Decay is the breakdown of complex organic compounds into simpler substances through the action of microorganisms. These microorganisms include many kinds of bacteria and fungi, which obtain nourishment by secreting enzymes that break down dead plant or animal tissues into food sources suitable for their nutrition. Decay-causing microorganisms thrive in moist, warm environments, and the species that live in airless environments produce a kind of decay usually called putrefaction. Putrefaction is often accompanied by unpleasant odors.
Decay-producing bacteria and fungi may attack food and produce toxins that make the food inedible to humans. Some animal proteins, including meat, eggs, and fish, decay into simpler substances that emit noxious gases. For example, rotten eggs give off hydrogen sulfide, a particularly foul-smelling gas.
Decay is important in maintaining the balance of nature. It breaks down complex carbon-containing and nitrogen-containing compounds into simpler forms that can be absorbed by green plants. Among the substances released by decay are free nitrogen, ammonia, methane, nitrites, and carbon dioxide. Without decay the world's carbon and nitrogen would remain locked up in unusable form, and plant growth would eventually cease.
