12.0.8 12.14: Fungi have some structures in common, but exploit an enormous diversity of habitats.

As we’ve seen, most fungi are multicellular and are composed of long strings of barely visible, thread-like hyphal cells. The hyphae interconnect to form a mass of tissue called a mycelium, the form in which a fungus spends most of its time, usually underground or in a decaying tree or log. Unless you dug down to reach the mycelium, you would not know a fungus was there.

The structure that most people associate with fungi is the mushroom. But a mushroom is just a temporary reproductive structure (or “fruiting body”), part of a complex reproductive cycle that includes both sexual and asexual reproduction in some fungi (FIGURE 12-31). This cycle consists of several steps.

• 1. Underground, genetically distinct haploid hyphal cells join together. But their nuclei don’t fuse, so instead of being diploid, they are “dikaryotic,” meaning that each cell in the hypha has two nuclei.
• 2. The dikaryotic mycelium can grow and spread for years.
• 3. At some point, tightly packed dikaryotic hyphae may form a mushroom.
• 4. The haploid nuclei in the dikaryotic hyphae fuse in some cells, putting the mushroom into a diploid state.
• 5. In meiosis, the diploid cells produce huge numbers of haploid spores (up to a billion in a single mushroom!), which are dispersed by wind or water or on the bodies of animals.
• 6. After landing in a hospitable place, the spores grow as haploid hyphae, and the cycle begins anew.

Fungi have an unusual and effective method of getting nutrition. Unlike humans, they digest their food outside their “body.” While growing underground, hyphae secrete strong enzymes that break down the organic molecules around them, and the hyphae then absorb the nutrient-rich fluid. How effective are fungi at absorbing nutrients and growing? Extremely good. In fact, in eastern Oregon, a yellow honey mushroom fungus covers an area of about 4 square miles (nearly 10 square kilometers) (FIGURE 12-32). This fungus is estimated to be at least 2,400 years old, and it may be more than 8,000 years old, which would place it in the category of the oldest living organism. Moreover, by area, it is the largest.

But huge as it is, the fungus is nonetheless hard to detect, because the mycelium is underground. The fungus appears on the surface only as mushrooms (edible, but not particularly tasty) and as hyphae in dying and dead trees. Indeed, no one even knew that this enormous fungus existed until 2000, when Oregon foresters sought an explanation for an epidemic of dead and dying trees in the Malheur National Forest. (They discovered that the fungus was killing trees by causing their roots to rot.)

Fungi can grow in many different habitats because, as decomposers, all they need for their nutrient supply is some sort of organic material that they can break down. Fungi play an enormously important ecological role in speeding the decay of organic material in forests. They don’t need light, so they can grow underground or inside dead trees and logs.

If you break down the tissues of dead organisms, you are a decomposer, and if you break down the tissues of living organisms, you are a parasite. Just as fungi can live off dying trees, they can also grow in and on people. “Mycosis” is a general term for a disease that is caused by a fungus, such as athlete’s foot, or related fungi that cause jock itch, beard itch, scalp itch, ringworm, and toenail fungus. These fungi get their nutrition by digesting some of the organic molecules of your body! And all of these fungal diseases are quite contagious, because they are spread by spores that can linger on moist surfaces and in clothing, so they tend to be a problem in places like dormitories, gymnasiums, and fitness centers.

Fungi can also thrive in poorly ventilated spaces in buildings (FIGURE 12-33). Molds are multicellular fungi that are responsible for many unpleasant effects. People living or working in a “sick building” can experience burning or watering eyes, a runny nose, and itchy skin—allergic reactions to the proteins in fungal spores. More severe effects, such as cancers and miscarriages, are probably produced by toxins released when the fruiting bodies of the fungi disintegrate. Curing a sick building can be so expensive that in some cases the entire building is destroyed and rebuilt.

It wouldn’t be fair to mention so many undesirable effects of fungi without describing some of the many fungi that are beneficial to humans, particularly one that has benefited millions of humans: the fungus that produces penicillin. Alexander Fleming was a researcher studying bacteria. In 1928, just prior to taking a month-long vacation, he stacked some of his used Petri dishes containing bacterial colonies in a corner of his lab. When he returned from his vacation, he found that some of the dishes had been contaminated by a fungus. Before discarding them, he noticed that near the fungal infection, the bacterial colonies were dead, while farther away from it, they continued to grow. Suspecting that the fungus might be producing something toxic to the bacteria, he cultured it and discovered that it did produce a substance that killed many different types of bacteria. He identified the fungus as Penicillium. Following the development by others of a method to purify the antibacterial compound produced by the fungus, penicillin became a “wonder drug,” responsible for saving millions of lives by helping to treat bacterial infections.

Many mushrooms are gastronomic delicacies, and commercially grown portobello and shiitake mushrooms, for example, command high prices. Truffles—the underground reproductive structures of a fungus called Tuber—can sell for as much as $3,500 per pound. Because truffles grow underground, there is nothing on the surface to indicate their presence, and truffle hunters use trained pigs or dogs to locate them by scent.