Unless you are a mushroom lover, a gardener or someone especially prone to skin infections, fungi probably don't hold much interest for you. The reason probably has something to do with the fungi temperament: They're shy, they generally keep themselves hidden and, when they do show up, it's often unwelcome. You end up with a discolored and misshapen toenail, the shrub in your yard has brown spots all over its leaves, there's a layer of slime on your ancient leftovers, and the frogs of the world start dying.
"Unfortunately, a lot of what the general public knows about fungi is bad," says Marin Brewer, associate professor of mycology — that's the study of fungi — in the Department of Plant Pathology at the University of Georgia. "We focus on the ones that are causing plant or human diseases. But in general, the vast, vast majority of fungi are just hanging out, breaking down organic matter, not killing anything."
Fungi are actually in their own taxonomic kingdom, meaning they've got something going on that's very different from every other type of organism on Earth. The immediately obvious thing that sets fungi apart from everyone else is that they reproduce exclusively via spores, little bits of DNA that float on the air or hitch a ride some other way, and then nestle into the soil or an old sandwich or something and just set up shop, creating a new fungus.
And although they are extremely diverse, all fungi have filamentous growth structures called hyphae (one filament is a hypha, a few of them are hyphae, and when there's a big mass of hyphae, we refer to it as mycelium). All fungi are eukaryotes — this means their cells are more similar to those of plants and animals than to bacteria and archaea (which are prokaryotes). Their cells have membrane-bound organelles and a nucleus where their DNA is stored. And while plant cell walls are made of cellulose and bacteria cell walls are comprised of glycans, all fungi have cell walls made of a tough, bendable polymer called chitin — it's also the main ingredient in the scales of fish and the exoskeletons of arthropods. Finally, their cell membranes are kept intact and healthy with ergosterol, which is basically the analog to cholesterol in animal cells.
As far as what fungi do with themselves every day, they seem a lot more like plants than animals. They can't move around, but because they don't photosynthesize — make their own food — fungi are actually more like animals: They've got to get out there and find their next meal. However, because they're pretty immobile, they've found a decent workaround: eating things that hold very still.
Fungi Love Dead Stuff
It is for this reason that fungi evolved a penchant for dead stuff: depending on the type of fungus, it could be dead wood or the hair, skin and teeth of an animal — you name it, there's probably a fungus out there that makes an enzyme that can break it down. Right now, billions of beneficial fungi can be found outside your window in the soil, decomposing organic matter. It's a living for them and it's also great for us, as what they're doing is of utmost importance to the health of the ecosystem. Not only are they responsible for turning organic matter — old plants and animal tissue — into soil again, but the vast majority of the world's plant families have some sort of symbiotic relationship with fungi, in which the fungi pass water and nutrients onto the roots of the plants and the plants make sugars for the fungi to eat.
Fungi eat by secreting enzymes out of the tips of their hyphae. Instead of engulfing food like an amoeba or ingesting and digesting it like an animal, they dump enzymes onto the food itself and after it breaks down into smaller molecules, they suck it back up through their hyphae.
"Which enzymes the specific fungus has determines what it can eat," says Brewer, "Not all fungi come with all the same enzymes. The ones that can break down cellulose are the ones that grow on plants or plant matter; the ones that break down keratin grow well on skin or hair or hooves."
Because of their eating style, fungi are the Great Decomposers, regardless of whether they're a mushroom on the ground, a bracket on a tree, a puffball, a plant pathogen or a film of mold on the wall of the forgotten tub of yogurt in the back of your refrigerator. There are several different phyla of fungi, but most of the ones we're familiar with fit into one of two of them: Basiodiomycota and Ascomycota.
Mushrooms (Basiodiomycota) Are Fungi
The phylum that houses most of the fungi we think of as "mushrooms" is the basiodiomycota — they're in the grocery store, making "fairy rings" in your yard, shelves on trees and sometimes causing plant diseases. Most of these have fruiting bodies that spring up from the mycelium inside a dead log or under the soil — in fact, the mycelium is where most of the mushroom business gets done, so a lot of the organism itself is always out of sight. What we think of as the "mushroom" is just the reproductive structure that the fungus sends up to release spores. Once a spore lands, the hyphae start growing out in all directions from the place the spore landed, which is why mushrooms often grow in a ring formation.
And Molds (Ascomycota) Are Fungi Too
The other group of fungi you would recognize is the ascomycota. Most molds, for instance, are in this phylum: they usually don't produce a large mushroom — they grow in circles like all fungi, so if you leave your coffee out for a few days, you'll notice the mold grows radially out from a single point. Yeasts, morel mushrooms, truffles and cup fungi are in this group. But after the Ascomycota, fungi become less immediately recognizable.
"They start getting weird," says Brewer. "There are lots of animal parasites out there like aquatic fungi that are flagellated so they can move around in water — which includes the chytrid fungi that are killing off all the planet's frogs. New phyla are being figured out all the time, so that's exciting."