| Abstract |
Endosymbiotic relationships between a heterotrophic host and a unicellular algal endosymbiont are observed across many eukaryotic lineages. Although these relationships are prevalent in oligotrophic environments, how they function and provide an advantage under such conditions remains largely unknown. To address these issues, we examined the behavior of the ciliate Paramecium bursaria hosting Chlorella endosymbionts under nitrogen- and prey-depleted conditions. The Paramecium host survived for up to 5 weeks while maintaining the number of Chlorella endosymbionts, whereas aposymbiotic Paramecium and free-living Chlorella either died or bleached, respectively, under the same conditions. In the symbiotic state, the host continuously fed on the endosymbionts without excreting nitrogenous waste into the medium, while the remaining endosymbionts continued to proliferate using heterotrophic metabolites from the host and light energy. Thus, the cyclical farming of endosymbionts by the host maintains a high concentration of nutrients within the closed system, providing a selective advantage in oligotrophic environments.
|
