Fungi are the planet's ultimate decomposers, playing a vital role in recycling nutrients by breaking down organic matter.

Unless you're a fan of mushrooms, an avid gardener, or someone prone to fungal infections, fungi probably aren't on your radar. Their elusive nature might be why: they prefer to stay hidden, and when they do make an appearance, it's rarely a pleasant one. You may find yourself dealing with a discolored, distorted toenail, brown spots on your shrub's leaves, a slimy film on your forgotten leftovers, or even witnessing the demise of frogs worldwide.

"Unfortunately, most people only associate fungi with negative aspects," says Marin Brewer, associate professor of mycology — the study of fungi — at the Department of Plant Pathology at the University of Georgia. "We tend to focus on the harmful ones that affect plants or humans. But in reality, the vast majority of fungi are harmless, simply decomposing organic matter without causing harm."

Fungi belong to their own unique taxonomic kingdom, distinguishing them from every other living organism on Earth. The most apparent characteristic that sets fungi apart is their method of reproduction: they propagate exclusively through spores, tiny DNA fragments that travel through the air or hitch a ride on other surfaces, eventually settling into environments like soil or even an abandoned sandwich, where they grow into new fungi.

Despite their vast diversity, all fungi share common structures known as hyphae. A single thread is a hypha, a group of them are hyphae, and when they form a dense network, it's called mycelium. Fungi are eukaryotes, meaning their cells are more similar to those of plants and animals than to bacteria or archaea, which are prokaryotes. These cells contain membrane-bound organelles and a nucleus where their genetic material is housed. Unlike plants, whose cell walls are made of cellulose, and bacteria, whose walls are made of glycans, fungi's cell walls are composed of a tough, flexible polymer called chitin — also found in the scales of fish and the exoskeletons of arthropods. Their cell membranes are kept intact by ergosterol, which is similar to cholesterol in animal cells.

When it comes to daily activities, fungi resemble plants more than animals. They can't move, but since they don't photosynthesize like plants, they are more like animals in that they need to seek out food. Being largely immobile, fungi have adapted by feeding on things that stay put.

Fungi Love Dead Stuff

This is why fungi have evolved a fascination with decomposing dead matter: from decaying wood to the remains of animals, including hair, skin, and teeth, fungi produce enzymes that can break down almost anything. Billions of these helpful fungi can be found outside, working tirelessly in the soil to decompose organic matter. Their work is not only crucial for the ecosystem's health but also benefits us: fungi play an essential role in returning old plants and animal tissues to soil, and the majority of plant families worldwide form a symbiotic relationship with fungi, exchanging water and nutrients with plant roots in exchange for sugars produced by the plants.

Fungi consume food by releasing enzymes from the tips of their hyphae. Rather than engulfing or ingesting their food like animals or amoebas, they excrete enzymes onto their food. Once the food breaks down into smaller molecules, they absorb it back through their hyphae.

"The enzymes a fungus produces determine what it can consume," says Brewer. "Not all fungi share the same enzymes. Those that can break down cellulose thrive on plant matter, while those that break down keratin are typically found on skin, hair, or hooves."

Thanks to their unique feeding method, fungi are the ultimate decomposers, whether they’re mushrooms on the forest floor, shelf fungi on trees, puffballs, plant pathogens, or even the mold growing on that forgotten tub of yogurt in the back of your fridge. While fungi exist across several phyla, most of the ones we encounter belong to one of two: Basidiomycota and Ascomycota.

Mushrooms (Basiodiomycota) Are Fungi

The phylum Basidiomycota is home to most of the fungi we think of as "mushrooms" — from those you find at the grocery store to the "fairy rings" in your yard, tree shelves, and even some plant pathogens. These fungi often have fruiting bodies that sprout from the mycelium, which may be hidden inside a decaying log or beneath the soil. The mycelium does most of the work, while what we recognize as the "mushroom" is just the reproductive structure releasing spores. Once a spore lands, hyphae spread out in all directions, which is why mushrooms tend to grow in circular patterns.

Ascomycota: Fungi That Mold

Another category of fungi you'll recognize are the Ascomycota. A majority of molds, for example, belong to this group. These fungi don’t form large mushrooms; instead, they grow in circular patterns. So, if you leave a cup of coffee unattended for a few days, you’ll likely see mold spreading outward from one spot. Yeasts, morel mushrooms, truffles, and cup fungi are all members of this phylum. Beyond the Ascomycota, fungi start becoming less familiar in appearance.

"They begin to get really strange," says Brewer. "You have numerous animal parasites, like aquatic fungi, which are flagellated and can move in water. This includes the chytrid fungi, which are responsible for the decline of frogs globally. New phyla are still being identified, and that’s thrilling."