Change in lipid composition in eastern oyster (Crassostrea virginica Gmelin) exposed to constant or fluctuating temperature regimes Fabrice Pernet a, ⁎ , Sophie Gauthier-Clerc a,b,c , Élise Mayrand b a Institut de Recherche sur les Zones Côtières, 232B rue de l'Église, Shippagan, Nouveau-Brunswick, Canada E8S 1J2 b Université de Moncton Campus de Shippagan, 218 boul. J.-D. Gauthier, Shippagan, Nouveau-Brunswick, Canada E8S 1P6 c Centre Aquacole Marin de Grande-Rivière, 6 rue du Parc, C.P. 340, Grande-Rivière, Québec, Canada G0C 1V0 Received 19 February 2007; received in revised form 15 March 2007; accepted 16 March 2007 Available online 28 March 2007 Abstract A temperature decrease usually induces an ordering effect in membrane phospholipids that can lead to membrane dysfunction. Ectotherms typically counteract this temperature effect by remodeling membrane lipids as stipulated in the homeoviscous adaptation theory (HVA). Previous studies mostly focused on the remodeling of membrane lipids during long-term acclimatization or acclimation at constant temperature regimes, whereas in nature, many organisms experience variations in temperature on a daily basis and must react to this changing thermal environment. The objective of this study was to examine the composition of membrane lipids in oysters subjected to long-term acclimation at constant temperatures (12 or 25 °C) or to environmentally realistic daily fluctuations in temperature between 12 and 25 °C for 7 d. The lipid composition of gill in oysters subjected to long-term acclimation at a constant temperature or to daily temperature fluctuations varied in a way consistent with HVA: oysters adjusted their phospholipid to sterol ratio in response to long-term acclimation to a constant temperature but not to daily temperature fluctuations. In contrast, the unsaturation index of polar lipids in oysters varied in response to both long-term acclimation to a constant temperature and to daily temperature fluctuations, mainly due to changes in 22:6n-3 and 20:5n-3. The 20:4n-6 levels in oyster gills increased as temperature rose, suggesting an increasing availability of this fatty acid for eicosanoid biosynthesis during stress responses. © 2007 Elsevier Inc. All rights reserved. Keywords: Acclimation; Fatty acid; Homeoviscous adaptation; Lipid class; Mollusk; Phospholipid; Temperature fluctuation 1. Introduction Because of its impact on all levels of biological organization, temperature is a crucial determinant of the biogeographical distribution and the performance of marine ectotherms. Temperature alters the velocity of chemical and enzymatic reactions, rates of diffusion, membrane fluidity, and protein structure (Hochachka and Somero, 2002). The thermal sensitivity of membrane processes is due to the strong effect of temperature on the physical properties of membrane lipids that in turn have a major influence on associated proteins. A decrease in temperature usually reduces membrane fluidity, which can lead to membrane dysfunction. Ectotherms usually counteract this temperature effect by remodeling membrane lipids, a phenomenon known as homeoviscous adaptation (HVA), via changes in phospholipid headgroups, fatty acid composition, and cholesterol content to compensate the effect of temperature on membrane structure (Sinensky, 1974; Hazel, 1995). Many intertidal organisms, which commonly withstand variations in temperature of 20–30 °C on a daily basis and encounter even wider thermal ranges on a seasonal basis, are able to regulate their membrane fluidity in response to temperature changes. For example, membrane fluidity in gill phospholipids of the sea scallop Placopecten magellanicus was shown to be negatively correlated with acclimation temperature and positively correlated with 20:5n-3, presumably helping to maintain membrane function at low temperatures (Hall et al., 2002). Furthermore, major lipid remodeling consistent with HVA has been observed in hard clams Mercenaria mercenaria (Pernet et al., 2006b) as well as blue mussels Mytilus edulis and eastern oysters Crassostrea virginica (Pernet unpublished data). However, all these studies focused on the remodeling of membrane lipids during long-term acclimatization or acclimation Comparative Biochemistry and Physiology, Part B 147 (2007) 557 – 565 www.elsevier.com/locate/cbpb ⁎ Corresponding author. Tel.: +1 506 336 6604; fax: +1 506 336 6601. E-mail address: fpernet@umcs.ca (F. Pernet). 1096-4959/$ - see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.cbpb.2007.03.009