Oxygen deprivation implicated in rapid coral mortality -- an emerging perilous threat to coral reefs

Tropical coral reefs are undergoing unprecedented degradation1, primarily due to the increasing intensity and frequency of marine heatwaves with climate change2-4. Coral bleaching is a well-known ramification of marine heatwaves, but rapid coral mortality is an emerging paradigm that visually manifests as the sloughing of tissue from the coral skeleton. Unlike coral bleaching, coral tissue sloughing precludes any prospect of holobiont recovery beyond the initial onset1,5,6, indicating a life-or-death tipping point. Here, we experimentally confirm this phenomenon occurs when temperatures increase within temporal windows of hours to days, consistent with field observations7,8. Through microscale measurements of dissolved oxygen in the diffusive boundary layers of two abundant, keystone reef-building corals, we demonstrate that rapid temperature increases coincide with intrinsic oxygen deprivation, occurring before gross tissue disintegration or coral tissue sloughing. We propose that this distinct phenomenon arises from rapid heating, rendering the coral holobiont incapable of engaging in reactive processes to counteract the combined effects of heightened aerobic demands and impaired photosynthetic function. The passive diffusion of O2 from the surrounding bulk water is likely insufficient to meet the holobionts requirements, as explained by the Einstein-Smoluchowski kinetic theory of gases and Brownian motion9,10. These insights into coral tissue sloughing underscore the complexity of holobiont responses to stress and biophysical consequences of heatwaves. A granular understanding of these mechanisms is urgently needed, particularly regarding how heating rates may change under future climate scenarios, to re-evaluate the potentially under-recognised threats facing coral reefs.