Direct evidence for symbiont sequestration in the marine red tide ciliate Mesodinium rubrum

The red tide ciliate Mesodinium rubrum (= Myrionecta rubra) is known to contain symbionts of cryptophyte origin. Molecular data have shown that the symbiont is closely related or similar to free-living species of the Teleaulax/Plagioselmis/Geminigera clade. This suggests that the symbiont of M. rubrum is either a temporary symbiont or a quite recently established symbiont. Here, we present data from a number of experiments in which we offered M. rubrum a phototrophic dinoflagellate and 8 different cryptophyte species belonging to 5 different clades. Mesodinium rubrum was only able to grow when fed the 2 cryptophyte species belonging to the genus Teleaulax, T. acuta and T. amphioxeia. Using the nucleomorph large subunit rDNA gene as marker, we were able to discriminate the 2 Teleaulax species, allowing monitoring of the exchange of the symbionts in M. rubrum. Over a period of 35 d, M. rubrum was able to exchange its symbionts from T. amphioxeia symbionts to T. acuta symbionts. This research suggests that M. rubrum can only utilize prey within the Teleaulax/Plagioselmis/Geminigera clade for sustained high growth rates and provides the first time-frame of endosymbiont replacement by M. rubrum.