A new conceptual model for the enhanced release of mucus in symbiotic reef corals during 'bleaching' conditions

Symbiotic reef corals exude large volumes of mucus when exposed to environmental conditions that challenge the integrity of the coral-algae endosymbiosis. Here, the physiological consequences of CO2-limitation within the 'dark' photosynthetic reactions of the algal endosymbionts ('zooxanthellae') are investigated as the possible cause of the release of 2 different forms of mucus: mucus-polysaccharide and mucus-lipid. This mechanism may explain why the experimental addition of specific host-derived free amino acids (commonly referred to as 'host factors') enhances photosynthate release and carbon fixation rates from in vitro zooxanthellae. Furthermore, it reinforces the often-ignored importance of the coral host in maintaining the stability and functioning of the intact symbiosis in the face of environmental stress, even supporting the possibility that disruption to hostcontrolled processes ultimately triggers the breakdown of the symbiosis leading to the mass expulsion of zooxanthellae ('coral bleaching').