ACID-BASED HOMEOSTASIS IN AQUATIC ANIMALS EXPOSED TO NATURAL AND PERTURBED ENVIRONMENTS

Keeping an appropriate acid-base state in the various body compartment s of animals is of prime importance for many basic living processes. W hat is preserved is not a constant pH value but rather a constant rela tionship between pH and body temperature, which tends to stabilize the protein electrical charge and, more generally, conformation and funct ion of macromolecules. Acid-base homeostasis requires a balance betwee n metabolic production and controlled excretion of two classes of acid s or bases : the volatile carbonic acid whose elimination depends on r espiratory regulations ; and fixed acids and bases, usually excreted i n association with ion exchanges. In aquatic animals, these functions are heavily challenged by large natural changes of respiratory gases, oxygen and carbon dioxide, as well as of total salinity or of particul ar ions in the environment. The effects of each of these factors in is olation have been well studied in laboratory conditions, but integrate d responses to the changes of many factors as it occurs in the natural setting are less well known. Variations of ambient or internal CO2 ar e not a strong stimulus to breathing in aquatic crustaceans and fishes , and respiratory compensations are thus of little importance in acid- base homeostasis. On the contrary, aquatic organisms are usually able to quickly get rid of large fixed acid or alkaline loads by coupling t heir excretion with gill ionic exchanges. Such excretory processes als o serve to compensate acid-base disturbances induced by changes of the respiratory qualities of the water. The well-known impact of various pollutants (heavy metals, ammonia, acid waters...) on gill structure a nd ionoregulatory mechanisms can also considerably disturb acid-base b alance in aquatic animals. Such disturbances may serve as very sensiti ve tests of sublethal toxicity.