Aluminum bioconcentration at the gill surface of juvenile Atlantic salmon in acidic media

Aluminum uptake by Atlantic salmon was examined in the laboratory at pH 4.5, under conditions similar to those found in running waters on the Canadian Precambrian Shield during spring snowmelt. Gill uptake of Al was slow, approaching steady state only after 3 d of exposure. The greatest fraction of gill-associated Al was sorbed not to the gill surface itself, but to the gill mucus. Mucus appears to retard Al transport from solution to the membrane surface, thus delaying the acute biological response of the fish. Strongly associated gill [Al] was never greater than 10% of total gill Al in the early stages (<3 d) of the experiment; however, longer experiments (7 d) indicated that this Al fraction could eventually exceed 50% of the total gill Al. In contrast to uptake, depuration of Al was extremely rapid; total gill [Al] of fish exposed to Al (pH 4.5) for 2 d decreased by 60% after only 2 h in an Al-free medium (t1/2 = 0.9 h). The effect of fluoride complexation on Al bio-concentration was also examined. For equivalent Al3+ concentrations, sorption of Al to the gill surface was higher in the presence of fluoride than in its absence, which suggests the formation of mixed ligand [F-Al-L-gill] complexes at the gill surface.