Optical feedback loop involving dinoflagellate symbiont and scleractinian host drives colourful coral bleaching

Coral bleaching, caused by the loss of brownish-coloured dinoflagellate photosymbionts from the host tissue of reef-building corals, is a major threat to reef survival. Occasionally, bleached corals become exceptionally colourful rather than white. These colours derive from photoprotective green fluorescent protein (GFP)-like pigments produced by the coral host. There is currently no consensus regarding what causes colourful bleaching events and what the consequences for the corals are. Here, we document that colourful bleaching events are a recurring phenomenon in reef regions around the globe. Our analysis of temperature conditions associated with colourful bleaching events suggests that corals develop extreme colouration within 2-3 weeks after exposure to mild or temporary heat stress. We demonstrate that the increase of light fluxes in symbiont-depleted tissue promoted by reflection of the incident from the coral skeleton induces strong expression of the photoprotective coral host pigments. We describe an optical feedback loop involving both partners of the association, discussing that the mitigation of light stress offered by host pigments could facilitate recolonization of bleached tissue by symbionts. Our data indicate that colourful bleaching has the potential to identify local environmental factors, such as nutrient stress, that can exacerbate the impact of elevated temperatures on corals, to indicate the severity of heat stress experienced by corals and to gauge their post- stress recovery potential.