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The sol–gel state of lipids, or their fluidity, has the potential to be profoundly altered by pressure, as it is with temperature. In fact, high pressure and low temperature have similar effects on membrane lipids: both tend to make them more crystalline, i.e. less fluid (Hazel and Williams 1990). Solutions to the problems posed by the ordering effects of hydrostatic pressure and low temperature are solved in a similar manner. In both cases, membrane lipids increase the incidence of double bonds, or their “kinkiness,” to increase fluidity.

Evidence supporting the contention that the membranes of deep‐sea species are more fluid than those of their shallower dwelling counterparts is more sparse than would be ideal (cf. Hazel and Williams 1990), but it is present, nonetheless. In a benchmark publication from 1984, Cossins and MacDonald found that membrane lipids isolated from the brains of a suite of fishes dwelling between 200 and 4800 m showed significant increases in fluidity with depth consistent with homeoviscous adaptation. Evidence was not conclusive for lipids isolated from other organs, notably liver and kidney, due largely to variability between samples, but trends were similar.

Further evidence supporting membrane adaptation to pressure comes from study of membrane‐bound enzymes, notably the ion‐pumping enzyme Na‐K ATPase, an important player in the osmoregulation of fishes. Gibbs and Somero (1989, 1990) first tested the pressure sensitivity of enzymes from fishes dwelling in a variety of habitats: shallow‐warm, shallow‐cold, hydrothermal vent (deep‐warm), and deep cold (Figure 2.21). As might be expected, they found that the order of pressure sensitivity (highest to lowest) was shallow‐warm, shallow‐cold, deep‐warm, and deep‐cold. The investigators then manipulated the membrane environment of the Na⁺ K⁺ ATPase to assess the influence of the membrane fraction on enzyme activity. They found that enzyme from a warm‐shallow species (the barracuda Sphyraena) placed in a membrane environment derived from the cold deep‐dwelling fish Coryphaenoides armatus was less pressure‐sensitive than the enzyme of a cold‐shallow species (the sablefish Anoplopoma fimbria) that was introduced into a highly ordered lipid environment derived from chicken eggs. In contrast, when enzymes from the three species were introduced into the same lipid environment, one derived from the deep‐cold species, Coryphaenoides armatus, the order of pressure sensitivity remained as stated above: warm‐shallow, cold‐shallow, and cold‐deep. The results of Gibbs and Somero provide evidence that membrane fluidity influences the function of membrane‐bound enzymes and also that changes in enzyme structure play an important role in adaptation to pressure.

Figure 2.21 Effects of lipid substitution on the pressure responses of Na⁺, K⁺‐ATPase of three species of fish from different depth‐temperature habitats. Native membrane lipids were removed and replaced with (a) chicken egg phosphatidylcholine or (b) phospholipids prepared from the gill of the abyssal grenadier Coryphaenoides armatus. Filled symbols indicate pressure responses before lipid substitution; open symbols were assays after substitution. Circle symbols represent Coryphaenoides armatus (deep sea, cold habitat); square symbols represent the sablefish Anoplopoma fimbria (shallow, cold habitat); triangles represent the barracuda Sphyraena barracuda (shallow, warm habitat).

Source: Gibbs (1997), figure 6 (p. 257). Reproduced with the permission of Academic Press.