The benefit of algae endosymbionts in Paramecium bursaria is temperature dependent

Background: Through the effects of kinetics, biochemical reaction rates, and phenotypes, changes in temperature could alter the costs and benefits that organisms experience when in a mutualistic relationship with other species. Yet the role of temperature in determining the net benefits of mutualisms is largely unknown. Question: How does the presence of a mutualistic relationship with endosymbiotic algae influence the temperature dependence of fitness in Paramecium bursaria? Organism: Paramecium bursaria. Methods: We developed paired strains of P. bursaria collected from the wild, one with the naturally occurring endosymbiont intact and the other with the endosymbiont removed by growing cells in the dark. We measured the per capita rate of population growth (average fitness) and per capita rate of biomass production for these two strains at seven temperatures and compared the resulting thermal performance curves. Results: The net benefit of the endosymbiotic algae on the thermal performance curves of their host P. bursaria depended considerably on temperature. Paramecium bursaria with the algae showed higher growth rates at cooler temperatures, while P. bursaria without the algae showed higher growth rates at warmer temperatures. The optimal temperature for P. bursaria without the algae was close to the typical optimal growth rate temperature of many bacteria species, suggesting that cells without algae can make more effective use of bacterial prey resources at high temperatures when these resources are plentiful. In contrast, P. bursaria that have endosymbiotic algae benefit more from them at cooler temperatures.