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.
Highlights
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
There was no additional 13C incorporation in FAs from either DI13C ASW treated aposymbiotic anemones, or nonenriched dissolved inorganic carbon (DIC) ASW treated symbiotic anemones as highlighted by the absence of additional isotopologues of docosahexaenoic acid (DHA C22:6: m/z 327) (Figure 1A and B) or FA lipogenesis profiles over time (Figure 2, 3, 4, and 5)
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
Summary
In particular fatty acids (FAs), are essential to cell and metabolic function and are typically associated with energy storage and structural fluidity of membranes. 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. 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]. The rates of specific metabolite synthesis resulting from translocation were not provided in these studies
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