A Lipidomic Approach to Understanding Free Fatty Acid Lipogenesis Derived from Dissolved Inorganic Carbon within Cnidarian-Dinoflagellate Symbiosis Simon R. Dunn 1 * ¤ , Michael C. Thomas2¤, Geoffrey W. Nette2, Sophie G. Dove1¤ 1 ARC Centre of Excellence for Coral Reef Studies, Global Change Institute, University of Queensland, St. Lucia, Brisbane, Queensland, Australia, 2 Independent Marine Biochemical Research, Moreton Bay Research Station, Dunwich, Queensland, Australia Abstract The cnidarian-dinoflagellate symbiosis is arguably one of the most important within the marine environment in that it is integral to the formation of coral reefs. However, the regulatory processes that perpetuate this symbiosis remain unresolved. It is essential to understand these processes, if we are to elucidate the mechanisms that support growth and resource accumulation by coral host, and conversely, recently observed reduction and/or mortality of corals in response to rapid environmental change. This study specifically focused on one area of metabolic activity within the symbiosis, that of free fatty acid synthesis within both the dinoflagellate symbionts and cnidarian host. The main model system used was Aiptasia pulchella and Symbiodinium sp. in combination with aposymbiotic A. pulchella, the symbiotic coral Acropora millepora system and dinoflagellate culture. Fatty acids (FAs) were selected because of their multiple essential roles inclusive of energy storage (resource accumulation), membrane structure fluidity and cell signaling. The study addressed free FA lipogenesis by using a new method of enriched stable isotopic (13C) incorporation from dissolved inorganic carbon (DI13C) combined with HPLC-MS. FAs derived from DI13C aligned with a mixture of known lipogenesis pathways with the addition of some unusual FAs. After 120 hr, 13C-enriched FA synthesis rates were attributed to only a complex integration of both n– 3 and n–6 lipogenesis pathways within the dinoflagellate symbionts. Furthermore, there was no detectible evidence of symbiont derived enriched isotope fatty acids, catabolized 13C derivatives or DI13C being directly utilized, in host late n–6 pathway long-chain FA lipogenesis. These findings do not align with a popular mutualistic translocation model with respect to the use of translocated symbiont photoassimilates in host long-chain FA lipogenesis, which has important connotations for linking nutrient sources with metabolite production and the dynamic regulation of this symbiosis. Citation: Dunn SR, Thomas MC, Nette GW, Dove SG (2012) A Lipidomic Approach to Understanding Free Fatty Acid Lipogenesis Derived from Dissolved Inorganic Carbon within Cnidarian-Dinoflagellate Symbiosis. PLoS ONE 7(10): e46801. doi:10.1371/journal.pone.0046801 Editor: Christian R. Voolstra, King Abdullah University of Science and Technology, Saudi Arabia Received June 13, 2011; Accepted September 10, 2012; Published October 24, 2012 Copyright: 2012 Dunn et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This study was supported by the Australian Research Council through the COE 561435 and LP110100566 grant funding. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: s.dunn2@uq.edu.au ¤ Current address: ARC Centre of Excellence for Coral Reef Studies, School of Biological Sciences, University of Queensland, St. Lucia, Brisbane, Queensland, Australia Introduction Lipids, in particular fatty acids (FAs), are essential to cell and metabolic function and are typically associated with energy storage and structural fluidity of membranes. In addition, FAs form multiple complex compounds, which have important roles in cell signaling processes [1,2]. FAs have a carbon based ‘backbone’ chain to their molecular structure, which may be elongated, shortened or altered by either elongase or desaturase enzymic activity respectively. As FA synthesis proceeds along a lipogenesis pathway, the longer a FA carbon chain will be. The size of the FA structure is also dependent upon the addition or removal of double hydrogen bonds and catabolic activity. The complement of lipogenesis pathways and associated enzymes, vary between animals (n (or v)-6 pathway), plants (n (or v)-3 pathway) and algae (both n–3 and n–6 pathways) resulting in a differential capacity to produce FAs within different organisms. The absence of certain functional lipogenesis pathways in animals can result in FA deficiency and affect health. However, an inability to synthesize FAs that are essential to animals, such as linoleic acid and docosahexaenoic acid (DHA), can be resolved by obtaining them from dietary sources [3]. For example, late lipogenesis pathway n(v)-3 FAs including ‘essential’ FAs, such as DHA, are synthesized by marine algae, but animals with the n(v)-6 lipogenesis pathway generally do not have the complement of enzymes to produce endogenous DHA [2]. Typically, animals acquire DHA through a food chain linked to marine algae or plants with the n–3 pathway. An alternative form of sequestering metabolites including lipids is through a symbiotic interaction with organisms that are able to synthesize the essential metabolites. In this case, metabolites including essential products are translocated and utilized by one or both partner organisms [4]. Specifically, FA translocation has been implied or reported to occur from the symbiotic dinoflagellates into a variety of hosts such as, the giant clams, tridacnids [5], and cnidarians including anthozoans [6] and scyphozoans [7]. However, the rates of specific metabolite synthesis resulting from translocation were not provided in these studies. PLOS ONE | www.plosone.org 1 October 2012 | Volume 7 | Issue 10 | e46801