Closed ecological system

A closed ecological system is one that does not exchange matter with anything outside itself in order to sustain life, although it still requires an external energy source such as sunlight. Within the system, the waste products of one organism, such as carbon dioxide, feces, and urine, must eventually be converted by another organism into usable resources like oxygen, food, and water, so a functioning system needs at least one autotrophic organism, typically green algae, capable of turning that waste back into usable material. Closed ecological systems are generally small, human-made environments built to test whether they could support life independently, as potential life-support systems for space habitats. Real attempts include Biosphere 2, the MELiSSA project, the Soviet BIOS series of experiments, and simple sealed bottle gardens or aquarium ecospheres containing shrimp and algae. So far, no artificial closed ecological system involving humans has managed to support life successfully over a long period. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/