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

Abstract

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 (Jmax) for both Na+ and Ca 2+ in soft, ion-poor water. The affinity constant (Km) did not vary significantly among treatments for either Na+ or Ca 2+. Jmax Na+ increased 30% in fish acclimated to HS for 5 weeks. Acclimation to low pH had no effect on Jmax 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. Jmax Ca2+ in cardinal tetras was high relative to that documented in other freshwater species acclimated to soft water (Jmax = 30 nmol g-1 h-1). Prolonged exposure of fish to pH 3.72 inhibited Jmax Ca2+ by 53%, although whole body Ca 2+ content remained unchanged relative to control. Acclimation of fish to HS + low pH resulted in an increase of Jmax 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 Jmax Ca2+) to ensure proper homeostasis. © 2007 The Fisheries Society of the British Isles.