Endocytosis of the symbiotic dinoflagellate Symbiodinium microadriaticum Freudenthal by endodermal cells of the scyphistomae of Cassiopeia xamachana and resistance of the algae to host digestion.

The ingestion and fate of four types of particles by endodermal cells of the scyphistomae of Cassiopeia xamachana were investigated by scanning and transmission electron microscopy. Ferritin was endocytosed pinocytotically by invagination of the plasmalemma. These small pinocytotic vesicles fuse with other similar vesicles to form larger ferritin-containing vacuoles, which eventually fuse with lysosomes. Such secondary lysosomes exhibit acid phosphatase activity. The co-occurrence of acid phosphatase activity and ferritin in secondary lysosomes achieved maximum frequency within 2 h of uptake of ferritin and was evident for at least 4 h following uptake. Artemia particles, live freshly isolated symbiotic algae (Symbiodinium microadriaticum), and heat-killed S. microadriaticum are phagocytosed by endodermal cells. Ferritin-labelled lysosomes fused with food vacuoles containing particles of Artemia. Vacuoles containing heat-killed S. microadriaticum also showed evidence of phago-lysosome fusion. S. microadriaticum in situ (i.e. in host cells) after 3 days exposure to the photosynthetic inhibitor, 3-(3-4-dichlorophenyl)-1,1-dimethylurea, appeared degenerate, and were found in loose-fitting host vacuoles, many in mid and apical portions of the host cell. More than 70% of these vacuoles with moribund algae contained the ferritin label, indicating that lysosome fusion had occurred. In contrast, live S. microadriaticum in control animals were almost always found at the base of the host cell in individual tight-fitting vacuoles with no evidence of lysosome fusion. Live S. microadriaticum apparently escape host digestion by prohibiting the fusion of lysosomes with the vacuole in which they reside. Vacuoles containing defunct algal symbionts, in contrast, were subject to lysosomal attack.