Localization of Perialgal Vacuoles beneath the Host Cell Surface is not a Prerequisite Phenomenon for Protection from the Host's Lysosomal Fusion in the Ciliate Paramecium bursaria

In a ciliate Paramecium bursaria cell, each symbiotic 3–4-μm-diameter Chlorella cell is enclosed within a perialgal vacuole membrane. It localizes near trichocysts beneath the host cell surface. Gomori's staining of this surface shows that it is an acid phosphatase activity-negative area to 5–10 μm depth. Trichocysts were removed by treatment with 1 mg/ml lysozyme to elucidate whether algal protection from the host lysosomal fusion is controlled by localization of the perialgal vacuole membrane to the acid phosphatase activity-negative area or by the capability of the perialgal vacuole membrane to give protection from lysosomal fusion. The trichocyst-free cell reduced the acid phosphatase activity-negative area to less than 3 μm depth at the dorsal surface. However, even though a part of the algal cell had been exposed in the acid phosphatase activity-positive area, the algae were able to attach beneath the host surface and to protect it from lysosomal fusion. Results of this study show that the perialgal vacuole membrane can give protection from host lysosomal fusion, and that the membrane does not require trichocysts for intracellular localization.