Effect of light and feeding on the fatty acid and sterol composition of zooxanthellae and host tissue isolated from the scleractinian coral Turbinaria reniformis

The fatty acid and sterol compositions of zooxanthellae and animal fractions of the scleractinian coral Turbinaria reniformis were investigated under different light and feeding conditions, to study the symbiont-host exchanges. Nubbins were maintained during 6 weeks under two light levels (100 mmol photons m22 s21 and 300 mmol photons m22 s21) and two feeding levels (starved and fed with zooplankton) in a factorial experiment. There were greater proportions of some polyunsaturated fatty acids (PUFA; e.g., C18:4 n-3, C20:5 n-3, C22:6 n-3) in the zooxanthellae than in the host, suggesting that these PUFA were synthesized by the algae and transferred to the animal. Conversely, C20:4 n-6 exhibited a greater proportion in the host and might have been synthesized by the animal. Light affected the chlorophyll content, the rates of photosynthesis, and the lipid production of all coral samples. Corals maintained in high-light conditions had lower relative phytol content but higher concentrations of fatty acids (FA) and sterols than the shaded corals. Feeding also affected coral metabolism, but differently according to the light level and despite the fact that the host did not directly incorporate the zooplankton lipids (PUFA and cholesterol). In low light, feeding resulted in an increase of growth rates and storage lipid concentrations, mainly saturated fatty acids (SAFA) and membrane constituents (PUFA and sterols). In high light, the lipid energy from the food was directed toward an increase in calcification, as well as in chlorophyll content and protein content. This study highlights the importance of feeding in sustaining coral metabolism, especially when light, or stress events, is limiting photosynthesis. Corals can meet their energetic requirements either via autotrophy through their symbiotic association with dinoflagellates called zooxanthellae (Muscatine et al. 1981) or via heterotrophy, i.e., capture of zooplankton and particulate organic matter (Goreau et al. 1971; Sebens et al. 1996). Zooxanthellae are known to transfer more than 90% of their photosynthates to their host (Muscatine et al. 1981), explaining the exceptional development of corals in oligotrophic environments. Corals are also heterotrophs that are able to catch large amounts of zooplankton (Goreau et al. 1971; Sebens et al. 1996; Yahel et al. 2005) as well as dissolved and particulate organic matter (Anthony and Fabricius 2000). Heterotrophy was proved to significantly enhance the zooxanthellae density, chlorophyll content, as well as the rates of growth and photosynthesis (Anthony and Fabricius 2000; Ferrier-Pagès et al. 2003; Houlbrèque et al. 2003). Both autotrophy and heterotrophy supply corals with major compounds, such as glycerol and other lipids (Crossland et al. 1980; Grottoli et al. 2006), which play an essential role in coral metabolism at all levels. Lipids are important energy reserves, mainly stored in the animal tissue as wax esters and triglycerides (Muscatine and Cernichiari 1969; Oku et al. 2002; Grottoli et al. 2004), or in the membranes as sterols and polyunsaturated fatty acids (PUFA; Tchernov et al. 2004). These reserves are, for example, used in the reproduction process (Ward 1995) or are oxidized to generate energy for survival during bleaching events (Yamashiro et al. 2005; Grottoli et al. 2006; Rodrigues and Grottoli 2007). Lipids are also respired to support metabolic needs or excreted as mucus (Crossland et al. 1980). Since lipid composition is often specific to particular groups of organisms (Volkman et al. 1989), the analysis of lipids, such as fatty acids (FA) and sterols, gives useful information about their autotrophic or heterotrophic origin (Napolitano et al. 1997). In the case of FA, animals cannot insert double bonds beyond the D9 position, and they cannot synthesize 18:2 n-6 and 18:3 n-3 (Bachok et al. 2006). These FA are essential for animals to further synthesize the n-6 and n-3 PUFA (Lehninger 1985; Papina et al. 2003). As a consequence, corals acquire PUFA through external diet (Meyers 1979; Al-Moghrabi et al. 1995) or through zooxanthellae photosynthates (Papina et al. 2003; Zhukova and Titlyanov 2003). Similarly, analysis of sterols may allow different food regimes to be distinguished because zooplankton contains large quantity of cholesterol, while algae mainly contain phytosterols (Volkman 1986). Many studies have revealed information on autotrophic FA production, by analyzing lipids in cultured and freshly isolated zooxanthellae (Al-Moghrabi et al. 1995; Zhukova and Titlyanov 2006) or in bulk symbiont samples (Yama2 Corresponding author ([email protected]).