Living things incorporate nutrients into their bodies. For example, plants absorb carbon dioxide from the air and water from the soil, then, using energy from the sun, synthesize carbon and hydrogen into carbohydrates. When you eat food, you break that food down into its basic units, and then reassemble those to build things like muscle. Living things tie up nutrients, and without decomposers those nutrients would stay tied up. Decomposers like bacteria, earthworms, and fungi release those nutrients from the dead bodies of plants and animals or from animal feces.