungi evolved efficient lignin-degrading enzymes, dead trees accumulated in vast quantities during the Carboniferous period (roughly 360–300 million years ago). This undecomposed wood was buried and compressed into coal — the fossil fuel that powered industrialization. The evolution of white-rot fungi ended this accumulation and established the modern nutrient cycling regime in which dead wood is recycled rather than permanently sequestered. Fungal biochemistry shaped geological history.
Why this matters
Understanding how fungi feed explains why they occupy such critical ecological positions. Every term in this lesson — extracellular digestion, cellulose, lignin, white rot, brown rot — appears throughout the mycology module because fungal nutrition is not a peripheral detail. It is the chemical basis of everything fungi do: decomposition, symbiosis, soil formation, and the nutrient cycling that sustains terrestrial life.