CARBON ISOTOPIC ANALYSIS OF THE FREE FATTY-ACIDS IN A TRIDACNID ALGALSYMBIOSIS - INTERPRETATION AND IMPLICATIONS FOR THE SYMBIOTIC ASSOCIATION

The transfer of photosynthates from zooxanthellae symbiotic with trida cnid clams contributes to the clam's survival. We used compound-specif ic isotope analysis to examine the contribution that zooxanthellar lip ids make to the fatty acid pool of the host. Bulk tissue delta(13)C va lues for zooxanthellae from two Tridacna gigas do not appear to exhibi t typical values expected for algae utilizing a sole C3 fixation pathw ay. Rather they are indicative of reduced isotope discrimination level s due to the high photosynthetic rates which result in depletion of th e inorganic carbon source. Fatty acids extracted from zooxanthellae ar e only minimally C-13-depleted or slightly C-13-enriched when compared to bulk tissue. We interpret this as evidence for the synthesis of ac ids from acetate, short-cutting the usual pathway via pyruvate. Palmit ic and 16:1 omega 7 acids in host tissues had similar delta(13)C value s to their respective zooxanthellae, suggesting direct translocation o f these acids to the host. Other fatty acids in the host were C-13-dep leted when compared to the corresponding acids in the zooxanthellae, i ndicating multiple sources for these acids.