Changes in 22Na influx and outflux in Daphnia magna (Straus) as a function of elevated Al concentrations in soft water at low pH

Abstract

The effects of aluminum (Al) on sodium (Na) regulation by the freshwater crustacean Daphnia magna were determined. 22Na influx and outflux experiments were conducted in soft water adjusted to pH 4.5, 5.0, and 6.5 (reference pH) with either ambient (0.02 mg/liter) or high (1.02 mg/liter) concentrations of total Al. The results indicate that Al toxicity was pH dependent. Aluminum increased the rate of morbidity of D. magna at pH 6.5, had no additional effect to those of hydrogen ions (H+) at pH 5.0, and reduced the rate of morbidity at pH 4.5. Both H+ and total Al concentrations interfered with Na regulation, although it was possible to distinguish between their respective effects by using 22Na. At pH 6.5, Al decreased 22Na influx (by 46%) and increased 22Na outflux (by 25%), which led to a net loss of Na. At pH 5.0, Al reduced 22Na influx (by 58%) but had no additional effect to that of H+ on 22Na outflux. At pH 4.5, 22Na influx was significantly inhibited (by 73%) compared with the reference pH 6.5 treatment even in the absence of Al. Aluminum decreased 22Na outflux (by 31%) at pH 4.5, which reduced the net loss of Na and temporarily prolonged survival of the daphnids. These results indicate that Al affects both 22Na influx and outflux in D. magna. The lower rate of Na uptake may involve a denaturation of the enzyme responsible for the active uptake of Na. At pH 6.5, the increased outflux of 22Na may be due to either increased membrane permeability or increased renal losses (or both). At pH 4.5, the reduced outflux of 22Na resembles the amelioration that occurs in the presence of elevated Ca concentrations.