Life in the Open Ocean. Joseph J. Torres
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in a species’ biochemical machinery that allows it to cope with seasonal change. MCA is a fundamental characteristic of a species’ biochemical power plant that allows it to survive in its particular climatic regime. Those adapted characteristics are achieved over the course of evolutionary time. The phenomenon of MCA simply stated is this: “When non cold‐adapted ectotherms are introduced to a given low temperature and allowed to acclimate, their metabolism tends to stabilize at a level below that of a species normally adapted to the low temperature” (Wohlschlag 1960). That is, cold‐adapted fishes and other cold‐adapted taxa have higher metabolic rates at low temperature than would their acclimated tropical counterparts (Figure 2.5). A helpful mental exercise is to extrapolate the metabolism vs. temperature (M‐T) curve for temperate fishes shown in Figure 2.5 to a temperature of 0 °C. Note that it is well below that of the M‐T curves of the polar species.
The rates in this classical figure are similar in the different zoogeographic locations. MCA is very important zoogeographically; it implies an advantage to elevated metabolic rates in cold‐adapted species and marked similarity of metabolic rates over the zoogeographic range of fishes (and other taxa). A great deal of thought and experimentation has gone into understanding MCA because it is of paramount importance to ectotherms.
Why should you care about temperature adaptation? The metabolic machinery of ectotherms has evolved to allow metabolic processes to proceed at similar rates over a widely disparate range of temperatures, ameliorating the tyranny of the Arrhenius concept. Temperature adaptation may be thought of as a biological intervention that allows life to proceed apace over the entire range of temperatures found in the open ocean. It requires modifications of the enzyme systems that underlie all metabolic processes. It is most important to embrace the fact that modifications take place in the long term, on an evolutionary time scale, between species from different regions and in the shorter term (seasonal time scale) within a species as described with the temperature polygon (Figure 2.2a).
Figure 2.5 Schematic representation of the relation between temperature and standard metabolic rates (log scale) of fish from different climatic zones. Dotted lines indicate the general range of variability within each zone. Adaptive metabolic compensation is shown for the different climatic regimes.
Source: Brett and Groves (1979), figure 1 (p. 292) with the permission of Academic Press.
Clearly, temperature not only sets boundaries for survival but also governs rate processes within those boundaries. The rate processes are, in turn, governed by enzyme systems: the biological catalysts that make life possible. Alterations in the quality or quantity of enzymes underlie much of the process of temperature adaptation. Thus, it is important to examine temperature adaptation in more depth.
In a comparison of enzymic activities in fishes from different climatic regimes, Kawall et al. (2002) utilized the activities of two important intermediary metabolic enzymes: lactate dehydrogenase (LDH), the terminal enzyme in anaerobic glycolysis, and citrate synthase (CS), the rate‐limiting step in the Kreb’s Cycle aerobic pathway. To keep the comparison meaningful among fishes of different lifestyles and locomotory abilities, the activity of enzymes within the brains of the fishes was compared at a common temperature (10 °C) instead of using enzyme activity in skeletal muscle. Those values were then extrapolated to habitat temperature (0 and 25 °C for polar and subtropical species, respectively) using Q10 values determined during the study (Figures 2.6 and 2.7). For both enzymes, the activity of the cold‐adapted Antarctic species was much higher than that of the subtropical species, showing a considerable degree of temperature compensation in the polar species, and echoing that observed in the metabolic rate determinations of Scholander et al. (1953) shown in Figure 2.5. A careful consideration of enzyme activities at the habitat temperatures within each climatic regime shows that compensation is not complete, i.e. the Antarctic species have an activity at 0 °C that is about 50% of that of the subtropical species at 25 °C. Temperature compensation is not perfect, but it is substantial! If you take the activity at a temperature of 25 °C and apply a Q10 of 2 over the 25 °C range, at 0 °C, the rate would be <20% of that at 25 °C.
Figure 2.6 Lactate dehydrogenase (LDH) activities (international units gWM−1) in brains of fishes from Antarctic and tropical/subtropical climatic zones in relation to environmental temperature. Curves were generated using the mean LDH activities obtained at 10 °C and adjusted using the experimentally determined Q10 of 2.1. Enzyme activities at 0 °C, 10 °C, and 25 °C are denoted by filled circles for Antarctic fish and open circles for tropical species. Activities at the approximate habitat temperatures of the two groups are given numerically next to the relevant symbols.
Source: Kawall et al. (2002), figure 1 (p. 283). Reproduced with the permission of Springer‐Verlag.
Figure 2.7 Citrate synthase (CS) activities (international units g WM−1) in brains of fishes from Antarctic and tropical/subtropical climatic zones in relation to environmental temperature. Curves generated as in Figure 2.8.
Source: Kawall et al. (2002), figure 2 (p. 283). Reproduced with the permission of Springer‐Verlag.
Temperature Compensation via Changes in Enzyme Concentration: The Quantitative Strategy for Short‐term Change
The easiest way to effect a change in rate, as measured by the accumulation of a reaction product, is to alter the concentration of reactants. In the case of an enzymatic reaction, if we assume a constant concentration of substrate and increase the amount of enzyme, the product of the reaction will accumulate more rapidly: an increase in activity. Surprisingly, few studies quantitatively address this issue. Most studies simply assume that short‐term changes in enzyme activities are due to enzyme concentration changes. The one study, consistently cited, that does address changes in enzyme activity as a function of enzyme concentration is Sidell et al. (1973). Figure 2.8 shows the activity of cytochrome oxidase, an important enzyme in the electron transport system, from goldfish skeletal muscle in fishes first acclimated to 15 °C and then transferred to either 5 or 25 °C. The enzyme activity per milligram protein in both groups was then monitored for approximately 30 days. Activity was much higher in the fishes transferred to the colder temperature, suggesting that the concentration of enzyme was much higher in the cold‐adapted fish.
It should be noted that accelerating or decelerating the activity of an enzymic pathway through differences in enzyme concentrations is best as a short‐term solution or for small