Apoptotic and necrotic stages of Symbiodinium (Dinophyceae) cell death activity:: bleaching of soft and scleractinian corals

Mechanisms which regulate abundances of Symbiodinium populations during bleaching are not yet fully understood. Currently proposed mechanisms include cell exocytosis, host-cell detachment, apoptosis and necrosis. Here we have analysed zooxanthellae of soft and scleractinian corals under thermal stress using light, fluorescence and transmission electron microscopy (TEM) and confirmed that different stages of apoptotic and necrotic cell death of Symbiodinium occur. The diagnostic characteristic of apoptosis within Symbiodinium cells included nuclear and organelle condensation. membrane blebbing, the formation of apoptotic bodies and mitochondrial swelling. Necrotic cells swell, cytoplasmic organelles fuse and burst, followed by Symbiodinium cell membrane disintegration and release of cytosolic contents. In addition, TEM showed that apoptotic and necrotic processes are initiated in situ. Symbiont cells in the process of dying in situ therefore reduce the likelihood of recolonization upon expulsion from the host.