Decomposers are organisms that break down dead or decaying organisms, and in doing so, carry out the natural process of decomposition. Like herbivores and predators, decomposers are heterotrophic, meaning that they use organic substrates to get their energy, carbon and nutrients for growth and development. Decomposers can break down cells of other organisms using biochemical reactions that convert the prey tissue into metabolically useful chemical products, without need for internal digestion. Decomposers use dead organisms and non-living organic compounds as their food source.
Bacteria are important decomposers; they are widely distributed and can break down just about any type of organic matter. A gram of soil typically contains 40 million bacterial cells, and the bacteria on Earth form a biomass that exceeds that of all living plants and animals. Bacteria are vital in the recycling of nutrients, and many steps in nutrient cycles depend on these organisms.
The primary decomposers of litter in many ecosystems are fungi. Unlike bacteria, which are unicellular organisms, most saprotrophic fungi grow as a branching network of hyphae. While bacteria are restricted to growing and feeding on the exposed surfaces of organic matter, fungi can use their hyphae to penetrate larger pieces of organic matter. Additionally, only wood-decay fungi have not evolved the enzymes necessary to decompose lignin, a chemically complex substance found in wood. These two factors make fungi the primary decomposers in forests, where litter has high concentrations of lignin and often occurs in large pieces. Fungi decompose organic matter by releasing enzymes to break down the decaying material, after which they absorb the nutrients in the decaying material. Hyphae used to break down matter and absorb nutrients are also used in reproduction. When two fungi's hyphae grow close to each other, they will then fuse together and form another fungus.
Various types of worms are also considered decomposers, as they act as scavengers. For example, a worm that begins to consume an apple helps to hasten its decay by removing parts of the skin and flesh, exposing the interior of the fruit to the elements and to other decomposers.