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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

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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 Ca2+ in soft, ion-poor water. The affinity constant (Km) did not vary significantly among treatments for either Na+ or Ca2+. 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 Ca2+ 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.

Keywords: Amazon; humic substances; ion transport; low pH; soft water

Document Type: Regular Paper


Affiliations: National Institute for Research in the Amazon (INPA), Laboratory of Ecophysiology and Molecular Evolution, Av. André Araújo, 2936, Aleixo, Manaus, Amazonas, 69083-000, Brazil

Publication date: 2007-04-01

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