Mechanisms of aluminium extraction and accumulation at the gills of rainbow trout, Oncorhynchus mykiss (Walbaum), in acidic soft water

Rainbow trout were fitted with latex masks for the measurement of ventilatory flow rate (V?_w) and with opercular catheters for sampling expired water from close to the branchial surface. Fish were exposed for 6 h to pH 5.1, 4.7 or 4.1 in the presence (138 μg l ¹) or nominal absence (1 μg 1^–1) of Al. Inspired and expired Al concentrations and water pH were measured via the opercular catheters. Gills were sampled for accumulated Al at the end of the experiments. V?_w increased during Al exposures at all three pHs. However, in the absence of Al, V?_w increased only at pH 4.1. Aluminium extraction from the water and Al accumulation on the gills were highest at inspired pH 5.1 and lowest at pH 4.1, and correlated well with expired pH, which was 0.2–0.7 pH units higher than inspired pH. Gill Al accumulations amounted to only about one-tenth of deposition calculated from Al extraction from the water and V?_w, and gill Al was tightly bound to the branchial surface. Calculations of Al solubility, oversaturation, and species composition were made using measured expired pH values, and were compared with Al extraction from the water and measured gill Al concentrations. In general, these analyses indicated that reduced Al solubility near the gills is a reasonable explanation of Al extraction from the water, and that Al(OH)₂⁺ and Al(OH)₃° are the Al species most likely to interact initially at the gills. It is suggested that mucus sloughing removes most precipitated Al, and that only the charged form persists, bound to structural elements on the gill surface. A model incorporating these results, and pH changes in the fish gill micro-environment in general, is presented to explain previously-reported ionoregulatory and respiratory effects of Al.