Helmuth, B.*; Hofmann, G.E.: Defining thermal stress in the rocky intertidal: linking ecology and physiology through biophysics

Thermal stress is thought to be an important determinant of organismal distribution, abundance, and productivity in the rocky intertidal zone. Yet, we know surprisingly little of how body temperatures change in space and time, and of how these changes translate into physiological stress. We monitored the body temperatures of intertidal mussels (Mytilus californianus) at a site in central California for a period of two years, at intervals of 5-10 minutes, on both horizontal and vertical, north-facing rock surfaces. Results showed strong effects of substrate, and mussels on north-facing slopes were often more than 10� cooler than those on adjacent horizontal surfaces. Body temperatures were only very poorly correlated with air temperatures, and differed substantially from temperature data recorded by adjacent loggers (Tidbits) placed directly on the rock surface or within mussel beds. We concurrently measured levels of two molecular chaperones from the 70 kDa heat shock family (hsp70/hsc70) at intervals of 3-4 months. These proteins served as biochemical indicators of stress to the protein pool and provided insight into protein homeostasis in each population of mussels. The data showed that mussels on horizontal surfaces had higher levels of Hsps than those from the north-facing locations, indicating that animals on horizontal surfaces were more frequently exposed to thermal conditions that would denature cellular proteins, disrupt protein homeostasis and thus, require more chaperone molecules to address the large population of non native proteins. Our results show that levels of thermal stress within an intertidal site vary substantially in space and time, and that only through a combined biochemical and biophysical approach can we understand patterns in thermal stress in the rocky intertidal zone.