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Carbon isotopic analysis of the free fatty acids in a tridacnid―algal symbiosis: interpretation and implications for the symbiotic association

The transfer of photosynthates from zooxanthellae symbiotic with tridacnid clams contributes to the clam’s survival. We used compound-specific isotope analysis to examine the contribution that zooxanthellar lipids make to the fatty acid pool of the host. Bulk tissue δ13C values for zooxanthellae from two Tridacna gigas do not appear to exhibit typical values expected for algae utilizing a sole C3 fixation pathway. Rather they are indicative of reduced isotope discrimination levels due to the high photosynthetic rates which result in depletion of the inorganic carbon source. Fatty acids extracted from zooxanthellae are only minimally 13C-depleted or slightly 13C-enriched when compared to bulk tissue. We interpret this as evidence for the synthesis of acids from acetate, short-cutting the usual pathway via pyruvate. Palmitic and 16 : 1 ω7 acids in host tissues had similar δ13C values to their respective zooxanthellae, suggesting direct translocation of these acids to the host. Other fatty acids in the host were 13C-depleted when compared to the corresponding acids in the zooxanthellae, indicating multiple sources for these acids.