Photosystem II breakdown induced by reactive oxygen species in freshly-isolatedSymbiodiniumfromMontipora(Scleractinia; Acroporidae)

Freshly-isolatedSymbiodinium(FIS) have been used to study cnidarian–alga symbiosis based on the assumption that their physiological performance is comparable to that of the algaein hospite. This assumption was tested with 15 species of scleractinian corals, using pulse-amplitude modulation (PAM) chlorophylla(chla) fluorescence to compare maximum quantum yields (F v/F m) of photosystem II (PSII) inSymbiodinium in hospiteand after isolation in seawater. FIS fromMontiporaspp. exhibited rapid and dramatic decreases (by up to >95%) in PSII activity within 30 min of isolation. In contrast, PSII activities of FIS from 8 other coral species decreased by only 5 to 21% after >4 h in seawater. To investigate possible reasons for this variation, the variation inSymbiodiniumgenetic type (ITS-2 types), transmission modes and several physiological indices were considered. The rapid loss of PSII activity in FIS fromMontiporaspp. was not correlated withSymbiodiniumITS-2 type, the mode of symbiont transmission, or ionic regulation capability, nor could consistent chemical effects in host extracts be demonstrated. PSII inactivation was correlated with increased levels of reactive oxygen species (ROS), degradation of pigments in peridinin-chlaprotein, and chloroplast disruption, indicating that the FIS was under physiological collapse. The specific mechanism(s) causingMontipora-associatedSymbiodiniumto fail during the isolation methods used here remain unknown. However, these data indicate that care should be taken when using FIS to representSymbiodinium in hospite, especially when comparing different species of corals.