The effect of salinity and temperature on solubility of oxygen and respiratory rate in oxygen-dependent marine invertebrates

Abstract

The diffusion rate of O 2 in sea water has been treated physico-chemically. It is shown that although the activity of oxygen in water remains constant when pO 2 and temperature are constant (equilibrium conditions), the diffusion rate of oxygen will nevertheless vary proportionally to the solubility of oxygen in sea water, which decreases with increasing salinity. The effect on the diffusion rate is due to a salt effect on the activity coefficient, which also implies that Ficks law can not be directly used in systems of varying salinity. It is shown that the respiratory rate of the oxygen-dependent prosobranch, Buccinum undatum L., increases with decreasing salinity when the measurements are made as usual, namely, at constant pO 2. The respiratory rate is independent of the salinity, however, if the plot is based upon equal O 2 concentrations in water of different salinity. Physico-chemical considerations make it likely that the diffusion rate of O 2 in water is a rate-limiting factor for respiration in Buccinum. The literature shows, however, that further experimental evidence is needed before a general conclusion as to the quantitative rôle of the diffusion rate of oxygen in sea water on the respiratory rate can be reached. Since the solubility of O 2 in water also varies with temperature, corresponding effects regarding metabolism/temperature curves have been considered. It is shown theoretically that when the respiratory rate depends on the diffusion rate of O 2 in water, the increase of the respiratory rate with temperature is less when measured at constant pO 2 than at constant O 2 concentration. Measurements of the respiratory rate of Buccinum at different temperatures conform to this statement.