Bleaching physiology: who's the 'weakest link' - host vs. symbiont?

Metabolic signatures of two scleractinian corals from the northern South China sea in response to extreme high temperature events

Coral bleaching events are becoming increasingly common worldwide, causing widespread coral mortality. However, not all colonies within the same coral taxa show sensitivity to bleaching events, and the current understanding of the metabolic mechanisms underlying thermal bleaching in corals remains limited. We used untargeted metabolomics to analyze the biochemical processes involved in the survival of two bleaching phenotypes of the common corals Pavona decussata and Acropora pruinosa, during a severe bleaching event in the northern South China Sea in 2020. During thermal bleaching, P. decussata and A. pruinosa significantly accumulated energy products such as succinate and EPA, antioxidants and inflammatory markers, and reduced energy storage substances like glutamate and thymidine. KEGG analysis revealed enrichment of energy production pathways such as ABC transporters, nucleotide metabolism and lipid metabolism, suggesting the occurrence of oxidative stress and energy metabolism disorders in bleached corals. Notably, heat stress exerted distinct effects on metabolic pathways in the two coral species, e.g., P. decussata activating carbohydrate metabolism pathways like glycolysis and the TCA cycle, along with amino acid metabolism pathways, whereas A. pruinosa significantly altered the content of multiple small peptides affected amino acid metabolism. Furthermore, the osmoregulatory potential of corals correlates with their ability to survive in heat-stress environments in the wild. This study provides valuable insights into the metabolic mechanisms linked to thermal tolerance in reef-building corals, contributes to the understanding of corals' adaptive potential to heat stress induced by global warming and lays the foundation for developing targeted conservation strategies in the future.

Triggers, cascades, and endpoints: connecting the dots of coral bleaching mechanisms

The intracellular coral-dinoflagellate symbiosis is the engine that underpins the success of coral reefs, one of the most diverse ecosystems on the planet. However, the breakdown of the symbiosis and the loss of the microalgal symbiont (i.e. coral bleaching) due to environmental changes are resulting in the rapid degradation of coral reefs globally. There is an urgent need to understand the cellular physiology of coral bleaching at the mechanistic level to help develop solutions to mitigate the coral reef crisis. Here, at an unprecedented scope, we present novel models that integrate putative mechanisms of coral bleaching within a common framework according to the triggers (initiators of bleaching, e.g. heat, cold, light stress, hypoxia, hyposalinity), cascades (cellular pathways, e.g. photoinhibition, unfolded protein response, nitric oxide), and endpoints (mechanisms of symbiont loss, e.g. apoptosis, necrosis, exocytosis/vomocytosis). The models are supported by direct evidence from cnidarian systems, and indirectly through comparative evolutionary analyses from non-cnidarian systems. With this approach, new putative mechanisms have been established within and between cascades initiated by different bleaching triggers. In particular, the models provide new insights into the poorly understood connections between bleaching cascades and endpoints and highlight the role of a new mechanism of symbiont loss, i.e. 'symbiolysosomal digestion', which is different from symbiophagy. This review also increases the approachability of bleaching physiology for specialists and non-specialists by mapping the vast landscape of bleaching mechanisms in an atlas of comprehensible and detailed mechanistic models. We then discuss major knowledge gaps and how future research may improve the understanding of the connections between the diverse cascade of cellular pathways and the mechanisms of symbiont loss (endpoints).