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 oftenbeen found to be more viable than aposymbiotic (without endosymbionts)counterparts during periods of starvation. However, the possiblebenefit of algal endosymbionts to the growth of ciliate hostshas rarely been quantified. Growth coordination between hostand symbionts is essential to maintain the symbiosis, but themechanism behind this is also uncertain. Our hypothesis is thatthe growth rate of the symbionts is always close to its maximum,irrespective of the nutritional status of the host. To testthis hypothesis we built a model based on a constant symbiontgrowth rate, and performed an experiment where we observed thegrowth rate of aposymbiotic and green Coleps cells under differentlight conditions and food concentrations. The results were ingood agreement with the model, and showed that at low food concentrationthe growth rate of green Coleps was clearly higher than thatof aposymbiotic Coleps, while there were no significant differenceswhen food was abundant. Our results indicate that algal grossgrowth rate is always close to maximum, and that growth coordinationbetween host and symbiont is obtained by a variable degree ofleakage of photosynthetic products from the symbionts to theciliate host.