The bloom-forming ciliate Mesodinium rubrum harbours a single permanent endosymbiont

Whether the red tide Mesodinium rubrum contains a permanent cryptophyte symbiont or whether it only sequesters chloroplasts from cryptophyte prey was addressed using electron microscopy and the dynamics of photosynthesis, chloroplasts and nuclei. Mesodinium rubrum contains a branched cryptophyte symbiont consisting of many chloroplasts, mitochondria, nucleomorphs, an endoplasmic reticulum and one nucleus. The volume of the symbiont constitutes 36% of the consortium and it is separated from its host by a single-cell membrane. The chloroplasts of Mesodium are larger and morphologically different from two Teleaulax species that served as prey. The symbiont nucleus is also much larger than Teleaulax nuclei. Although M. rubrum is functionally a phototroph, sustained growth beyond two to four generations requires ingestion of prey, but less than one prey cell per generation suffices for maximum growth. This suggests that either the ciliate or its symbiont needs an essential growth factor for continuous growth.