Coral bleaching occurs when corals expel their symbiotic algae (Symbiodinium). This happens especially at high temperatures, and can be fatal to the coral. However, different clades of Symbiodinium seem to provide different levels of thermotolerance, although the cause is unknown. This project tested the hypothesis that an enzyme from a more heat-tolerant clade (D) is better able to resist denaturation at high temperatures than a more heat-sensitive clade (C). Furthermore, we tested the effects of amino acid differences between orthologs of the enzyme, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), in an attempt to determine which are responsible for the observed difference in heat stability. Using site-directed mutagenesis, we altered 4 potentially functionally significant amino acids and tested residual activity after heat exposure. We found that neither mutant N25G nor R58K affected the temperature sensitivity of clade C GAPDH. This project was made possible by funding from F&M's Eyler Fund and Leser Scholars Grant.