Infectivity of Chlorella species for the ciliate Paramecium bursaria is not based on sugar residues of their cell wall components, but on their ability to localize beneath the host cell membrane after escaping from the host digestive vacuole in the early infection process.

Paramecium bursaria cells harbor several hundred symbiotic algae in their cytoplasm. Algae-free cells can be reinfected with algae isolated from algae-bearing cells or cultivated Chlorella species through the digestive vacuoles. To determine the relationship between the infectivity of various Chlorella species and the nature of their cell wall components, algae-free P. bursaria cells were mixed with 15 strains of cultivated Chlorella species and observed for the establishment of endosymbiosis at 1 h and 3 weeks after mixing. Only 2 free-living algal strains, C. sorokiniana C-212 and C. kessleri C-531, were maintained in the host cells, whereas free-living C. sorokiniana C-43, C. kessleri C-208, C. vulgaris C-27, C. ellipsoidea C-87 and C-542, C. saccharophila C-183 and C-169, C. fusca var. vacuolata C-104 and C-28, C. zofingiensis C-111, and C. protothecoides C-150 and C-206 and the cultivated symbiotic Chlorella sp. strain C-201 derived from Spongilla fluviatilis could not be maintained. These infection-incapable strains could escape from the host digestive vacuole but failed to localize beneath the host cell membrane and were eventually digested. Labeling of their cell walls with Alexa Fluor 488-conjugated wheat germ agglutinin, GS-II, or concanavalin A, with or without pretreatment with 0.4 N NaOH, showed no relationship between their infectivity and the stainability with these lectins. Our results indicate that the infectivity of Chlorella species for P. bursaria is not based on the sugar residues on their cell wall and on the alkali-insoluble part of the cell wall components, but on their ability to localize just beneath the host cell membrane after escaping from the host digestive vacuole.