Nitrite may be more toxic to certain fish because of the way in which their gills remove chloride ions from the water, says research published today.
Tomasso and Grosell of the Department of Biological Sciences, Clemson University, South Carolina, recently conducted experiments on Channel catfish, Ictalurus punctatus, and Bluegills, Lepomis macrochirus, to determine the underlying physiological basis for nitrite poisoning in fishes, and have just published their findings in the latest issue of the journal Environmental Science and Technology.
Scientists already knew that freshwater take in nitrite via their gills, as it has previously been shown to be actively transported into the blood via a chloride uptake pathway that occurs within special cells in the gills. However, some fish appear to be resistant to the effects of nitrite and have a mechanism that doesn't allow the nitrite to become as concentrated within their plasma.
Previous experiments have indicated the Mummichog killifish, Fundulus heteroclitus and the Eel, Anguilla anguilla, have no chloride uptake activity in their gills, or only a very weak activity, and are thought to get most of the chloride they need from their diet.
Tomasso and Grosell's study compared Bluegills, a species which doesn't concentrate nitrite in its blood to Channel cats, which do, and are therefore more sensitive to the pollutant. The study showed that Channel catfish actively transported chloride into their plasma, while the Bluegills did not, proving the link between toxicity and chloride uptake at the gills.
Since nitrite enters the gills via these chloride pathways, adding more chloride to the water can reduce its toxicity. For more details on calculating the level of chloride required, see our Nitrite Toxicity Calculator.
For more details see the paper: Tomasso JR Jr, Grosell M. (2005) - Physiological basis for large differences in resistance to nitrite among freshwater and freshwater-acclimated euryhaline fishes. Environ Sci Technol. 2005 Jan 1;39(1):98-102.