For the fixation of 24 moles of CO₂ (288 grams of C) produced per man per day, the production of about 640 grams dry bacterial mass is required. At an average N-content of 12 percent, the nitrogen requirement would be some 100 grams. A comparison of daily output (urine) and daily requirement by the bacterial suspension reveals that only 10 to 33 percent of this amount could be recovered from average urine. To obtain a material balance, either the man must be fed a protein-rich diet or the bacterial suspension must be grown under conditions which lead to the production of a cell mass relatively low in protein content. Experiments have indicated that nitrogen starvation of the bacterial culture might be a promising solution. Culture "staging" (cultivation under nitrogen-rich conditions, followed by cultivation in the absence of substrate nitrogen and subsequent harvesting for food processing) will probably be the most promising means of nitrogen economy in the closed environment. As discussed in a following section, a biomass of relatively high lipid content can be obtained under conditions of nitrogen starvation. Continuous Culture of _Hydrogenomonas_ Bacteria Growth of hydrogen bacteria in a batch culture, after an initial period of adjustment, becomes steady and rapid during the exponential growth phase. This steady state of growth is temporary and ceases when nutrient substrate or gas concentrations drop to limiting values. For long periods a continual supply of nutrients must be provided. Growth then occurs under steady-state conditions for prolonged periods, and such factors as pH, concentration of nutrient, oxygen, and metabolic products (which change during batch culture) are all maintained constant in continuous culture. Two methods can be used for control of continuous cultures: the turbidostat and the chemostat. In the turbidostat, regulation of medium input and cell concentration is controlled by optically sensing the turbidity of the culture. The dilution rate varies with the population density of the culture and maintains the density within a narrow range. Organisms grow at the maximum rate characteristic of the organism and the conditions. The growth rate can be changed by modifying the nutrient medium, gas concentration, or incubation temperature. A disadvantage of the turbidostat is that all nutrient concentrations in the culture chamber are necessarily higher than the minimum, resulting in inefficient utilization of nutrients. The turbidostat system for continuous culture of _Hydrogenomonas_ bacteria, developed by Battelle Memorial Institute ([ref.194]), includes electrolysis of water in a separate unit. Hydrogen and oxygen are fed separately up to the point of injection into the culture vessel, and the mixed volume is kept very small to minimize am possibility of explosion. However, the two gases may be injected simultaneously if there is a demand for both.