Meeting Abstract
Octocorals remain one of the understudied groups of cnidarians. In addition to cell death or exiting the colony, colonial octocorals unlike anemones provide a third option for perturbed symbionts (Symbiodinium spp.)—migrating deeper into the colony. Cell death pathways typically initiate with the disruption of photosynthesis, shifting photosystem redox state in the direction of reduction and the formation of reactive oxygen species (ROS), which in turn trigger programmed cell death in symbiont and host cells (“canonical” bleaching pathway). Disrupting photosynthesis, however, has other consequences; specifically, a decrease in photosynthesis increases the available CO2 as bicarbonate (HCO3–). This can then activate ciliary action and symbiont migration via the soluble adenylyl cyclase (sAC)/cyclic adenosine monophosphate (cAMP) “non-canonical” pathway. Migrating symbionts may exit the colony or move deeper into the coenenchyme. The former contributes to bleaching, while the latter can result in symbiont retention and recovery and thus determines colony fate. In this context, several species of octocorals, Phenganax parrini, Sarcothelia sp., and Sympodium sp., have been adapted for microscopic experimentation. The effects of these pathways can be examined by thermal stress in the presence and absence of light. If light energy is present and photochemistry is blocked downstream of photosystem I and II, water may split but the resulting electrons will usually form ROS, leading to cell death. In the dark, photochemistry is abolished, and no light energy is available to provide electrons for ROS formation. Results from light/dark comparisons and experiments with exogenous HCO3– support the existence of non-canonical pathways.
