Diverse Strategies for Ion Regulation in Fish Collected from the Ion‐Poor, Acidic Rio Negro

Abstract

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 J(max) values, ranging from 1,150 to 1,750 nmol g(-1) h(-1), while K(m) values varied to a greater extent. Three species had K(m) values <33 micromol L(-1), while the rest had K(m) values >or=110 micromol L(-1). Because of the extremely low Na(+) concentration of Rio Negro water, the differences in K(m) 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 micromol L(-1)) by most species. Unlike other species with high K(m) values, the catfish Corydoras julii maintained high rates of Na(+) uptake in dilute waters by having a J(max) 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, J(max) fell >50%, and K(m) dropped about 70%. The unusual acclimatory drop in K(m) 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.

Calcium fluxes in Hoplosternum littorale (tamoatá) exposed to different types of Amazonian waters

“…The same kind of transfers in tamoatás led to net Na + , Cl -and K + loss through 4 h (except Cl -and K + in tamotás transferred from white water to black water) (Baldisserotto et al, 2008). Net Na + , Cl -and K + losses after transfer from black water (pH 5.5) or slightly acid well water (pH 6.5) to acidic black water (pH 3.5-3.75) were also observed by Gonzalez et al (1998), Wood et al (1998) and Wilson et al (1999) in tambaqui (Colossoma macropomum) and by (Gonzalez et al, 2002) in Geophagus sp. and Pimelodus sp.…”

“…Tamoatá juveniles (9-26 g) were raised in a 200-m 3 earth pond at the Embrapa fish culture sector (Manaus, Amazonas, Brazil). One month before the experiments fish were transferred to a 2000 L indoor fiberglass tank and fed three times a day with a commercial pelleted feed with 45% crude protein (TR 45;Nutron,São Paulo,Brazil (Gonzalez et al, 2002) and 2.0 mg.L -1 in well water and white water . This first exposure lasted four hours, and then the water in the chambers was replaced by a different type of water and the fish remained in the chamber for an additional 4 h (second exposure).…”

“…Several studies of ion fluxes between the fishes and their external media were performed in samples collected from the Negro River (Gonzalez et al 1997(Gonzalez et al , 2002Wood et al, 1998;Wilson et al, 1999;Gonzalez & Wilson, 2001) and some demonstrated that they are more resistant to acidic waters than those that do not live in black waters (Wilson et al, 1999;Gonzalez et al 1997Gonzalez et al , 2002. However, only two studies analyzed Ca 2+ fluxes in Amazon fishes, and they were performed with modified well water, which is similar to the "clear water" (Wilson et al, 1999;Matsuo et al, 2005).…”

Growth of silver catfish ( Rhamdia quelen ) exposed to acidic pH at different humic acid levels

“…Humic substances may be stimulant and protector of paracellular junctions of the gill epithelium, thus preventing ion efflux in some fish from the natural acidic black waters of Rio Negro (GONZALEZ et al, 2002;2005;WOOD et al, 2003). This protective action could spare energy to be used in fish growth, explaining the greater development of green sword tail exposed to 180mg L -1 humic substance compared to control (0mg L -1 humic substance) (MEINELT et al, 2004).…”

Effect of humic acid on survival, ionoregulation and hematology of the silver catfish, Rhamdia quelen (Siluriformes: Heptapteridae), exposed to different pHs

“…GONZALEZ et al (2002) and MATSUO & VAL (2007) observed that the presence of DOC in acidic water reduced both Na + influx inhibition and diffusive efflux stimulation in teleosts native to Amazonia. The role of DOC to bind fish gills at low pH and promote physiological benefits (CAMPBELL et al 1997) may be comparable with the above-mentioned action of elevated waterborne levels of Ca 2+ , that is, stabilization of tight junctions and prevention of ion losses.…”