Ciliates within the Mesodinium rubrum/M. major species complex harbour chloroplasts and other cell organelles from specific cryptophyte species. M. major was recently described, and new studies indicate that blooms of M. major are just as common as blooms of M. rubrum. Despite this, the physiology of M. major has never been studied and compared to M. rubrum. In this study, growth, food uptake, chlorophyll a and photosynthesis were measured at six different irradiances, when fed the cryptophyte, Teleaulax amphioxeia. The results show that the light compensation point for growth of M. major was significantly higher than for M. rubrum. Inorganic carbon uptake via photosynthesis contributed by far most of total carbon uptake at most irradiances, similar to M. rubrum. M. major cells contain ~four times as many chloroplast as M. rubrum leading to up to ~four times higher rates of photosynthesis. The responses of M. major to prey starvation and refeeding were also studied. M. major was well adapted to prey starvation, and 51 days without prey did not lead to mortality. M. major quickly recovered from prey starvation when re-fed, due to high ingestion rates of >150 prey predator-1 day-1.