Algal Symbiont Diversity and Host Fitness Variation in Amoebozoan Photosymbiosis

Photosymbioses, the symbiotic relationships between microalgae and non-photosynthetic eukaryotes, are sporadically found in many eukaryotic lineages. Only a few taxa, such as cnidarians and ciliates hosting algal endosymbionts, have been actively studied, which has hindered understanding the universal mechanisms of photosymbiosis establishment. In Amoebozoa, few species are reported as photosymbiotic, and how the photosymbioses are established is still unclear. To investigate the extent to which one of the photosymbiotic amoebae, Mayorella viridis, depends on their symbionts, the amoebae were treated with reagents known to induce the collapsing of photosymbioses in other species. We succeeded in removing algal symbionts from the hosts with 2-amino-3-chloro-1,4-naphthoquinone. While the apo-symbiotic amoebae grew to the same extent as the symbiotic state when they fed on prey, their survival rates were lower than those of the symbiotic ones during starvation, suggesting that the impact of the photosymbiosis on fitness is condition-dependent. Furthermore, we showed that the photosymbiotic state was reversible by feeding two strains of the green alga Chlorella to the apo-symbiotic amoebae. The efficiencies of ingesting algal cells significantly differed between algal strains. These results suggest that the photosymbiotic relationship in the amoeba is facultative and that different algal strains have discrete symbiotic abilities to the amoeba.