Acid-base homoestasis in aquatic animals exposed to natural and perturbed environments.

Keeping an appropriate acid-base state in the various body compartments of animals is of prime importance for many basic living processes. What is preserved is not a constant pH value but rather a constant relationship between pH and body temperature, which tends to stabilize the protein electrical charge and, more generally, conformation and function of macromolecules. Acid-base homeostasis requires a balance between metabolic production and controlled excretion of two classes of acids or bases : the volatile carbonic acid whose elimination depends on respiratory regulations ; and fixed acids and bases, usually excreted in 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 particular ions in the environment. The effects of each of these factors in isolation have been well studied in laboratory conditions, but integrated responses to the changes of many factors as it occurs in the natural setting are less well known. Variations of ambient or internal CO2 are 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 their excretion with gill ionic exchanges. Such excretory processes also 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 and ionoregulatory mechanisms can also considerably disturb acid-base balance in aquatic animals. Such disturbances may serve as very sensitive tests of sublethal toxicity.