Ecological significance of endosymbionts in a mixotrophic ciliate - An experimental test of a simple model of growth coordination between host and symbiont

Ciliates with endosymbiotic algae (green ciliates) have often been found to be more viable than aposymbiotic (without endosymbionts) counterparts during periods of starvation. However, the possible benefit of algal endosymbionts to the growth of ciliate hosts has rarely been quantified. Growth coordination between host and symbionts is essential to maintain the symbiosis, but the mechanism behind this is also uncertain. Our hypothesis is that the growth rate of the symbionts is always close to its maximum, irrespective of the nutritional status of the host. To test this hypothesis we built a model based on a constant symbiont growth rate, and performed an experiment where we observed the growth rate of aposymbiotic and green Coleps cells under different light conditions and food concentrations. The results were in good agreement with the model, and showed that at low food concentration the growth rate of green Coleps was clearly higher than that of aposymbiotic Coleps, while there were no significant differences when food was abundant. Our results indicate that algal gross growth rate is always close to maximum, and that growth coordination between host and symbiont is obtained by a variable degree of leakage of photosynthetic products from the symbionts to the ciliate host.