We measured unidirectional ion fluxes of fish collected directly from the Rio Negro, an extremely dilute, acidic blackwater tributary of the Amazon. Kinetic analysis of Na+ uptake revealed that most species had fairly similar Jmax values, ranging from 1,150 to 1,750 nmol g-1 h-1, while Km values varied to a greater extent. Three species had Km values <33 μmol L-1, while the rest had Km values ≥ 110 μmol L-1. Because of the extremely low Na+ concentration of Rio Negro water, the differences in Km values yield very different rates of Na+ uptake. However, regardless of the rate of Na+ uptake, measurements of Na+ efflux show that Na+ balance was maintained at very low Na+ levels (<50 μmol L-1) by most species. Unlike other species with high Km values, the catfish Corydoras julii maintained high rates of Na+ uptake in dilute waters by having a Jmax value at least 100% higher than the other species. Corydoras julii also demonstrated the ability to modulate kinetic parameters in response to changes in water chemistry. After 2 wk in 2 mmol L-1 NaCl, Jmax fell >50%, and Km dropped about 70%. The unusual acclimatory drop in Km may represent a mechanism to ensure high rates of Na+ uptake on return to dilute water. As well as being tolerant of extremely dilute waters, Rio Negro fish generally were fairly tolerant of low pH. Still, there were significant differences in sensitivity to pH among the species on the basis of degree of stimulation of Na+ efflux at low pH. There were also differences in sensitivity to low pH of Na+ uptake, and two species maintained significant rates of uptake even at pH 3.5. When fish were exposed to low pH in Rio Negro water instead of deionized water (with the same concentrations of major ions), the effects of low pH were reduced. This suggests that high concentrations of dissolved organic molecules in the water, which give it its dark tea color, may interact with the branchial epithelium in some protective manner.
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