Detritus is dead and decaying matter including the wastes of organisms. It is composed of organic material resulting from the fragmentation and decomposition of plants and animals after they die. Detritus is decomposed by bacterial activity, which can help cycle nutrients back into the food chain. In aquatic environments, detritus may make up a substantial percentage of the particulate organic carbon (POC) that is suspended in the water column. Animals that consume detritus are called "detritivores". Although detritus is available in large quantities in most ecosystems, it is usually not a very high quality food, and may be lacking in essential nitrogen or carbon compounds. Detritivores generally must expend a larger amount of energy to assimilate carbon and nutrients from detritus than from sources of food based on living plant or animal material. Some detritivores harbor beneficial bacteria or fungi in their guts to aid in the digestion of compounds that are difficult to degrade.