Comparative Physiological and Ecological Characterization of the Photosymbiotic Amoeba Mayorella viridis Across Symbiotic and Aposymbiotic States

Symbiosis is a fundamental biological strategy that enables metabolic complementation between distinct organisms. The amoeba Mayorella viridis establishes a photosymbiotic relationship with the green alga Chlorella, yet the physiological basis of this association remains poorly understood. Here, we compared symbiotic (green) and aposymbiotic (white) strains to quantify phenotypic effects of the symbiotic state. Cell size did not differ when the cells were fed but was significantly reduced in the white strain under starvation, indicating that Chlorella contributes to host stability under nutrient limitation. Motility was unaffected by symbiosis. In contrast, photoresponses, including positive phototaxis and photophobic responses, were completely abolished in the white strain, demonstrating that symbiotic Chlorella is essential for host photoresponse. Growth and survival were significantly reduced in the white strain under both constant light and constant dark conditions, indicating that the symbiont enhances both proliferation and starvation tolerance. Notably, under dark and nutrient-limited conditions, intracellular Chlorella declined, suggesting digestion and utilization as an internal nutrient source. Photosymbiosis was re-established by reintroducing isolated Chlorella into aposymbiotic cells, restoring green phenotype and photoresponse. These findings establish a reversible system and clarify the physiological roles of photosymbiosis in M. viridis.