Cnidarian-Dinoflagellate Symbiosis Lipidomics Figure 4. See Figure 2 legend. doi:10.1371/journal.pone.0046801.g004 (24.91% 6 Std Dev 2.06; f = 34.7, P,0.05). However, both of these uncommon FA displayed a similar but unique multinomial isotopologue distribution compared to the other FAs. Transfer of methods between model systems. The transferability of the isotopic incorporation method to track the source of carbon to FA synthesis was shown to function in other model systems by the incorporation profiles of DHA (C22:6) for the symbiotic coral, Acropora millepora (10.47% 6 Std Dev 2.08; n = 3) (Figure 6A) and collated cultured clade B dinoflagellates (10.73%) (Figure 6B). The incorporation was below that of the symbiotic A. pulchella for the same time point (21.42% 6 Std Dev 6.59) (Figure 5B). Discussion Fatty acids are synthesized from Acetyl CoA by the action of elongase enzymes that add 2 units of carbon at a time to form a carbon chain. A previously synthesised fatty acid of one type, for example, palmitic acid (C16:0, 16 carbons and no double bonds), may be used as a foundation for the synthesis of the next product in a lipogenesis pathway, such as stearic acid (C18:0, 18 carbons and no double bonds). Fatty acid (FA) chains may be reduced in carbon or altered further by a change in the number of double bonds by the action of other enzymes such as desaturases. For example, the monosaturated FA palmiteolic acid C 16:1, n–7 is synthesized from palmitic acid C16:0 by the insertion of one double bond seven carbon atoms (n–7) from the terminal methyl group. This study applied a new method for tracking assimilation of a known carbon source into a variety of FAs. The method revealed intermediate products and their specific rates of synthesis within the cnidarian-dinoflagellate symbiosis that could be mapped into known FA lipogenesis pathways. The differential rates of known FAs synthesis generally aligned with the known lipogenesis pathways, and indicated candidate FAs, with potentially important functions within the symbiosis, in addition to transitory products for downstream metabolites. The potential lipogenesis of FAs and of unusual size and previously identified in other studies, such as the long-chain fatty acids (LCFA), OHA and OOA are discussed in more detail later. The findings of this study were unexpected in that the dinoflagellate symbiont, Symbiodinium sp. displayed a complex integration of both n–3 and n–6 LCFA pathways and not just n–3 ‘‘plant-like’’ pathways that were active throughout lipogenesis process (Figure 7). In addition, there was a surprising lack of evidence of host utilization of symbiont-derived FAs over a 120 hr incubation period, highlighted by the absence of additional 13C incorporation in late n–6 pathway lipogenesis. This was surprising in that it contravenes previous studies using 14C and an assumed automated translocation and utilization of photoassimi- lated metabolites in a mutualistic relationship regarding FA lipogenesis within the host [36,37,38,39]. However, as mentioned in the introduction, although the influence of temporal variability on metabolite production and potential translocation should not be ignored [23] it equally needs to be put into perspective that different metabolic and cell division rates occur in both host and PLOS ONE | www.plosone.org 8 October 2012 | Volume 7 | Issue 10 | e46801