Zooxanthellales

Many marine organisms possess endosymbiotic dinoflagellates called zooxanthellae. Bleaching is the loss of zooxanthellae, but bleached hosts can be repopulated. Little is known about how repopulation is controlled. The sea anemone Aiptasia pallida was used to study the effects of feeding on repopulation. Two groups of aposymbiotic anemones were reinfected with algae from symbiotic clones. One group was fed and the other starved, and counts of algae within each anemone were made at 3 day intervals for 6 weeks. The experiment was repeated, and results showed that the rate of algal growth did not differ between fed and starved groups. However, starved anemones reached plateau more quickly and lost algal cells after plateau was reached, whereas fed anemones contained more algae and protein. This suggests that starved anemones can support an algal population initially, but cannot maintain it. Space and nutrients are implicated in having a limiting effect on maintenance of symbionts.

Erythropodium caribaeorum is an octocoral known for its production of novel anticancer agents, such as eleutherobin and its analogs as well as for its complex association with symbiotic dinoflagellates (Zooxanthellae; genus Symbiodimum). With this in mind, two sets of experiments were conducted using the extracts of the symbiotic algal cells from Erythropodium caribaeorum: an isolation and characterization of a novel tri-hydroxy sterol and a bioassay-guided isolation of three additional compounds. All compounds exhibit significant bioactivity against the following three cell lines: the human melanoma cell line M14P, the colon cancer cell line RKO and the breast cancer cell line MDA-MB-231. Importantly, this study reports the first isolation of a bioactive polyhydroxy sterol from a gorgonian's symbiont and further suggests that these algae represent a promising sustainable resource for drug discovery.

Coral bleaching involves the loss of symbiotic algae (zooxanthellae) from reef corals and other cnidarians during periods of environmental stress, particularly elevated temperature. In this study the thermal bleaching responses of the zoanthid Palythoa caribaeorum from three populations along the southeast coast of Florida were compared. Winter (2002--2003) and summer (2003) samples from three geographically separate sites were exposed to increased temperatures and the loss of zooxanthellae was measured. Zooxanthellae populations were sampled and identified using PCR-DGGE. The results showed that samples of P. caribaeorum from reefs that experience smaller annual thermal ranges released the most zooxanthellae. Seasonal comparisons revealed winter samples experienced a greater amount of zooxanthellae loss than summer samples. P. caribaeorum was found to harbor two genetic strains of zooxanthellae, C1 and D1a. Colonies either contained monotypic populations of these, or a combination of both.