Physiological effects of aluminum on rainbow trout in acidic soft water, with emphasis on the gill micro-environment
This thesis examined the physiological and toxicological effects of Al (-100 μg.Lˉ¹) in acidic soft water (pH 4.0-6.5) on the rainbow trout (Salmo gairdneri = Oncorhynchus mykiss), and the mechanisms of Al deposition on the gills. Cannulated trout exposed for 66 h to Al in synthetic soft water (Ca²⁺ = 45 or 410 μequiv.Lˉ¹) showed highest mortality at pH 5.2, intermediate mortality at pH 4.8, and least mortality at pH 4.4. Aluminum caused losses of Na⁺ and Clˉ from the plasma at pH 5.2 and 4.8, where there were no ion losses in the absence of Al. Respiratory disturbances (decreases in arterial O₂ tension, increases in CO₂ tension) occurred in fish exposed to Al at pH 5.2 and 4.8.
Experiments on trout fitted with ventilation masks and opercular catheters showed that acidic soft water is rendered more basic as it passes over fish gills, and basic inspired water is made more acidic. A model was generated in which these effects were adequately explained by measured ammonia, base, and CO₂ release at the gills.
Exposure to acidic soft water containing Al resulted in hyperventilation during both short (2-3 h) and longer term (44 h) exposures. Alkalinization of the gill micro-environment was modified but not abolished in the presence of Al. This alkalinization was large enough to exceed the theoretical solubility limit for Al, so that Al precipitation from solution onto the gills would occur. The alkalinization also altered the theoretical speciation of Al near the gills, resulting in shifts in the direction Al³⁺ → Al(OH)²⁺ → Al(OH)₂⁺ → Al(OH)₃°.
Aluminum extraction at the gills, and Al accumulation on the gills, was greatest at higher inspired and expired pH, in agreement with an Al precipitation explanation of Al deposition on fish gills. Aluminum precipitation experiments indicated that precipitation of Al from solution was fast enough to occur during the short (<2 >s) residence time of water at the gills. Measured accumulation on trout gills was about 10% of the deposition calculated from measurements of ventilation volume and Al extraction; the difference was explained as extensive sloughing of Al from the gills.
A model of Al interactions at fish gills is presented, in which precipitation of Al onto the gills causes the respiratory effects of Al. A smaller proportion of Al deposition is proposed to be positively charged Al species interacting with the negatively charged gill surfaces, causing the ionoregulatory effects of Al.