THE BIOLOGY OF THE ASSOCIATION OF SYMBIODINIUM MICROADRIATICUM WITH AIPTASIA PALLIDA: AN ANEMONE-ALGA SYMBIOSIS

Scanning and transmission electron microscopy demonstrated that the symbiotic dinoflagellate Symbiodinium microadriaticum Freudenthal occurs inside vacuoles of gastrodermal cells in the sea anemone Aiptasia pallida (Verrill).;The photosynthsis-irradiance response of the A. pallida-S. microadriaticum symbiosis (phycozoan) and its component bionts was examined. The initial rate of photosynthetic oxygen evolution at subsaturating light intensities ((alpha)) for symbionts in hospice was significantly greater than that for symbionts ex hospice (includes freshly isolated, cultured and extruded symbionts) but the maximum rate of photosynthesis at saturating light intensities (P(,max)) for symbionts in hospice and ex hospice was not significantly different. It was also demonstrated that the A. pallida phycozoan exhibits net oxygen production at radiant flux intensities above 63 (mu)E m('-2)s('-1).;Viable symbionts were extruded by A. pallida as mucous-bound pellets. Extruded symbionts were photosynthetically active and capable of cell division ex hospice; their ultrastructure was similar to that of in hospice symbionts. Both symbiont mitotic cell division in hospice and symbiont extrusion by light fed anemones were minimal during the light and reached maximum levels during the dark. It is suggested that dark-extruded symbionts may represent supernumerary symbionts produced by cell division during the dark, and extrusion of symbionts by A. pallida may result from a physical limitation of space for symbionts within the gastrodermal cells.;Ultrastructural analysis revealed that the symbiont amphiesma is composed of an outer pellicle (trilaminar + amorphous layer), two inner amphiesmal membranes, an inner trilaminar pellicle and a cytoplasmic membrane. The amorphous layer of symbionts in hospice (in host) and from culture varies greatly in thickness, but in general, it appears better developed in cultured symbionts. In the initial stages of symbiont cytokinesis the inner pellicle and cytoplasmic membranes invaginate at each pole of the parental cell. The two inner amphiesmal membranes do not actively participate in cytokinesis.