Forget a slice of bread or a piece of cheese for long enough and you may find fuzzy growth on it. That fuzz is mould, and mould is a fungus. Fungi like mushrooms and mould are not plants, and they have no green chlorophyll, so they cannot make their own food from sunlight. That leaves a question: if the fuzz cannot make food from sunlight, where does its food come from?

Mould was once the star of a famous lab discovery. In 1928 Alexander Fleming saw that mould on a lab dish had a bacteria-free circle around it. The mould made penicillin. The dish had mould on it, and all around the fuzzy growth there was a clear space with no bacteria in it. Mould is easy to see as only a sign of spoiled food, but that dish shows it can be more than that.

Both fungi and bacteria count as decomposers, and their work begins the moment a plant or another living thing dies. Take the fungi first. Fungi grow thin threads called hyphae into wood and leaves, and together the threads are called mycelium. A fungus that sends its threads into wood has put them right where its food is, and the threads do their work out of sight, inside the wood or the leaf.

Fungi produce special chemicals known as enzymes. Enzymes split the wood apart, and after that the fungus absorbs the nutrients. An enzyme is a molecule that makes a chemical reaction go faster, and decomposers use enzymes to cut dead plants into bits small enough to absorb. Think of the enzymes as the scissors and the nutrients as the pieces that come off. The enzymes do the snipping, and the decomposer takes in the bits that are left.

Earthworms help too. They leave behind nutrient-rich castings that plants can use, and worm composting makes a soil booster called vermicompost. More widely, when things rot, nutrients go back into the soil, and living plants use them to grow. Decomposers turn dead things back into food for new plants. So a pile of dead leaves and a patch of living plants are linked by the same nutrients.

Rot goes faster in some places than in others. Decomposers work best when it is warm and wet, so the weather and the place matter a great deal. Tropical rainforests are warm and wet, and dead things decay very fast there. Picture a place that is both warm and wet at once, with the workers in their best conditions. A leaf that falls there lands where things suit the workers well, and the result is quick decay.

Decomposers can even warm a pile. The heart of a compost pile warms up because the decomposers inside it are busy. A compost pile warms in stages. Microbes that like mild heat start first, and then heat-loving ones that thrive at 115 to 160 degrees F take over. So the microbes that start the job are not the same ones working when the pile is at its hottest. The heat tells you that many living things are at work in there.

Carbon takes a trip, and rot is part of it. Carbon keeps moving among the air, plants, soil and rocks, and that movement has a name: the carbon cycle. Rot is one of its steps. Where there is no air, the story changes. In a landfill there is no air, so food and yard waste rot without oxygen and give off methane, a powerful greenhouse gas. That makes air one more thing that shapes how rot goes, and it shows why the gas matters for the air around Earth.

Words to know

• enzymes: chemical speeders
• mycelium: fungus threads
• methane: a greenhouse gas
• hyphae: thin threads