CH2O + O2 ———————> CO2 + H2O
(30) + (32) ———————> (44) + (18) (3)
The metabolic data in [table VIII] show that the CO2 of the astronaut and the bacteria must balance at about 1.056 kg per day.
The water relations are not completely balanced, but are fairly close. About 2.6 liters per day of water are split by electrolysis. The astronaut has an intake of 3.5 liters of water per day, 2.5 liters for drinking and 1 liter for preparing dehydrated food. The output is about 1.6 liters of urine and 2.1 liters of water of respiration and perspiration per day, or a total output of 3.7 liters, with the 0.2-liter excess due mainly to water of metabolism. The bacteria-produced water, amounting to 2.2 liters per day, and the excess from the astronaut would supply 2.4 liters toward balancing the 2.6 liters of water electrolyzed.
Bacterial Culture
Hydrogen bacteria are characterized by their ability to metabolize and multiply in a strictly inorganic medium, when supplied with H2, CO2 and O2 in required amounts. They can be grown in batch culture or in continuous culture using different methods of supplying entire medium or components on a demand feed system.
A medium was developed for batch culture of Hydrogenomonas eutropha by Repaske ([ref.187]) with quantitation of a number of components including trace minerals. Experiments by Bongers ([ref.188]) showed that a simplified medium, using laboratory-grade chemicals, could be used. A definite requirement was found for magnesium and ferrous iron (Fe++). The optimal growth requirements observed for Hydrogenomonas eutropha are shown in [table X].
| Culture parameter | Optimum value |
|---|---|
| Cell density, g (dry weight)/liter | 10 |
| Temperature, °C | 35 |
| Pressure, atm | 1 |
| pH (phosphate buffer) | 6.8 (6.4-8.0) |
| H2, percent | 75 |
| O2, percent | 15 |
| CO2, percent | 10 |
| Urea CO(NH2)2, g/liter | 1 |
| MgSO4·7H2O, g/liter | 0.1 |
| Fe(NH4)2(SO4)2, g/liter | 0.008 |
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.