The possibility of using animals in the closed ecological system is open to question, particularly in the absence of gravity, and much work remains to be done on using plant materials as animal food and on the disposal of wastes. Animals which have been considered are crustaceans, fish, chickens, rabbits, and goats. Algae Algae have the fastest growth rate and are among the most efficient plants for oxygen and food production. It has been amply demonstrated by Myers ([ref.179]) and other workers that _Chlorella_ can be used in a closed ecological system to maintain animals such as mice and a monkey. The use of algae for supplying O₂ and food, and for removing CO₂ and odors has been considered by many authors for use in spacecraft, space platforms, and for establishing bases on the Moon or Mars. Estimates of total efficiency are based on extrapolated laboratory data and vary widely, since many different types of data have been used as a basis for these estimates. The respired air containing about 4-5 percent CO₂ is bubbled into the _Chlorella_ culture, at either atmospheric or increased pressure. Air containing a high percentage of oxygen and saturated with moisture is released from the algal system. The use of algae for several purposes might require from one to three separate algal systems. For food production, _Chlorella_ produces 50 percent protein and 50 percent lipids in high-nitrogen media. In low-nitrogen media, it produces 85 percent lipids. Proper choice of _Chlorella_ strains and media will produce not only the necessary calories but also the necessary specific nutrients required. Certain strains are more effective in O₂ production, and others in the use of urine and other wastes. Some of the early estimates, using _Chlorella_ grown at 25° C, for supplying these requirements for a single man in space include the following: 168 kg of algal suspension ([ref.179]), 200 kg of algal suspension and 50 kg of equipment including pumps (refs. [ref.180] and [ref.181]), and 100 kg of algal suspension and 50 cubic feet for equipment and gas exchange ([ref.182]). Using the blue-green alga _Synechocystis_, 600 kg of algal suspension would be required, according to Gafford and Craft. These estimates are based on preliminary studies, are quite high, and are not of real practical value.