Acclimation to humic substances prevents whole body sodium loss and stimulates branchial calcium uptake capacity in cardinal tetras Paracheirodon axelrodi (Schultz) subjected to extremely low pH

Kinetics of sodium (Na⁺) and calcium (Ca²⁺) uptake were studied in cardinal tetras Paracheirodon axelrodi acclimated to humic substances (HS, 35 mg C l^?1) and low pH (pH 3·72), parallel to analysis of whole body Na⁺ and Ca²⁺ content. This species had a high uptake capacity (J_max) for both Na⁺ and Ca²⁺ in soft, ion‐poor water. The affinity constant (K_m) did not vary significantly among treatments for either Na⁺ or Ca²⁺. J_max Na⁺ increased 30% in fish acclimated to HS for 5 weeks. Acclimation to low pH had no effect on J_max Na⁺ but this treatment was associated with a 32% decrease on whole body Na⁺ content, suggesting that fish were unable to compensate for the increased Na⁺ loss induced by extreme acidity. Exposure of fish to HS + low pH, the treatment most closely approximating to the conditions experienced by the species in its native environment, resulted in an increase in whole body Na⁺ by 31% relative to acclimation to low pH alone. J_max Ca²⁺ in cardinal tetras was high relative to that documented in other freshwater species acclimated to soft water (J_max= 30 nmol g^?1 h^?1). Prolonged exposure of fish to pH 3·72 inhibited J_max Ca²⁺ by 53%, although whole body Ca²⁺ content remained unchanged relative to control. Acclimation of fish to HS + low pH resulted in an increase of J_max Ca²⁺ by 166% relative to low pH alone. Collectively, these results suggest that HS protect cardinal tetras acclimated to soft, acidic waters by preventing excessive Na⁺ loss (as indicated by whole body Na⁺ content) and by stimulating Ca²⁺ uptake (as indicated by increased J_max Ca²⁺) to ensure proper homeostasis.