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 co lonial radiolaria including prasinophytes, prymnesiophytes, and dinofl agellates. Among the symbiotic dinoflagellates the typical size is 6 t o 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 structu re of the symbiont and possible correlates with function. The globose cell has a single layer of plastids distributed at the periphery of th e cell, a mesokaryotic nucleus with puffy chromosomes characteristic o f some dinoflagellates, a very large mass of osmiophilic matter that f ills most of the cytoplasm, peripheral chloroplasts with lamina contai ning 2-3 thylakoids, and a very reduced chondriome. The relatively sma ll amount of mitochondria, the large mass of reserve material, and the nearly continuous layer of plastids at the periphery suggest that thi s symbiont is maximally active as a photosynthetic unit with minimal r espiratory activity, thus enhancing its role as a source of nutrients for the host. This is the first report of a dinoflagellate symbiont wi th these properties and raises the interesting question of why this ho st-algal symbiosis is so different from previously reported dinoflagel late associations with radiolaria.