Plastid Sequestration in Mesodinium rubrum: Physiological, Molecular, and Ecological Aspects

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Mesodinium rubrum is a one-celled member of the marine plankton that can form extensive red-tides. It is unusual in that although it is a "Protozoa" (animal-like cell) it contains numerous chloroplasts (the organelles in which photosynthesis occurs) and can make its own food from carbon dioxide and water using the energy in sunlight, and thus is plant-like. Although Mesodinium rubrum appears to derive its plastids from ingestion of algae, it only occasionally requires algal prey and the plastids appear to be capable of synthesizing chlorophyll, the primary pigment used to capture light in photosynthesis. It also temporarily harbors nuclei of algae. Because of its unusual attributes, it may be a model for how ancestors of some algal groups acquired their photosynthetic organelles. The objectives are to describe the relationship of Mesodinium rubrum to its algal prey and to determine if photosynthesis and growth depend on acquisition of new chloroplasts or nuclei from algae and/or on digestion of prey as food. Another goal is to determine if Mesodinium rubrum permanently harbors algal organelles or genes. These questions will be approached using a combination of experimental studies of cultured Mesodinium rubrum and of natural populations in the plankton. A combination of microscopic, chemical, and isotopic techniques will be used to study the physiological ecology of Mesodinium rubrum. Molecular techniques will be used to measure algal gene content and expression in Mesodinium rubrum and to determine relationships among different populations of Mesodinium rubrum and their plastids. These parallel physiological and molecular investigations will elucidate the possible role of organelle uptake from algae by protozoa in cellular evolution.