Dinoflagellates

Bleaching of reef corals and other cnidarians symbiotic with zooxanthellae can be attributed to the stress response of the host, algae or both. To determine if zooxanthellae are involved in the bleaching process, I infected a single strain of sea anemone, Aiptasia pallida with zooxanthellae from different hosts. I measured expulsion of the algae from the host during 24-hour incubations at 25, 32 and 34C, as well as photosynthetic rates at these temperatures. Photosynthesis and expulsion of zooxanthellae were inversely and directly proportional to elevated temperatures, respectively. Photosynthesis and expulsion of zooxanthellae isolated from Condylactis gigantea showed the greatest sensitivity to elevated temperature when compared to other zooxanthellae tested. These results suggest that zooxanthellae have a function in the bleaching process and that this function may be related to their photosynthetic response. Thus, the differential tolerance of zooxanthellae to stress could partly explain the spatial variability characteristic of coral-bleaching episodes.

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.

Grazing on bioluminescent and nonluminescent prey by the copepod Acartia tonsa was examined to determine the effect dinoflagellate bioluminescence has on copepod feeding preferences and rates. The percentage of cells ml-1 remaining after 30 minutes of copepod grazing on the dinoflagellates Lingulodinium polyedrum and Pyrodinium bahamense in their bioluminescent and nonbioluminescent phases was used to compare results for concentrations of 10, 1,000, and 3,000 cells ml-1. The nonluminescent diatom Thalassiosira eccentrica was later offered along with each of the dinoflagellates. When diatoms were offered with P. bahamense, the copepods consumed them in equal amounts regardless of bioluminescence at the lower concentration, while the higher concentration showed decreased grazing with bioluminescence. Nonbioluminescent L. polyedrum was consumed at a high rate for all concentrations, but grazing decreased once they were bioluminescent. Copepods switched to T. eccentrica once the dinoflagellates became bioluminescent, except at 3,000 cells ml-1, at which all grazing was reduced. These results indicate there may be a threshold concentration separating two functions of dinoflagellate bioluminescence.

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.