Temperature is one of the most important environmental factors for the regulation of animal physiology. This is especially true for ectotherms, a group of animals that cannot regulate their body temperature. Indeed,ectothermic animals display body temperatures similar to their surroundings. The fact that the rate of metabolic reactions doubles for a 10°C increase in temperature illustrates the profound impact of temperature on ectotherms'lives. Mitochondria are the powerhouses of the cell, and the effect of temperature on mitochondrial metabolism in ectotherms has been a subject of intense research for many years. Indeed, studies need to be carried out on many different species such that general principles can emerge. One fundamental question regards the comparison of changes in mitochondrial function that occur during seasonal temperature changes versus those occurring due to adaptation to life in different thermal habitats. Angela Sommer and Hans Otto Pörtner took up that challenge and examined mitochondrial functions during seasonal acclimatization in the lugworm, as well as mitochondrial characteristics during latitudinal adaptation, by comparing subpolar lugworms of the White Sea close to the Arctic ocean with boreal specimens from the North Sea.
Mitochondrial respiration is often increased in ectotherms exposed to cold temperature. This is thought to occur in order to maintain the level of metabolic activity present at warmer temperatures. Indeed, if there were no increase in respiration upon cold exposure, the effect of temperature on biochemical reactions would be such that oxygen consumption would be reduced compared with that at warmer temperatures. And the team's results supported this idea; mitochondria from both cold-acclimatized lugworms and subpolar specimens displayed increased oxygen consumption rate.
In order to determine which mitochondrial reactions caused the elevated oxygen consumption rate, the authors examined two important states of mitochondrial respiration: state 3 and state 4. State 3 represents the maximal oxygen consumption rate associated with ATP production, whereas state 4 represents basal respiration associated with maintenance costs. The ratio between state 3 and state 4 respiration indicates the coupling of respiration and mitochondrial metabolic efficiency. The team found that increased mitochondrial respiration in cold-acclimated lugworms as well as in subpolar animals was reflected in an elevation of both respiration states. However,different changes in mitochondrial properties occurred during latitudinal adaptation and seasonal acclimatization. During latitudinal adaptation, the increase in state 3 respiration was accompanied by a slightly larger increase in state 4 respiration, leading to a reduction in mitochondrial metabolic efficiency. In other words, the elevated capacity to produce ATP during latitudinal adaptation was paralleled by increased maintenance costs. However,during seasonal acclimatization, state 3 respiration increased more than state 4 respiration, resulting in elevated mitochondrial metabolic efficiency and lowered maintenance costs.
The different responses of mitochondrial metabolism to the two cold challenges examined in the present study illustrate nicely the plasticity of mitochondrial functions. The next fundamental question emerging in the field of temperature physiology is: how do mitochondria integrate all external stimuli into a finely balanced metabolic program?