SUMMARYEarlier studies demonstrated that oscars, endemic to ion-poor Amazonian waters, are extremely hypoxia tolerant, and exhibit a marked reduction in active unidirectional Na + uptake rate (measured directly) but unchanged net Na + balance during acute exposure to low P O2 , indicating a comparable reduction in whole body Na + efflux rate. However, branchial O 2 transfer factor does not fall. The present study focused on the nature of the efflux reduction in the face of maintained gill O 2 permeability. Direct measurements of 22 Na appearance in the water from bladder-catheterized fish confirmed a rapid 55% fall in unidirectional Na + efflux rate across the gills upon acute exposure to hypoxia (P O2 =10-20 torr; 1 torr=133.3 Pa), which was quickly reversed upon return to normoxia. An exchange diffusion mechanism for Na + is not present, so the reduction in efflux was not directly linked to the reduction in Na + influx. A quickly developing bradycardia occurred during hypoxia. Transepithelial potential, which was sensitive to water [Ca 2+ ], became markedly less negative during hypoxia and was restored upon return to normoxia. Ammonia excretion, net K + loss rates, and 3 H 2 O exchange rates (diffusive water efflux rates) across the gills fell by 55-75% during hypoxia, with recovery during normoxia. Osmotic permeability to water also declined, but the fall (30%) was less than that in diffusive water permeability (70%). In total, these observations indicate a reduction in gill transcellular permeability during hypoxia, a conclusion supported by unchanged branchial efflux rates of the paracellular marker [ 3 H]PEG-4000 during hypoxia and normoxic recovery. At the kidney, glomerular filtration rate, urine flow rate, and tubular Na + reabsorption rate fell in parallel by 70% during hypoxia, facilitating additional reductions in costs and in urinary Na + , K + and ammonia excretion rates. Scanning electron microscopy of the gill epithelium revealed no remodelling at a macro-level, but pronounced changes in surface morphology. Under normoxia, mitochondria-rich cells were exposed only through small apical crypts, and these decreased in number by 47% and in individual area by 65% during 3 h hypoxia. We suggest that a rapid closure of transcellular channels, perhaps effected by pavement cell coverage of the crypts, allows conservation of ions and reduction of ionoregulatory costs without compromise of O 2 exchange capacity during acute hypoxia, a response very different from the traditional osmorespiratory compromise.

The so-called “blackwaters” of the Amazonian Rio Negro are rich in highly coloured dissolved organic carbon (DOC), but ion-poor and very acidic, conditions that would cause fatal ionoregulatory failure in most fish. However these blackwaters support 8% of the world’s ichthyofauna. We tested the hypothesis that native DOC provides protection against ionoregulatory dysfunction in this extreme environment. DOCs were isolated by reverse-osmosis from two Rio Negro sites. Physico-chemical characterization clearly indicated a terrigenous origin, with a high proportion of hydroxyl and phenolic sites, high chemical reactivity to protons, and unusual proteinaceous fluorescence. When tested using zebrafish (a model organism), Rio Negro DOC provided almost perfect protection against ionoregulatory disturbances associated with acute exposure to pH 4.0 in ion-poor water. DOC reduced diffusive losses of Na+ and Cl−, and promoted a remarkable stimulation of Na+ uptake that otherwise would have been completely inhibited. Additionally, prior acclimation to DOC at neutral pH reduced rates of branchial Na+ turnover, and provided similar protection against acid-induced ionoregulatory disturbances, even if the DOC was no longer present. These results reinforce the important roles that DOC molecules can play in the regulation of gill functions in freshwater fish, particularly in ion-poor, acidic blackwaters.

Roundup Original® (RD) is a glyphosate-based herbicide used to control weeds in agriculture. Contamination of Amazon waters has increased as a consequence of anthropogenic pressure, including the use of herbicides as RD. The central goal of this study was to evaluate the toxic effects of RD on juveniles of tambaqui (Colossoma macropomum). Our findings show that biomarkers in tambaqui are organ specific and dependent on RD concentration. Alterations in gills structural and respiratory epithelium were followed by changes in hematological parameters such as concentration of hemoglobin, particularly in fish exposed to the higher concentration tested (75% of RD LC50 96 h). In addition, both RD concentrations affected the biotransformation process in gills of tambaqui negatively. Instead, liver responses suggest that a production of reactive oxygen species (ROS) occurred in fish exposed to RD, particularly in the animals exposed to 75% RD, as seen by imbalances in biotransformation and antioxidant systems. The increased DNA damage observed in red blood cells of tambaqui exposed to RD is in agreement with this hypothesis. Finally, both tested sub-lethal concentrations of RD markedly inhibited the cholinesterase activity in fish brain. Thus, we can suggest that RD is potentially toxic to tambaqui and possibly to other tropical fish species.

Copper oxide nanoparticles (nCuO) are widely used in boat antifouling paints and are released into the environment, potentially inducing toxicity to aquatic organisms. The present study aimed to understand the effects of nCuO and dissolved copper (Cu) on two ornamental Amazon fish species: dwarf cichlid (Apistogramma agassizii) and cardinal tetra (Paracheirodon axelrodi). Fish were exposed to 50% of the LC for nCuO (dwarf cichlid 58.31μgL and cardinal tetra 69.6μgL) and Cu (dwarf cichlid 20μgL and cardinal tetra 22.9μgL) for 24, 48, 72 and 96h. Following exposure, aerobic metabolic rate (ṀO), gill osmoregulatory physiology and mitochondrial function, oxidative stress markers, and morphological damage were evaluated. Our results revealed species specificity in metabolic stress responses. An increase of ṀO was noted in cardinal tetra exposed to Cu, but not nCuO, whereas ṀO in dwarf cichlid showed little change with either treatment. In contrast, mitochondria from dwarf cichlid exhibited increased proton leak and a resulting decrease in respiratory control ratios in response to nCuO and Cu exposure. This uncoupling was directly related to an increase in reactive oxygen species (ROS) levels. Our findings reveal different metabolic responses between these two species in response to nCuO and Cu, which are probably caused by the differences between species natural histories, indicating that different mechanisms of toxic action of the contaminants are associated to differential osmoregulatory strategies among species.

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To assess how the quality and properties of the natural dissolved organic carbon (DOC) could drive different effects on gill physiology, we analysed the ionoregulatory responses of a native Amazonian fish species, the tambaqui Colossoma macropomum, to the presence of dissolved organic carbon (DOC; 10 mg l−1) at both pH 7.0 and pH 4.0 in ion‐poor water. The DOC was isolated from black water from São Gabriel da Cachoeira (SGC) in the upper Rio Negro of the Amazon (Brazil) that earlier been shown to protect a non‐native species, zebrafish Danio rerio against low pH under similar conditions. Transepithelial potential (TEP), net flux rates of Na+, Cl− and ammonia and their concentrations in plasma and Na+, K+ ATPase; v‐type H+ ATPase and carbonic anhydrase activities in gills were measured. The presence of DOC had negligible effects at pH 7.0 apart from lowering the TEP, but it prevented the depolarization of TEP that occurred at pH 4.0 in the absence of DOC. However, contrary to our initial hypothesis, SGC DOC was not protective against the effects of low pH. Colossoma macropomum exposed to SGC DOC at pH 4.0 experienced greater net Na+ and Cl− losses, decreases of Na+ and Cl− concentrations in plasma and elevated plasma ammonia levels and excretion rates, relative to those exposed in the absence of DOC. Species‐specific differences and changes in DOC properties during storage are discussed as possible factors influencing the effectiveness of SGC DOC in ameliorating the effects of the acid exposure.

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