Exploring the digestive processes of symbiotic Chlorella sp. in non-endosymbiotic Paramecium species, Paramecium multimicronucleatum.

Chlorella spp. live in mutual symbiosis with Paramecium bursaria. In the present study, we investigated the digestive processes of Chlorella variabilis isolated from P. bursaria ingested by P. multimicronucleatum, a species that does not have the ability to undergo endosymbiosis with algae. The digestion of algae within the digestive vacuole (DV) of P. multimicronucleatum began within 5 min, and complete digestion occurred within 12 h. The digested algae were retained in P. multimicronucleatum even after 72 h of incubation. Additionally, after 0.5 h, some single green alga appeared in the P. multimicronucleatum cytoplasm by budding from the DV membrane. Comparing the re-endosymbiosis process between P. bursaria and Chlorella sp., some algae exhibited temporary lysosomal enzyme resistance in P. multimicronucleatum DVs and appeared from the DVs by budding the DV membrane one cell at a time. However, the differentiation of the DV membrane surrounding a single green alga into a symbiosome membrane, called the perialgal vacuole membrane, localized beneath the P. bursaria cell cortex was not observed in P. multimicronucleatum. These findings provide insights into the digestive process of symbiotic algae in Paramecium species incapable of endosymbiosis and highlight the unique adaptations required for the establishment of endosymbiosis between P. bursaria and Chlorella spp.