Or. Anderson et al., FINE-STRUCTURE OF A LARGE DINOFLAGELLATE SYMBIONT ASSOCIATED WITH A COLONIAL RADIOLARIAN (COLLOZOUM SP.) IN THE BANDA SEA, Symbiosis, 24(2), 1998, pp. 259-269
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.