Carbon transfer from zooxanthellae to the coral host comes in a diverse array of small molecules, such as glycerol, amino acids, glucose and organic acids. Corals rely on lipids as their main long term energy store and are commonplace in many cellular processes. The direct transfer of lipids from zooxanthellae to the host is not well described anywhere, with only a few studies hinting that zooxanthellae influence lipid ratios in their hosts.
Lipids are uniquely composed of fatty acids (FA), which have had their compositions profiled in some zooxanthellae-harbouring corals. However the research body on the FA compositions of zooxanthellae and their host is minimal and narrow.
Polyunsaturated fatty acids (PUFAs) are synthesised very differently between plants and animals. Only plants can insert double bonds beyond the ninth carbon and synthesise linoleic acid and alpha-linoleic acid. Zooxanthellae and their hosts are therefore expected to have distinct PUFA profiles, and specific marker FAs may be selected as indicators of lipid transfer within symbioses.
Gamma-linoleic acid, stearidonic acid and octadecapentaenoic acid are PUFAs characteristically synthesised by zooxanthellae and were used as the marker FAs for this study. FA markers for host lipid transfer to symbionts were not studied and whether hosts can influence zooxanthellae lipid synthesis remains unresolved.
Dinoflagellate FA synthesis can vary massively due to environmental conditions and between hosts, which has been attributed to differences in zooxanthellae strains in corals. Since this study found no connection between FA composition and clade classification in zooxanthellae, the data contradict this idea. The influence of the host on zooxanthellae lipid synthesis has been previously rejected; the symbionts of this study shared similar FA profiles and synthesis, so it suggests a modifying effect originating from the host.
Fatty acid compositions of zooxanthellae between some major coral taxa (Octocorallia, Hexcorallia, Hydrozoa) were found to be starkly distinct. Fatty acids closely related to those previously mentioned were identified as the most variable between host taxa. A handful of FAs were highlighted for further study as potential biomarkers for symbiont to host lipid transfer, and vice versa.
An important suggestion from this is the host influence of symbiont lipid synthesis. The mechanisms of this can only be speculated about; does the host provide the initial metabolite needed for certain PUFA syntheses? Or is it just signals from host which induce synthesis? And why is the synthesis of certain lipids controlled by input from either partner in the symbiosis; what functions do they serve that require their association with host-microbe interactions? This study never fully explains why they did not look at marker FAs to detect lipid transfer from the host to the symbiont; it would made their results more definite. I am also curious about the mechanisms of lipid transfer within symbioses, which are poorly understood as they are limited by knowledge of how lipids behave in cells/tissues. Does the host or symbiont simply leak excess lipids or are their specialised transfer routes for fatty acids?
Imbs, A. B., Yakovleva, I. M., Dautova, T. N., Bui, L. H., & Jones, P. (2014). Diversity of fatty acid composition of symbiotic dinoflagellates in corals: Evidence for the transfer of host PUFAs to the symbionts. Phytochemistry.