Physiological Chemistry of Pseudopterosin Biosynthesis in Symbiotic Dinoflagellates from the Soft Coral Pseudopterogorgia elisabethae

The pseudopterosins (Ps) are unique diterpene glycosides with potent anti­ inflammatory properties which were isolated from the gorgonian Pseudopterogorgia elisabethae. The relationship of the anti-inflammatory effects of Ps to their endogenous function in the coral-symbiont association is not known. In this study the biosynthetic origins and the potential endogenous physiological function of the Ps within the coral-symbiont complex are investigated.Significant levels of endogenous Ps A, B, C and D were identified within the dinoflagellate symbiont, Symbiodinium sp, isolated from P. elisabethae. Biosynthetic studies using tritiated geranylgeranyl bisphosphate, yielded radiochemically pure Ps A, 8, C and D and the first committed intermediate, elisabethatriene. The photosynthetic production of organic carbon in Symbiodinium sp. was traced with the aid of radiolabelled 14C02 uptake studies. Results indicated that photosynthate is a significant carbon source for the production of the pseudopterosins and their intermediates. Ps constitute 10-15% of the intracellular lipids of Symbiodinium sp. and are biosynthesized at a low daily rate. This is the first report of Ps biosynthesis occurs within the algal symbiont of P. elisabethae, and thus prompted further study of the physiological implications of this biosynthesis.In comparative physiological studies of Symbiodinium sp. isolated from P. elisabethae and in the free-living dinoflagellate Heterocapsa pygmaea, physical sonic injury induced a significant oxidative burst of highly reactive oxygen species (ROS). Symbiodinium sp. cells from P. elisabethae had an attenuated oxidative burst in response to these injuries when compared to H. pygmaea and other related Symbiodinium species. Exogenously added Ps inhibited ROS release in a dose­ dependant manner in physically stressed cells of H. pygmaea and Symbiodinium sp. isolated from P. americana. The reductions in ROS by Ps were not due to direct anti­-oxidant effect indicating that they may provide a protective role to the cell membrane.In studies with gorgonian corals, hydrogen peroxide was identified as signal molecule in response to physical and heat stress. In these experiments P. elisabethae also exhibited a muted stress response when compared to related gorgonian corals. Overall this study demonstrates that the biosynthetic production of Ps in Symbiodinium sp. cells from P. elisabethae occurs in sufficient quantities to confer beneficial effects to both symbiont and host cells