Physiological Responses of Mesodiniummajor to Irradiance, Prey Concentration and Prey Starvation

Abstract

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