Exploring the coral bleaching tipping point with ¹³C metabolomics

A key driver of global coral reef loss is thermal stress, resulting in coral bleaching. Bleaching is the breakdown of the coral symbiosis (the holobiont) and loss of internal dinoflagellate symbionts (Symbiodiniaceae). Severe, repeated, and prolonged bleaching events have major fitness implications for corals, which can ultimately lead to death. With climate change and continued warming of the world’s oceans, greater insight is critically needed into metabolic and acclimatory responses to severe thermal stress in the coral holobiont. With the application of a stable isotope tracer (13C), analyzed within individual compounds using GC-MS metabolomics, we mapped alterations to symbiont biosynthesis and downstream metabolic pathways in both symbiont and coral host. An untargeted metabolomics approach allowed us to investigate changes not only to bulk carbon fixation and translocation during thermal stress and bleaching, but also complex metabolic responses to thermal and oxidative stress. Multiple time points (early- vs late-phase bleaching) matched to traditional monitoring parameters (symbiont photosystem health) allowed us to explore the fitness implications of both initial and pronounced damage, the tipping point that can often result in coral death.