Seasonal changes in stress-70 protein levels reflect thermal tolerance in the marine bivalve Mytilus edulis L.

Fluctuating concentrations of cellular stress proteins may be especially significant in the environmental adaptation of eurythermal ectotherms. This study has demonstrated that endogenous levels of stress-70 proteins in M. edulis vary seasonally and are positively correlated with seasonal changes both in environmental temperature and thermal tolerance. The levels of stress-70 protein isoforms of 70, 72 and 78 kDa were analyzed in M. edulis L. collected at six consecutive bimonthly intervals from the River Exe (Devon, UK). Significant seasonal differences in endogenous levels of the stress-70 proteins in M. edulis were observed. Levels of these protein isoforms correlated positively with seasonal changes in environmental temperature. Levels of the 70 kDa protein showed a larger seasonal variation, more closely related to natural temperature change, than levels of the 72 and 78 kDa proteins. Additional mussels from the same samples were heat-stressed at 28.5 degrees C, a temperature at which M. edulis is unable to acclimate. M. edulis survived longer at 28.5 degrees C during the warmer months of the year. There were also positive correlations between time of survival at 28.5 degrees C and the coincident levels of endogenous stress-70 proteins. Our results provide evidence that environmental stress may have been sufficient to cause at least partial protein denaturation during a significant proportion of the year, and suggest that high natural concentrations of stress-70 proteins may promote thermal tolerance. Evidence from other studies suggests that maintaining high levels of stress proteins have significant energetic costs and may be detrimental to organismal fitness. We discuss our findings in terms of the costs and benefits associated with stress protein synthesis, and consequences for the ecology and distribution of marine ectotherms. (C) 1998 Elsevier Science B.V. All rights reserved.