Differential sensitivity to high temperature in glyceraldehyde-3-phosphate dehydrogenases of coral and algal symbionts

Abstract

Acute environmental stresses including high temperature can lead to the breakdown of the mutualism between reef-building corals and their symbiotic algal partners, ultimately leading to bleaching. However, variation in the thermal tolerance of different coral-algal symbioses has been described repeatedly, suggesting differences in sensitivity to heat among the partners. Since high temperature perturbs the structure and function of macromolecules, these differences in whole-organism temperature sensitivity may occur in part because of differences in the thermal stability of such molecules, including proteins. Here, we examine temperature sensitivity of the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in three coral species – Acropora millepora, Orbicella faveolata, and Astrangia poculata – and three genera of Symbiodiniaceae – Breviolum, Cladocopium, and Durusdinium – to explore whether differences exist between coral and algal orthologs in stability and function at high temperatures. In addition, we examine whether differences in thermal sensitivity exist within GAPDHs of Symbiodiniaceae taxa that have been suggested to possess different levels of thermal tolerance. We use recombinant GAPDH to examine both thermal stability (melting temperature measured via differential scanning fluorimetry) and functional measures of thermal sensitivity (Michaelis constant Km, activation energy Ea) in each ortholog. Our data, complemented by molecular dynamics simulations, indicate that GAPDHs in coral are less sensitive to high temperature perturbation than are algal orthologs, both in kinetics and stability. These results suggest that in some coral-algal symbioses, certain enzymes of the dinoflagellate are more sensitive to heat stress than the host, which may contribute to derangement of the symbiosis and resultant bleaching.