Bleaching of Reef Organisms: Current Knowledge, Challenges, and Future Directions into the Anthropocene

Coral reefs worldwide are threatened by climate change and other anthropogenic stressors. A global factor contributing to coral reef decline is the increasing frequency, duration, and intensity of bleaching events leading to extensive mortality of keystone species, which is caused by the loss of their photosymbionts under stressful conditions. The disruption of the symbiosis between scleractinian corals and their dinoflagellate algae has been the primary focus of bleaching studies. However, phototrophic microorganisms live in symbiosis with a diversity of non-scleractinian taxonomic groups, which have received less attention in the past. Here, we provide an overview of the current knowledge on the bleaching responses of a diversity of reef organisms, focusing on four symbiotic metazoan groups: scleractinian corals, sea anemones, giant clams, and sponges. We summarize the methods currently used to detect bleaching events at different spatial and temporal scales, the physiological and molecular mechanisms of bleaching, and the major hypotheses of bleaching. We provide future directions for the implementation of traditional and emerging protection strategies to increase the resilience of coral reefs to increasingly frequent and severe bleaching events. Finally, we discuss the potential for integrating state-of-the-art multi-omics tools to target multiple levels of organismal organization for future research on bleaching in complex coral reef holobionts.