Effect of season on the acute thermal tolerance and thermal inactivation of membrane (Mg super(2+) + Na super(+) + K super(+))- and Mg super(2+)-adenosine triphosphatase activity of the Baltic Sea amphipods, Monoporeia affinis and Gammarus spp

This paper presents a study of the acute upper thermal tolerance and thermal inactivation (IA50) of the membrane-bound (Mg super(2+) + Na super(+) + K super(+))- and Mg super(2+)-adenosine triphosphatases of the Baltic amphipods, Monoporeia affinis, inhabiting a stable cold environment (ca. 3.5 degree C), and Gammarus spp., living in annually fluctuating temperatures (0-20 degree C). For M. affinis the lethal temperatures (LT50) in a continuously rising temperature were 29.8 degree C in winter, 29.5 degree C in spring and 30.2 degree C in summer. For Gammarus spp. the LT50 was 31.0 degree C in spring, 32.8 degree C in summer and 30.2 degree C in autumn. Seasonality in the LT50 values was observed in Gammarus spp., but in M. affinis only the summer value differed from the other seasons. The LT50 of M. affinis did not change after temperature acclimation of the animals at 13.5 degree C. The thermal inactivation values (IA50) of the ATPases of M. affinis (44.3-47.4 degree C) were higher than those of Gammarus spp. (40.3-45.2 degree C). In Gammarus differences in the IA50 values between the seasons were found while in M. affinis only between spring and summer.