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SummaryTwo sources of oxygen for man in closed-cycle space system environinerit have been considered in previous studies: ( 1 ) photosyiit,hesis using algae, and ( 2 ) electrolysis of water. The latter system appears to be the most promising from the standpoints of energy and weight requirements and ability to operate in a zero gravity field. The surplus hydrogen produced by t,he electrolysis of water may be utilized together with waste carbon dioxide, part of the oxygen, and waste urea by bacteria of t.he genus Hydrogenomonns to produce cellular protein which might be used as a source of food. ,4 continuous culture systeni for the propagation of hydrogen-fixing bwteria consists of a baffied borosilicate glass culture vessel provided with an impeller, a reservoir vessel for the culture medium, and an overflow vessel for collecting the bacterial cells removed from the culture vessel. Complete feedback control of all parameters affechg growth can be provided by hydrogen, oxygen, and carbon dioxide sensors, and a pH electrode in the culture medium.Cell density is controlled in the optimum range by means of a photoelectric cell which dictates the amount of fresh medium t.o be added and the amount of cells to be removed. Operating data indicate that some of the key parameters are the ratio o f hydrogen, oxygen, and carbon dioxide in solution. The harvested bacterial substance is high in protein, which contains all the essential amino arids.In addition, total pressure is nionit,ored.The microbial utilization of hydrogen is a promising niethod for closing the oxygen cycle in a continuous process for replacing the oxygen consuiiied by the occupants of a manned space &ation. I , 2

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