The closed nutrient recycling system in the <i>Paramecium</i> - <i>Chlorella</i> photosymbiosis contributes to survival under oligotrophic conditions

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 <i>Paramecium bursaria</i> hosting <i>Chlorella</i> endosymbionts under nitrogen- and prey-depleted conditions. The <i>Paramecium</i> host survived for up to 5 weeks while maintaining the number of <i>Chlorella</i> endosymbionts, whereas aposymbiotic <i>Paramecium</i> and free-living <i>Chlorella</i> 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.