Abstract
The influence of temperature on the activity of the respiratory electron transport system (ETS) was measured in one clone ofDaphnia hyalina x galeata and one ofDaphnia cucullata x galeata, isolated from Lake Bled (Slovenia). The ETS activity of ovigerous females acclimated to 7, 20 and 25° C, was measured at 5, 10, 15, 20, 25 and 30°C. Population growth experiments showed that D. cucullata × galeata grew better at high rather than low temperatures. Daphnia hyalina x galeata, however, grew more successful at low temperatures than did D. cucullata x galeata. The highest Q 10 of ETS activity of D. cucullata x galeata at the lowest temperature range of 5-15°C indicated the absence of enzymes that could function sufficiently well at low temperatures. The ETS activity of the warm-acclimated hybrid D. hyalina x galeata reached a maximum at an incubation temperature of 20°C, while D. cucullata x galeata had maximal ETS activity at 25°C. Thus D. cucullata x galeata has a more efficient enzyme system than D. hyalina x galeata at the higher temperature. The higher Arrhenius activation energy (E a ) for D. cucullata x galeata than for D. hyalina x galeata indicates that enzymes from D. cucullata x galeata are more temperature sensitive than those from D. hyalina x galeata. In conclusion, the ETS of D. cucullata x galeata is adapted to a higher temperature and to narrower temperature fluctuations than that of D. hyalina x galeata.
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