Decomposer
A decomposer is an organism, mainly fungi and bacteria, that breaks down dead organisms and waste, returning nutrients to the soil and water.
Definition
A decomposer is a living thing that breaks down dead plants, animals, and waste, turning them back into soil, and mushrooms, mold, and many bacteria are nature's recycling crew, since without them dead leaves, logs, and animals would pile up and the nutrients locked inside them would never return. More formally, a decomposer obtains energy by breaking down dead organic matter into simpler inorganic substances such as carbon dioxide, water, and mineral nutrients, mainly through fungi and bacteria that secrete digestive enzymes outside their bodies and absorb the products, completing the cycling of nutrients such as nitrogen and phosphorus back to producers, with detritivores such as earthworms and millipedes helping by shredding material into smaller pieces first. At the biochemical level, decomposers are saprotrophic heterotrophs that mineralize dead organic matter by secreting extracellular hydrolytic and oxidative enzymes, converting organic carbon, nitrogen, phosphorus, and sulfur into forms autotrophs can use, with decomposition rate depending on temperature, moisture, oxygen, and substrate quality, especially the carbon-to-nitrogen ratio and lignin content; fungi dominate the breakdown of lignin and cellulose in terrestrial litter, bacteria dominate in aquatic and nutrient-rich systems, and the detrital food web based on decomposers often processes more energy than the grazing food web.
Example
Leaf litter in a temperate forest breaks down within about $1$ to $3$ years, returning nitrogen to the soil for trees to reuse, and fresh sawdust has a carbon-to-nitrogen ratio near $400:1$, so microbes decomposing it immobilize soil nitrogen and can starve nearby plants, while material near $25:1$ decomposes quickly and releases nitrogen instead. In cold, waterlogged peatlands decomposition is so slow that dead plant matter accumulates for thousands of years, storing about a third of the world's soil carbon.
Key Insight
Energy flows one way through an ecosystem and is lost as heat, but matter cycles, and decomposers are the link that closes that loop, so decomposition is roughly the largest flux of carbon dioxide from land back to the atmosphere, closely matching photosynthesis in the other direction, which is why a shift in that balance under warming is one of the central uncertainties in climate projections.