Symbiodinium photosynthesis in Caribbean octocorals

Symbioses with the dinoflagellate Symbiodinium form the foundation of tropical coral reef communities. Symbiodinium photosynthesis fuels the growth of an array of marine invertebrates, including cnidarians such as scleractinian corals and octocorals (e.g., gorgonian and soft corals). Studies examining the symbioses between Caribbean gorgonian corals and Symbiodinium are sparse, even though gorgonian corals blanket the landscape of Caribbean coral reefs. The objective of this study was to compare photosynthetic characteristics of Symbiodinium in four common Caribbean gorgonian species: Pterogorgia anceps, Eunicea tourneforti, Pseudoplexaura porosa, and Pseudoplexaura wagenaari. Symbiodinium associated with these four species exhibited differences in Symbiodinium density, chlorophyll a per cell, light absorption by chlorophyll a, and rates of photosynthetic oxygen production. The two Pseudoplexaura species had higher Symbiodinium densities and chlorophyll a per Symbiodinium cell but lower chlorophyll a specific absorption compared to P. anceps and E. tourneforti. Consequently, P. porosa and P. wagenaari had the highest average photosynthetic rates per cm²but the lowest average photosynthetic rates per Symbiodinium cell or chlorophyll a. With the exception of Symbiodinium from E. tourneforti, isolated Symbiodinium did not photosynthesize at the same rate as Symbiodinium in hospite. Differences in Symbiodinium photosynthetic performance could not be attributed to Symbiodinium type. All P. anceps (n = 9) and P. wagenaari (n = 6) colonies, in addition to one E. tourneforti and three P. porosa colonies, associated with Symbiodinium type B1. The B1 Symbiodinium from these four gorgonian species did not cluster with lineages of B1 Symbiodinium from scleractinian corals. The remaining eight E. tourneforti colonies harbored Symbiodinium type B1L, while six P. porosa colonies harbored type B1i. Understanding the symbioses between gorgonian corals and Symbiodinium will aid in deciphering why gorgonian corals dominate many Caribbean reefs.