Life in the Open Ocean. Joseph J. Torres
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for metabolic rates, it is imperative to also provide the temperature range over which the measurements were taken.
The fact that metabolic rate doubles with a temperature change of 10 °C (or halves with a 10 °C drop) has a profound effect on ectothermic species. A vertical migrator in the tropical ocean swimming from a depth of 500 m to near surface waters at sunset (a common occurrence, as we shall see) will encounter changes of >10 °C on its way up and again on its way back to depth. As a consequence, it will endure profound changes in its metabolic rate during each leg of its excursion.
In 1914, August Krogh, the father of comparative physiology, first attempted to define a pattern for the change in Q10 with temperature by subjecting a narcotized goldfish to temperatures ranging between 0 and 25 °C and measuring its oxygen consumption rate. The curve he derived is called the “Normal Curve.” It was popularized considerably in later years when it was found that a similar relationship between metabolism and temperature existed for many species, with the exception of the large Q10 in the 0–5 °C temperature interval (e.g. Winberg 1956). Even Krogh stated that his Q10 value of 10.9 between 0 and 5 was “obviously erroneous.” The general trend was remarkably accurate though, as were the numbers generated above 5 °C. The Q10’s this curve represents is shown below.
The Q10 approximation is a fundamental molecular response to temperature: it applies to chemical reactions taking place in a beaker as well as to rate processes in ectothermic species. However, it is not intuitively obvious why reaction rates should double for every increase in a temperature of 10 °C. The answer is in a concept termed “activation energy,” which was pioneered by the Swedish physical chemist Svante Arrhenius, and which earned him a Nobel Prize in 1903.
In the realm of physical chemistry, temperatures are expressed in the absolute temperature scale, in degrees Kelvin. A Kelvin degree is equal to a degree Celsius, but the scale begins at absolute zero, the temperature where all molecular motion ceases: −273 °C. Thus, a temperature of 0 °C is equal to 273 K, and 20 °C is equal to 293 K; by convention, the degree symbol is not used for degrees Kelvin. The temperature range most relevant to the pelagic fauna, −2 to 40 °C (271–313 K), only covers about a 10% change of temperature on the absolute scale. In our range of concern, a change of 10 °C is roughly 3% of the absolute temperature. Why, then, do reaction rates double?
The breakthrough of Arrhenius was his idea that within a population of molecules, only a fraction have sufficient energy to be reactive: those that exceed the activation energy threshold (Figure 2.3). The average thermal energy of the molecules gives us the temperature, but it is not the average that is most important, it is the proportion of molecules that have enough energy to exceed the activation energy threshold and be competent to react. When heat is added to a system, the proportion of molecules that exceeds the activation energy increases more quickly than the average temperature. In fact, an increase in temperature of 10 K results in a doubling of the fraction of molecules exceeding the activation energy. The activation energy concept thus explains the Q10s we observe with biological rates.
Experimentation in the 1940s, 1950s, and 1960s further defined temperature responses as a function of time and acclimation period. Three general time courses were identified.
1 Direct responses of rate functions to changes in temperature persisting for hours: acute measurements
Figure 2.3 Energy distribution curves for a population of molecules at two different temperatures. Only those molecules having energies equal to or greater than the activation energy are reactive.
Source: Hochachka and Somero (2002), figure 7.1 (p. 296). Reproduced with the permission of Oxford University Press.
Rates measured in this way, with no acclimation period, reflect the short‐term flexibility of biological systems. In some cases, acutely measured responses to temperature can show Q10 values greatly different from 2. When metabolism is the rate being measured, such a response is termed “metabolic overshoot.” It would correspond to a “type 5” response described below. It is the result of a system that is still in the process of adjusting to a new temperature and it can happen in a transition to a warmer or colder temperature.
1 Compensatory acclimation to days or weeks of exposure: the acclimated response
An animal is acclimated only when its rate processes have stabilized to the new temperature. Acclimated animals were utilized in constructing the temperature tolerance polygon shown in Figure 2.2a.
1 Evolutionary adaptation through natural selection: climatic adaptation
Patterns of Thermal Acclimation
Animals moved to a new temperature and kept there for several days often show some compensation in their rate functions, especially metabolic rate. One of the classic treatments of how individuals adjust to a new temperature is that of Precht (1958) who identified five responses (Figure 2.4). Animals were maintained initially at temperature T2 and moved to T1 or T3. Metabolic rates were monitored during the acclimation process.
Type 4 acclimation: no change in rate occurs after time for acclimation (Q10 = 2–3).
Type 2 acclimation: the animal’s rate falls or rises to original rate (Q10 = 1)
Type 3 acclimation: somewhere in between (Q10 = 1–2)
Type 1 acclimation: overcompensation – rate lower at higher temperature (Q10 < 0)
Type 5 acclimation: reverse compensation (Q10 > 3)
Figure 2.4 Precht’s patterns of temperature acclimation. Animals are acclimated at temperature T2 then transferred to a lower temperature (T1) or higher temperature (T3). The five acclimation types are described in the text.
Source: Prosser (1973), figure 9‐13 (p. 375). Reproduced with the permission of Saunders Publishing.
In reality, all five curves are rarely seen. In most cases, the response is type 3 or 4. However, type 2 has been seen in rare cases; it has been documented in some intertidal animals (Newell and Northcroft 1967). The Precht patterns are the simplest way of classifying trends in metabolism vs. temperature, but other useful schemes exist. Prosser (1973) describes species’ response to temperature using a family of metabolism vs. temperature (M‐T) curves instead of Precht’s three points. The advantage of Prosser’s scheme is that it can differentiate varied responses to lowered and raised temperature, and the reader is encouraged to examine Prosser’s curves.
Climatic Adaptation in Ectotherms
Scholander et al. (1953) first described another important way that ectotherms achieve some degree of freedom from their environment: the phenomenon of metabolic cold adaptation or “MCA.” We may think of the changes in lethal limits that