Experimental bleaching of photosymbiotic amoeba revealed strain-dependent differences in algal symbiosis ability

Photosymbioses, the symbiotic relationships between photosynthetic algal symbionts and non-photosynthetic eukaryotic hosts, are sporadically found in a lot of eukaryotic lineages, but only a few taxa, such as cnidarians and ciliates hosting algal endosymbionts, have been actively studied for a long time. That has hindered understanding the universal mechanisms of the photosymbiosis establishment. Especially in Amoebozoa, only two species,Mayorella viridisandParachaos zoochlorella, are reported as photosymbiotic in nature, and their mechanisms of establishing symbiotic relationships are still unclear. To investigate the extent to which and how photosymbiotic amoebae depend on the symbiotic relationships,M. viridiswere treated with reagents that are known to induce the collapsing of photosymbiotic relationships, or bleaching, in other photosymbiotic species. As a result, we succeeded in artificially removing algal symbionts from hostM. viridiscells with an herbicide, 2-amino-3-chloro-1,4-naphthoquinone. The apo-symbiotic state amoeba cells were able to survive and grow to the same extent as the symbiotic state cells when they fed microbial prey, indicating that the algal symbionts are not essential for the host growth under certain conditions. Furthermore, to see whether the photosymbiotic state is reversible, we fed two strains of algal symbionts to the apo-symbiotic amoeba host. The result showed that the apo-symbiotic hosts were able to ingest symbiont cells and re-establish the symbiotic state. The efficiencies of ingesting algal cells were significantly different depending on algal symbiont strains, indicating that different algal strains possess discrete symbiotic abilities toM. viridis. To our knowledge, we provide first insights on the establishment and collapse of photosymbiosis in Amoebozoa, which pave the way to understand the universal mechanism of photosymbiosis utilizingM. viridisas a model system.