The rich organic matter that gives the largest tributary of the Amazon River, the Rio Negro, its characteristic black hue may provide fish with some protection from the negative effects of copper toxicity (Sci. Total Environ. 2022, DOI: 10.1016/j.scitotenv.2022.157032). This matter may also provide the Amazon ecosystem with resilience against other kinds of metal pollution. “Copper is acutely toxic to fish,” says Anne Crémazy, a professor of environmental chemistry at the University of Quebec.
Once copper gets into the gills, it can affect the way that fish regulate sodium ions in their bodies. If Na+ levels drop by 30%, the fish’s cardiovascular system can collapse, leading to death, says Chris Wood, adjunct professor of zoology at the University of British Columbia and professor emeritus of biology at McMaster University, who authored the new study along with Crémazy and colleagues.
Many of the streams and rivers around Manaus—a city on the Amazon River—are heavily contaminated with copper because of pollution from industries that generate copper as a by-product.
Previous studies had revealed that dissolved organic matter in water can provide fish some protection from the harmful effects of copper. Other research had looked at how the organic material in the Rio Negro protects fish from the river’s high acidity. The Rio Negro has among the highest concentrations of dissolved organic matter in the world, Wood says, and this study found exactly how the compounds in the river protect fish from copper toxicity.
To determine this, the researchers used two species of fish common in the Amazon—the cardinal tetra (Paracheirodon axelrodi) and the dwarf cichlid (Apistogramma agassizii). They put these fish in two tanks in the lab: one contained water taken directly from the Rio Negro with a high concentration of dissolved organic matter and the other had a low concentration. Both tanks had the same acidity as the Rio Negro, pH 5.9.
The researchers added 1,000 µg/L of copper to both tanks and monitored the effects on the gills of the fish for 3 h—enough to elicit an acute response in the fish and a level found in some of the waterways around Manaus, but lower than the background content found upstream in the Rio Negro. They found that fish absorbed less copper in the tank with more organic matter.
The organic material likely protect the fish by binding to the copper before the metal has a chance to bind to the gills. As a result, copper accumulates less in the gills, Crémazy says.
Though the researchers just looked at copper, the composition of the Rio Negro also may help reduce negative effects from other metals. “Dissolved organic matter can bind to many metals and reduce their bioavailability to aquatic organisms,” Crémazy says.
Chris Mebane, Deputy Director for Studies at the US Geological Survey’s Idaho Water Science Center who was not involved in the work, calls it “a pretty elegant study.” He would like to see future studies on how copper might affect larger fish in the Amazon—species like freshwater stingrays or even bull sharks.
Edison Barbieri, a professor of the Fishery Institute of São Paulo, said in an email that high acidity waters like those of the Rio Negro usually help metals like copper dissolve better, which makes them easier for fish to absorb. “This article, in an unprecedented way, found that dissolved organic matter protects the gills from the action of Cu,” wrote Barbieri, who also was not involved in the work.
This story was updated on Aug. 1, 2022, to correct the concentration of copper added to the fish tanks. It is 1,000 µg/L, not 1,000 mg/L.
This story was updated on Aug. 5, 2022, to correct Chris Wood's affiliation. Wood is an adjunct professor of zoology, not a professor emeritus, at the University of British Columbia. He is a professor emeritus of biology at McMaster University.