Ammonia toxicity, tolerance, and excretion

Abstract

Abstract Ammonia is an unusual toxicant in that it is produced by, as well as being poisonous to, animals. In aqueous solution ammonia has two species, NH3 and NH4+, total ammonia is the sum of [NH3] + [NH4+] and the pK of this ammonia/ammonium ion reaction is around 9.5. The NH3/NH4+ equilibrium both internally in animals and in ambient water depends on temperature, pressure, ionic strength, and pH; pH is most often of greatest significance to animals. Elevated ammonia levels in the environment are toxic. Temperature has only minor effects on ammonia toxicity expressed as total ammonia in water, and ionic strength of the water can influence ammonia toxicity, but pH has a very marked effect on toxicity. Acid waters ameliorate, whereas alkaline waters exacerbate ammonia toxicity. The threshold concentration of total ammonia ([NH3] + [NH4+]) resulting in unacceptable biological effects in freshwater, promulgated by the EPA (1998), is 3.48 mg N/liter at pH 6.5 and 0.25 mg N/liter at pH 9.0. There is only a relatively small saltwater data set, and a paucity of data on ammonia toxicity in marine environments, particularly chronic toxicity. The national criteria promulgated in the EPA (1989) saltwater document is a criterion continuous concentration (chronic value) of 0.99 mg N/liter total ammonia and a criterion maximum concentration (half the mean acute value) of 6.58 mg N/liter total ammonia, somewhat less than the equivalent freshwater pH 8.0 values of 1.27 and 8.4 mg N/liter total ammonia, respectively. This is consistent with marine species being somewhat more sensitive to ammonia than freshwater species. Toxicity studies are usually carried out on unfed, resting fish in order to facilitate comparison of results. Based on recent studies, however, environmental stresses, including swimming, can have dramatic effects on ammonia toxicity. It is also clear that feeding results in elevated postprandial body ammonia levels. Thus, feeding will probably also exacerbate ammonia toxicity. Fish may be more susceptible to elevated ammonia levels during and following feeding or when swimming. Thus, present ammonia criteria may fail to protect migrating fish and may be inappropriate for fish fed on a regular basis. Most teleost fish are ammonotelic, producing and excreting ammonia by diffusion of NH3 across the gills. They are very susceptible to elevated tissue ammonia levels under adverse conditions. Some fish avoid ammonia toxicity by utilizing several physiologic mechanisms. Suppression of proteolysis and/or amino acid catabolism may be a general mechanism adopted by some fishes during aerial exposure or ammonia loading. Others, like the mudskipper, can undergo partial amino acid catabolism and use amino acids as an energy source, leading to the accumulation of alanine, while active on land. Some fish convert excess ammonia to less toxic compounds including glutamine and other amino acids for storage. A few species have active ornithine—urea cycles and convert ammonia to urea for both storage and excretion. Under conditions of elevated ambient ammonia, the mudskipper P. schlosseri can continue to excrete ammonia by active transport of ammonium ions. There are indications that some fish may be able to manipulate the pH of the body surface to facilitate NH3 volatilization during aerial exposure, or that of the external medium to lower the toxicity of ammonia during ammonia loading. Future investigation of these aspects of “environmental ammonia detoxification” may produce new information on how fish avoid ammonia intoxication.