Other studies have indicated an extremely efficient algal system which offers a real potential for a practical and effective gas exchanger ([ref.183]). A thermophilic strain of _Chlorella_ with an optimum growth temperature of 39° C and an optimum temperature for photosynthesis of about 40° C can increase its cell mass 10 000-fold per day. When operating at one-half maximum efficiency, this alga produces 100 times its cell volume of oxygen per hour. Burk et al. ([ref.183]) state: "Future engineering development should lead to a space requirement, per adult person, of no more than 3 to 5 cubic feet of algal culture, equipment, and instrumentation for adequate purification of air." The requirements of this system would require additional energy in the form of light and of small amounts of nitrogenous and mineral material for the algae. The light source used by Burk et al. ([ref.183]) is a tungsten filament quartz lamp the size of a pencil, which has a long life, produces a luminous flux 5-10 times greater than sunlight on Earth, and operates at a 10-12 percent light efficiency. Research is being carried out on algal regenerative systems by about 40 or 50 laboratories in the United States. NASA is supporting several basic studies on photosynthesis, the physiology of algae, and engineering pilot-plant development. Much of the research on algae is being supported by the Air Force. Most algal studies have been carried out in small units and the data obtained have been used as a basis for extrapolating logistic values for the use of these organisms in manned space vehicles. Myers ([ref.179]) has shown that the quantity of algae necessary to support a man (with an assumed O₂ requirement of 625 liters per day) would yield about 600-700 grams dry weight of new cells per day. If algal growth in mass cultures could be maintained in a steady-state concentration of 2.5 gram dry weight per liter with such a growth rate as to yield 10 grams weight per liter per day, the volume of algal culture would be 60-70 liters and the total mass of the system would approximate 200-250 pounds. Using an 8-liter system, Ward et al. ([ref.176]) have produced algal concentrations of 5-7 grams of dry algae per liter with a high-temperature algal strain. The maximum growth rate observed with the culture was 0.375 gram dry weight per liter per hour, or 9 grams dry weight per liter per day. This was accomplished by using 1-centimeter layers of culture and a light intensity of 8000 foot-candles. The culture system consisted of a rectangular plastic chamber having an area of 0.5 square meter and illuminated on each side to an intensity of 4000 foot-candles (cool-white). To produce 25 liters of oxygen per hour, an area of 8.3 square meters (85 square feet) would be required.