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. Table X.—_Optimum Growth Requirements of_ Hydrogenomonas eutropha --------------------------------------------------------- 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 ---------------------------------------------------------