The phase behavior and physical properties of lipids in biological membranes are exquisitely sensitive to changes in temperature (50). Because membranes (a) act as physical barriers to solute diffusion, (b) mediate the transmembrane movement of specific solutes, (c) regulate the utilization of energy stored in transmembrane ion gradients, Cd) provide an organizing matrix for the assem­ bly of multicomponent metabolic and signal transduction pathways, and (e) supply precursors for the generation of lipid-derived second messengers, tem­ perature-induced perturbations in membrane organization pose a serious chal­ lenge to the maintenance of physiological function in poikilotherms. However, poikilotherms exploit the diversity of lipid structure to fashion membranes with physical properties appropriate to their thermal circumstance and, in this way, restore membrane function following thermal challenge. Based on the finding that membrane lipids of Escherichia coli grown at 43 and 15°C dis­ played similar physical properties when compared at their respective growth temperatures, Sinensky concluded that membrane fluidity was defended as growth temperature changes and referred to this cellular homeostatic response as homeoviscous adaptation CHV A) (94). Since the original exposition of this hypothesis, HV A has emerged as the most commonly employed paradigm to assess the efficacy of thermal adaptation in biological membranes and to explain patterns of temperature-induced change in membrane lipid composi-

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Thermal adaptation in biological membranes: is homeoviscous adaptation the explanation?

Preface.- Introduction.- Algal Lipids and Effect of the Environment on their Biochemistry.- Formation and transfer of fatty acids in aquatic microbial food webs - role of heterotrophic protists.- Ecological significance of sterols in aquatic food webs.- Fatty acids and oxylipins as semiochemicals.- Integrating lipids and contaminants in aquatic ecology and ecotoxicology.- Crustacean zooplankton fatty acid composition.- Fatty acid ratios in freshwater fish, zooplankton and zoobenthos - are there specific optima?- Preliminary estimates of the export of omega-3 highly unsaturated fatty acids (EPA+DHA) from aquatic to terrestrial ecosystems.- Biosynthesis of polyunsaturated fatty acids in aquatic ecosystems: general pathways and new directions.- Health and condition in fish: the influence of lipids on membrane competency and immune response.- Lipids in marine copepods: latitudinal characteristics and perspective to global warming.- Lipids in marine copepods: latitudinal characteristics and perspective to global warming.- Tracing aquatic food webs using fatty acids: from qualitative indicators to quantitative determination.- Essential fatty acids in aquatic food webs.- Human life: caught in the food web.

Lipids in aquatic ecosystems

Lipid reserves in teleost eggs are stored in lipoprotein yolk and, in some species, a discrete oil globule. Lipoprotein yolk lipids are primarily polar lipids, especially phosphatidylcholine and phosphatidylethanolamine (PE), and are rich in (n−3) polyunsaturated fatty acids, especially 22:6(n−3) (docosahexaenoic acid, DHA). Oil consists of neutral lipids and is rich in monounsaturated fatty acids (MUFA). Egg lipids are derived from dietary fatty acid, fatty acid mobilized from reserves and possibly fatty acid synthesized de novo. There is selective incorporation of essential fatty acids, particularly DHA, into yolk lipids and discrimination against incorporation of 22:1(n−11). Lipid is delivered to the oocyte by vitellogenin, which is rich in polar lipids, and likely also by other lipoproteins, especially very low density lipoprotein, which is rich in triacylglycerol (TAG). All classes of lipid may be used as fuel during embryonic and larval development and MUFA are preferred fatty acids for catabolism by embryos. Catabolism of oil globules is frequently delayed until latter stages of development. In some species, DHA derived from hydrolysis of phospholipid may be conserved by transfer to the neutral lipid. Recent work has expanded knowledge of the role of DHA in membrane structure, especially in neural tissue, and molecular species analysis has indicated that PE containing sn-1 oleic acid is a prime contributor to membrane fluidity. The results of this type of study provide an explanation for the selection pressures that influence yolk lipid composition. Future work ought to expand knowledge of specific roles of individual fatty acids in embryos along with knowledge of the ecological physiology of ovarian recrudescence, environmental influences on vitellogenin and yolk lipid composition, and the control of yolk lipid accumulation and utilization.

Composition, accumulation and utilization of yolk lipids in teleost fish

Traditionally fisheries biologists have used various metrics to indicate the condition and, by implication, health of fish. These indices are usually based on relationships between length and weight (Anderson and Neumann 1996). Although such metrics can, under some circumstances, provide a quick estimate of a fish’s condition, their ability to shed light on the underlying cause-and-effect relationship(s) governing a fish’s health and nutritional status are limited. Biochemical measures (e.g. lipids including fatty acids (FA) and sterols, proteins and their constituent amino acids, and trace elements) offer complimentary measures to assess, in a more specific way, the condition and underlying health of fish. Fatty acids and other lipids affect the health of fish in many ways; including, but not limited to, their effects on growth, reproduction, behavior, vision, osmoregularity, membrane fluidity (thermal adaptation), and immune response. In this review, we focus on the latter two roles that lipids play in mediating the health and condition of fish.

Health and condition in fish: the influence of lipids on membrane competency and immune response

Lipids provide the densest form of energy in marine ecosystems. They are also a solvent and absorption carrier for organic contaminants and thus can be drivers of pollutant bioaccumulation. Among the lipids, certain essential fatty acids and sterols are considered to be important determinants of ecosystem health and stability. Fatty acids and sterols are also susceptible to oxidative damage leading to cytotoxicity and a decrease in membrane fluidity. The physical characteristics of biological membranes can be defended from the influence of changing temperature, pressure, or lipid peroxidation by altering the fatty acid and sterol composition of the lipid bilayer. Marine lipids are also a valuable tool to measure inputs, cycling, and loss of materials. Their heterogeneous nature makes them versatile biomarkers that are widely used in marine trophic studies, often with the help of multivariate statistics, to delineate carbon cycling and transfer of materials. Principal components analysis has a strong following as it permits data reduction and an objective interpretation of results, but several more sophisticated multivariate analyses which are more quantitative are emerging too. Integrating stable isotope and lipid data can facilitate the interpretation of both data sets and can provide a quantitative estimate of transfer across trophic levels.

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Lipids in Marine Ecosystems

The compositions and physical states of the liver phospholipids of marine and freshwater fish adapted to relatively constant but radically different temperatures were investigated. Fish adapted to low temperature (5-10 degrees C) accumulated more unsaturated fatty acids than those in a warm (25-27 degrees C) environment. There were no measurable differences in the gross fatty acid compositions of the total liver phospholipids from identical thermal environments. Docosahexaenoic acid (22:6) did not seem to participate in the process of adaptation. Cold adaptation was coincidental with oleic acid (18:1) accumulation, preferentially in the phosphatidylethanolamine. Determination of the molecular species composition of phosphatidylethanolamine revealed a 2- to 3-fold and 10-fold increase in the level of 18:1/22:6 and 18:1/20:5 species, respectively. ESR spectroscopy revealed a 7-10% compensation in the ordering state of native phospholipids with temperature. Combination of 16:0/22:6 phosphatidylcholine with phosphatidylethanolamines of cold-adapted marine fish showed a drastic fluidization near the C-2 segment of the bilayer, but not in the deeper regions. An appropriate combination (75:25) of phosphatidylcholines from warmth-adapted marine fish with phosphatidylethanolamines from cold-adapted marine fish mimicked a 100% adaptational efficacy in the C-2 segment as compared with the phosphatidylethanolamines of warmth-adapted marine fish. A specific role of 18:1/22:6 phosphatidylethanolamine in controlling membrane structure and physical state with thermal adaptation is proposed.

https://www.pnas.org/content/pnas/90/16/7498.full.pdf

T Wiik

Papers

Molecular and structural composition of phospholipid membranes in livers of marine and freshwater fish in relation to temperature.