When faced with extreme stress, coral reefs can undergo bleaching: a breakdown of the relationship between the animal host and their colourful internal photosynthetic symbionts. Bleached corals that are unable to recover their symbiont populations face starvation, disease and death. Bizarrely, bleaching sometimes results in vibrantly green-, yellow- or purple–blue-coloured coral, rather than white. These colours are known to arise from pigments produced by the coral host, but the triggers, mechanisms and meaning behind colourful bleaching remain unclear.
Elena Bollati from the University of Southampton, UK, and colleagues show that colourful bleaching is a global phenomenon, affecting multiple key reef-building species. Their experiments reveal that upregulation of the colourful pigments by the coral host is a natural response to increased light stress. Loss of light-shielding symbionts due to heat- or nutrient-induced bleaching can lead to massive active upregulation of the host pigments, which can then protect and promote symbiont recolonization.
Colourful bleaching can act as an identifier for local environmental stressors but can also indicate post-stress coral recovery potential.
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Armarego-Marriott, T. Bleach me colourful. Nat. Clim. Chang. 10, 599 (2020). https://doi.org/10.1038/s41558-020-0845-6