Assessment of controller platforms on consistency of acute short-term thermal stress assays and coral physiology.
Supervisor: Christian R Voolstra (Univ Konstanz)
Increase in sea surface temperatures and extreme heat stress events pose an ongoing threat to coral reef ecosystems and are linked to mass mortality events around the world. Consequences affect the species depending on them and the livelihood of almost a billion people relying on its ecosystem services. Despite rapid degradation, coral colonies, populations, and regions have been reported to exhibit outstanding tolerance to sea surface temperatures of several degrees above their maximum monthly mean (MMM) water temperatures. Moreover, it has become increasingly acknowledged that highly variable temperatures may promote thermal resilience. Identifying corals with exceptional tolerance and understanding the processes underlying their thermal limits is crucial for effective conservation efforts. Standardized short-term acute thermal assays, such as the 18 h heat-stress run with the Coral Bleaching Automated Stress System (CBASS), are currently being used to resolve thermal limits of corals across a wide range of spatial scales. Two popular temperature control platforms have been employed during acute thermal stress assays, the custom-built Arduino controller and the readily accessible Inkbird controller. While the Arduino is highly customizable, allowing unlimited programmable steps with a precision of ±0.1°C, the Inkbird is a simpler 12-step but highly robust temperature controller, offering a precision of ±0.5 °C, rendering heating profiles

more variable. The aim of the study was to assess whether such fluctuations would affect modeled coral thermal thresholds. Temperature controllers showed variation in heating consistency, with the Arduino controller yielding a more precise temperature profile compared to the Inkbird controller, yet less accurate. Despite the differences in precision and consequent fluctuations caused by the Inkbird controller, none of the response variables led to significant differences across experiments. Most notably, thermal thresholds were almost identical for both temperature controllers, ~ 36°C being the temperature at which 50% of the photosynthetic efficiency was lost relative to control conditions. Lack of recovery of physiological traits and no significant differences in the measurements at the end of the experiment for both setups, showed no detectable effects of these fluctuations in coral fragments run using the Inkbird controller setup. However, the fine-scale variation in temperature profiles observed in the study does not discard the effect of greater fluctuations in reef settings, where it is commonly assumed that more variable temperature settings yield increased thermal resilience. The higher complexity and cost of the Arduino controller need to be weighed against the robustness and simplicity of operation of the Inkbird controller, where the latter likely is adopted by a larger audience. Moreover, accessibility could pose a problem for assays in remote areas, and the fragile structure of the Arduino controller compared to the Inkbird controller could be an issue when performing assays in the field under unstable conditions. Users should therefore consider the experimental needs and the limitations of controllers for each study. We hope our results promote further analysis of corals with exceptional thermotolerance using short-term acute thermal stress assay platforms.