Water-breathing animals, including fish, are more
susceptible to a rise in environmental CO2 concentration
than terrestrial animals, because of the lower CO2
partial
pressures (PCO2
) of their body fluids [Ultsch and Jackson,
1996]. Ventilation of water-breathing animals is usually
governed by O2 stimuli because of the low O2
concentrations in water, even in equilibrium with air (O2
concentration in water is 1/30 of that in air at the same PO2
[Dejours, 1988]). This situation necessarily results in hyperventilation of fish in terms of CO2 excretion, because
CO2 solubility in water far exceeds O2 solubility. As a
result, thePCO2of fish body fluids is only a few torr, which
is one order lower than in air-breathing animals (for
example 40 torr in man). Thus elevations of CO2
(hypercapnia) in water will easily reverse the normal outward
diffusion of CO2 from the fish body. CO2
, as a small,
uncharged gaseous molecule, rapidly crosses cell membranes, and acidifies the body fluids through its rapid
conversion into carbonic acid catalyzed by a ubiquitous
enzyme, carbonic anhydrase. As almost all cellular processes
are highly dependent on pH, CO2 undoubtedly has farreaching impacts on living organisms.