Cadmium effects on mitochondrial function are enhanced by elevated temperatures in a marine poikilotherm, Crassostrea virginica Gmelin (Bivalvia: Ostreidae)

Marine intertidal mollusks, such as oysters, are exposed to multiple stressors in estuaries, including varying environmental temperature and levels of trace metals, which may interactively affect their physiology. In order to understand the combined effects of cadmium and elevated temperature on mitochondrial bioenergetics of marine mollusks, respiration rates and mitochondrial volume changes were studied in response to different cadmium levels (0-1000 mumol l(-1)) and temperatures (15, 25 and 35degreesC) in isolated mitochondria from the eastern oyster Crassostrea virginica acclimated at 15degreesC. It was found that both cadmium and temperature significantly affect mitochondrial function in oysters. Elevated temperature had a rate-enhancing effect on state 3 (ADP-timulated) and states 4 and 4+ (representative of proton leak) respiration, and the rate of temperature-dependent increase was higher for states 4 and 4+ than for state 3 respiration. Exposure of oyster mitochondria to 35degreesC resulted in a decreased respiratory control and phosphorylation efficiency (P/O ratio) compared to that of the-acclimation temperature (15degreesC), while an intermediate temperature (25degreesC) had no effect. Cadmium exposure did not lead to a significant volume change in oyster mitochondria in vitro. Low levels of cadmium (1-5 mumol l(-1)) stimulated the rate of proton leak in oyster mitochondria, while not affecting ADP-stimulated state 3 respiration. In contrast, higher cadmium levels (10-50 mumol l(-1)) had little or no effect on proton leak, but significantly inhibited state 3 respiration by 40-80% of the control rates. Elevated temperature increased sensitivity of oyster mitochondria to cadmium leading to an early inhibition of ADP-stimulated respiration and an onset of complete mitochondrial uncoupling at progressively lower cadmium concentrations with increasing temperature. Enhancement of cadmium effects by elevated temperatures suggests that oyster populations subjected to elevated temperatures due to seasonal warming or global climate change may become more susceptible to trace metal pollution, and vice versa.