Fungi: Decomposers, Partners and Pathogens

The hidden kingdom

When you see a mushroom pop up on a lawn after rain, you are seeing only a tiny part of the organism, like seeing an apple but not the tree. The real fungus is a web of threads in the soil, sometimes spreading for many metres. Fungi rot fallen leaves, feed trees, raise bread, make penicillin and, occasionally, cause disease.

In the domains and kingdoms lesson you saw that fungi are eukaryotic heterotrophs with chitin walls that absorb their food. Now we look inside the kingdom.

How a fungus is built

• Most fungi are made of hyphae: thin threads, one cell wide. In many fungi, cross-walls called septa divide hyphae into cells; in others (such as bread mould) the hyphae have no cross-walls and many nuclei share one long cell.
• Hyphae branch into a mycelium, which grows through soil, wood or food. A mycelium has a huge surface area for absorbing nutrients.
• Hyphae release digestive enzymes into their surroundings, then absorb the small molecules.
• Cell walls are made of chitin. Fungi store food as glycogen.
• To reproduce, many fungi grow a fruiting body (a mushroom, puffball or shelf fungus) that releases millions of spores. Yeasts are unicellular fungi that usually reproduce by budding: a small cell grows out of the parent and pinches off.

Reproduction and the main groups

Fungi reproduce asexually (spores made by mitosis, budding, or pieces of mycelium breaking off) and sexually (hyphae of two compatible mating types fuse, and their nuclei combine; the resulting cells make spores by meiosis). Biologists group fungi mainly by the structure that makes their sexual spores.

• Zygomycetes (bread moulds such as Rhizopus): fuzzy hyphae with round black sporangia on stalks. Sexual reproduction forms a tough zygospore.
• Sac fungi (ascomycetes): sexual spores form in a tiny sac called an ascus, usually 8 per sac. Includes baker's yeast (Saccharomyces cerevisiae), Penicillium, morels and the fungus that causes Dutch elm disease.
• Club fungi (basidiomycetes): sexual spores form on club-shaped basidia, usually 4 per club, often lining the gills of a mushroom. Includes mushrooms, shelf (bracket) fungi, puffballs, and rusts that attack wheat.

DNA studies have rearranged some of these groups (zygomycetes are now split into several lineages), but these three spore structures are still the best way to recognize the main kinds of fungi.

Why fungi matter

Decomposers. Fungi (with bacteria) break down dead wood and leaves, returning carbon and nutrients to the soil. Fungi are among the few organisms that can break down lignin, the tough material in wood. Without them, forests would be buried in dead logs.

Partners.

• Mycorrhizae: most land plants, including sugar maples and white pines, have fungal partners on their roots. The hyphae act like extra roots, bringing water, phosphorus and nitrogen; the plant pays with sugar.
• Lichens: a fungus living with a green alga or cyanobacterium. The fungus gives shelter and moisture; the partner makes food by photosynthesis. Lichens grow on bare rock and help start soil. In northern Canada, caribou depend on lichens as winter food.

Food and medicine. Yeast raises bread; Penicillium species ripen blue cheeses; in 1928 Alexander Fleming noticed that a Penicillium mould killed bacteria, leading to penicillin, one of the first and most important antibiotics.

Pathogens. Athlete's foot and ringworm are fungal skin infections. Dutch elm disease killed a large share of the American elms that once lined streets in Ontario and Québec. White-nose syndrome, caused by a fungus that grows on hibernating bats, has killed huge numbers of little brown bats in Ontario since it arrived around 2010.