Possible mechanisms of aluminum toxicity in a dilute,acidic environment to fingerlings and older life stages of salmonids

The respiratory, acid-base, and ionoregulatory responses of juvenile rainbow trout (Salmo gairdneri) were monitored during exposure of the fish in the laboratory to inorganic Al (2.8 μM) over the pH range 4.0 to 6.5. Responses to Al were most severe at pH 6.1 and 4.5, mortality being primarily due to asphyxia at pH 6.1 and to electrolyte loss at pH 4.5. Competition between the H⁺-ion and Al for binding at the gill surface is offered as an explanation for the decreased toxicity of Al at pH 4.0, one which is compatible with the free-ion toxicity model that has been developed for other metals. The physiologically distinct response of S. gairdneri to Al at pH 6.1 is less amenable to unambiguous interpretation. If a mixed ligand hydroxo-Al complex is incorporated in the free-ion model, and if it is assumed that the two Al species, Al-L-gill] and HO-Al-L-gill], provoke distinct toxicological responses, then a bimodal toxicological response to Al is indeed predicted. An alternative explanation of the apparent toxic action of Al at pH 6.1, i.e., at pH values close to that of minimum Al solubility, is the precipitation of solid Al(OH)₃ at the gill surface, i.e., a ‘physical’ effect rather than a biochemical one.