A project undertaken at the University of Technology, Sydney, and supervised by Dr Katherina Petrou and Dr Daniel A Nielsen.
Coral bleaching, although often described as an ecosystem wide event, is fundamentally a process occurring at the single-cell scale. For this reason, single-cell techniques are required to effectively separate the response of the different cell-types during the bleaching process. This project aimed to resolve the biomolecular fingerprint of host and symbiont cells across different species of corals during the bleaching process using synchrotron-based InfraRed (IR) microspectroscopy. This fast, non-destructive technique can detect variations in biomolecular composition at the single cell level, providing a snapshot of the metabolic state of individual cells within coral symbiont populations. In this project, we aimed to elucidate the metabolic “fingerprint” of coral bleaching and thus expose the key physiological changes that occur within the coral leading up to the expulsion of its life-giving symbionts.
Figure 1. Coral colonies collected from Heron Island reef flat before fragmentation
We undertook two field campaigns. The first was to complete a controlled manipulative study looking at high time resolved changes to biomolecular profiles of symbionts from four species of heat-treated corals. We were able to show strong and consistent biomolecular responses to heat stress and reveal, for the first time, several distinct metabolic profiles within coral symbiont populations, with prevalence of individual profiles corresponding with the progression of heat stress. This highlights the heterogeneity in symbiont health during a bleaching event at the colony level.
Figure 2. Close up of coral fragments Stylophora pistillata, Acropora millepora and Pocillopora damicornis, in aquaria for thermal stress experiment
Our second field campaign was used to apply these findings in a field setting, sampling quarterly over an annual cycle with the aim to capture a natural warming event. We sampled 29 corals covering 6 species, returning in each season to sample the same corals and analyse their biomolecular profiles and symbiont populations, again, finding high levels of response diversity independent of phylogenetic relationships.
Figure 3. Monitoring temperature in coral tanks
Through this work we have discovered and characterised a number of ‘hidden’ endosymbiont metabolic profiles. Our work highlights strong consistency in the biomolecular makeup of endosymbiotic algae during healthy conditions, as well as a uniform response pattern in the pathophysiology of bleaching, pointing to similar physiological mechanisms across coral symbiont species.