Photosynthetic algae are able to provide astronauts in space with oxygen and remove CO2.
A development program for bioregenerative life support systems for the space applications, called CELSS (Controlled Ecological Life Support Systems), has been established by NASA in the late 1970's. CELSS operation is based on photosynthetic organisms, such as plants or algae, to produce food, oxygen, and potable water, and to remove carbon dioxide exhaled by the crew.
Autotrophs such as plants and algae take up carbon dioxide expelled by heterotrophes, including man, and produce the oxygen such organisms consume, e.g., oxygen is produced from carbon dioxide by algae, e.g., Chlorella, Spirulina, Scenedesmus, Anacystis, according to the photosynthesis equation:
CO2 + H2O +Light = O2 + Biomass + Heat
A part of the edible biomass is transformed to CO2 and H2O again by the human metabolism. The rest of the edible biomass will be partly oxidized and appears in the secretions, i.e., urine, feces, and transpiration. If these products are later completely oxidized, together with the inedible biomass, a completely closed ecosystem, an "artificial biosphere", can be achieved.
The advantages of algae with respect to an application for life support in space are: rapid growth, controllable metabolism, high harvest index, gas exchange characteristics compatible to human requirements.