Symbiodinium sp cells produce light-induced intra- and extracellular singlet oxygen, which mediates photodamage of the photosynthetic apparatus and has the potential to interact with the animal host in coral symbiosis

- Coral bleaching is an important environmental phenomenon, whose mechanism has not yet been clarified. The involvement of reactive oxygen species (ROS) has been implicated, but direct evidence of what species are involved, their location and their mechanisms of production remains unknown. - Histidine-mediated chemical trapping and singlet oxygen sensor green (SOSG) were used to detect intra- and extracellular singlet oxygen (1O2) in Symbiodinium cultures. - Inhibition of the CalvinBenson cycle by thermal stress or high light promotes intracellular 1O2 formation. Histidine addition, which decreases the amount of intracellular 1O2, provides partial protection against photosystem II photoinactivation and chlorophyll (Chl) bleaching. 1O2 production also occurs in cell-free medium of Symbiodinium cultures, an effect that is enhanced under heat and light stress and can be attributed to the excretion of 1O2-sensitizing metabolites from the cells. Confocal microscopy imaging using SOSG showed most extracellular 1O2 around the cell surface, but it is also produced across the medium distant from the cells. - We demonstrate, for the first time, both intra- and extracellular 1O2 production in Symbiodinium cultures. Intracellular 1O2 is associated with photosystem II photodamage and pigment bleaching, whereas extracellular 1O2 has the potential to mediate the breakdown of symbiotic interaction between zooxanthellae and their animal host during coral bleaching.