Changes in temperature could alter the costs and benefits that organisms experience when in a mutualistic relationship with other specie through the effects of kinetics, biochemical reaction rates, and phenotypes. Yet the role of temperature in determining the net benefits of mutualisms is largely unknown. Paramecium bursaria, a single-celled protist with endosymbiotic algae, is an ideal model system to tackle this question. 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 then measured the per capita rate of population growth (average fitness) and estimated per capita rate of biomass production for these two strains at seven temperatures and compared the resulting thermal performance curves.
Results/Conclusions
The net benefit of the endosymbiotic algae on the thermal performance curves of its host P. bursaria depended considerably on temperature. P. bursaria with algae showed higher growth rates at lower temperatures, while P. bursaria without the algae showed much higher growth rates at warmer temperatures. The optimal temperature for P. bursaria without the algae was close to the typical optimal growth temperature of many bacteria, suggesting that cells without algae may 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 algae’s presence at cooler temperatures when most bacteria species are less productive. With more ecological research being conducted on how individual species respond to increasing temperatures, it is important to realize that species interactions will change as well.