ENERGY-BALANCE CALCULATIONS AS A TOOL TO DETERMINE MAINTENANCE ENERGY-REQUIREMENTS UNDER STRESS CONDITIONS

The increase in maintenance energy requirements in response to changed osmotic stress were studied in the yeast Saccharomyces cerevisiae. The yeast was aerobically grown at different growth rates in chemostats with glucose as the limiting substrate. The choice of growth rates served to induce different metabolisms, i.e. respiratory and mixed respiratory-fermentative catabolism at a low (0.09 h-1) and high (0.18 h-1) growth rate, respectively. The total energy demand for growth at the different conditions was determined by energy balance measurements, which provided a test of the reliability of the data. Measurement of catabolic energy flux at the different growth rates and salinities (0 M and 0.9 M NaCl) were used for the calculation of specific ATP production rates. The difference in specific ATP production rate between the two salinities at the respective growth rate, indicated an additional maintenance energy requirement of 2 to 3 mmol ATP g-1 h-1 (or 0.6 - 0.8 mmol g-I ks-1) imposed by salt stress.