Fine structure of a large dinoflagellate symbiont associated with a colonial radiolarian (Collozoum sp.) in the Banda Sea

A wide variety of algal symbionts has been reported in solitary and colonial radiolaria including prasinophytes, prymnesiophytes, and dinoflagellates. Among the symbiotic dinoflagellates the typical size is 6 to 12 mu m. During a plankton collection in the Banda Sea, we obtained a skeletonless colonial radiolarian (Collozoum sp.) with an unusually large dinoflagellate symbiont (c. 25 mu m). We report the fine structure of the symbiont and possible correlates with function. The globose cell has a single layer of plastids distributed at the periphery of the cell, a mesokaryotic nucleus with puffy chromosomes characteristic of some dinoflagellates, a very large mass of osmiophilic matter that fills most of the cytoplasm, peripheral chloroplasts with lamina containing 2-3 thylakoids, and a very reduced chondriome. The relatively small amount of mitochondria, the large mass of reserve material, and the nearly continuous layer of plastids at the periphery suggest that this symbiont is maximally active as a photosynthetic unit with minimal respiratory activity, thus enhancing its role as a source of nutrients for the host. This is the first report of a dinoflagellate symbiont with these properties and raises the interesting question of why this host-algal symbiosis is so different from previously reported dinoflagellate associations with radiolaria.