Exploring the application of high-temperature isothermal heating experiments to examine protein degradation in Porites coral skeleton

In addition to being a time-dependent process, amino acid racemisation is also temperature-dependent. In light of this, high-temperature isothermal heating of modern biominerals has traditionally been used as a convenient method of accelerating protein degradation. Reactions such as racemisation, that usually progress over geological time-scales at burial temperatures, occur over much shorter periods. In this study, modern Porites coral skeleton from Jarvis, Indonesia, was subjected to isothermal heating experiments at three different temperatures, for a variety of time intervals (up to 5760hrs). Subsequently, the intra-crystalline protein fraction was isolated, and the extent of protein degradation examined using reverse-phase high-performance liquid chromatography (RP-HPLC). In order to compare degradation of the experimentally heated modern samples with diagenetic reactions occurring over geological time, the intra-crystalline protein fraction of fossil Porites coral over the last four centuries was also analysed using RP-HPLC. Our results suggest that protein degradation occurring at temperatures in excess of 110ºC does not simulate that occurring at normal burial temperatures. However, heating at the lower temperature of at 80ºC does appear to mimic that occurring in fossils. Further analyses (including X-ray diffraction and Fourier transform infrared spectroscopy) were performed to examine whether heat-induced changes in skeletal mineralogy may account for the differences in degradation patterns following heating. These findings also present practical applications. In modern corals, for which a high-resolution record of the environmental conditions during growth is available, the concentration of D-enantiomers is usually too low to measure. However, our results suggest that heating at 80ºC can amplify this signal. We suggest that anomalies in racemisation values may coincide with environmental perturbations, providing a “coral-stress” biomarker.