Respiratory electron transport system (ETS) activity as an estimator of the thermal tolerance of two Daphnia hybrids

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.