The effects of temperatures ranging from 20° to 42.5° C on the growth rates of _Hydrogenomonas eutropha_ were studied by Bongers ([ref.189]), and the optimal temperature was found to be about 35° C. Experiments at 25° and 35° C indicated that the efficiency of energy conversion was essentially identical at both temperatures. _Hydrogenomonas_ requires, as part of its substrate, a mixture of three gases: hydrogen, oxygen, and carbon dioxide. Experiments were performed by Bongers ([ref.189]) to determine the toleration limits of the three gases. Growth rates were found to be identical when hydrogen varied from 5 to 80 percent. Nearly identical growth was obtained when CO₂ partial pressures were 5 to 60 percent, being slightly lower at higher partial pressures. The organism was highly sensitive to oxygen concentration. Dissolved oxygen concentrations above 0.13 mM were found to inhibit cell division; energy utilization was also affected by oxygen concentration. At 0.2 mM oxygen concentration, the efficiency of energy conversion was approximately half the value observed with 0.05 mM. Another parameter of importance is the total volume of suspension which would be required to balance the metabolic needs of one man. The volume of suspension is determined by the conversion capacity of a unit volume. This capacity is a function of the cell concentration; hence, the more cells that can be packed in a unit volume of suspension (and adequately provided with H₂, O₂, and CO₂), the less the volume of suspension required. Results of experiments by Bongers (refs. [ref.190] and [ref.191]) on conversion capacity-density relationships show that the rate of CO₂ conversion obtained with suspensions up to approximately 10 grams (dry weight) per liter is linear with relation to density. This indicates that the supply of H₂, O₂, and CO₂ is adequate. Upon a further increase in cell concentration, the conversion rate still increases but not linearly. The highest amount of CO₂ taken up per liter of suspension was approximately 2 liters per hour. At these very high cell concentrations, the relationship between rate of conversion and density is no longer linear. This is demonstrated when the conversion rate is calculated per unit cell weight instead of per unit suspension volume. The rate per gram dry weight per liter decreases from 146 to 68 ml of CO₂ per hour. With a suspension at a density of approximately 10 grams, the conversion of 1.1 liters of CO₂ per liter per hour is obtained. At a CO₂ output of 22 liters per man per hour, 20 liters of suspension would be sufficient to balance the gas exchange needs of one man.