Page 775 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

734 SECTION | X Avian and Aquatic Toxicology

VetBooks.ir Nitrogen Cycle and Chew, 2010). Ammonia gas can radially diffuse
across cell membranes. The toxicity of NH 3 in water is
The nitrogen cycle in aquatic systems is critically impor-
dependent on water pH, temperature, and the species of
tant especially in closed systems. The nitrogen cycle in an
aquatic animal. The pK of ammonia is approximately 9.2
aquatic system is primarily regulated by biota. An excel-
and at a pH of 9.2, NH 3 is in equilibrium with
lent review on the geobiochemistry of the nitrogen cycle 1 2
NH 1 OH . Lowering pH below the pK increases the
4
has been written (Hargreaves, 1998). The nitrogen cycle 1
concentration of ionized NH . Most of the NH 3 in fish
4
transforms organic nitrogen, ammonia, and other forms of 1
blood exists as NH 4 ion because the pH of blood is
nitrogen to nitrogen gas (N 2 ). The most common ionic approximately 2 pH units below the pK of ammonia. Most
2
1
forms of nitrogen are ammonium (NH ), nitrite (NO ), fish species excrete excess nitrogen from deaminization of
4
2
2
and nitrate (NO )(Camargo et al., 2005). Sources of amino acids as NH 3 instead of uric acid (Wilkie, 2002;
3
these ions are atmospheric deposition, presence in surface
Bucking, 2017). The gill excretion of NH 3 (80% 90%)
and groundwater, nitrogen fixation, and degradation of
occurs by simple diffusion down a concentration
organic matter and agricultural fertilizers. An important
gradient from the fish to the ambient water. Fish have
reactive gaseous form of nitrogen is ammonia (HN 3 ). 1
mechanisms to acidify (excretion of H ) the boundary
Important sources of reactive nitrogen in the aquatic sys-
layer on the apical surface of the gill to favor both the for-
tem are feedstuff residues, fish feces, and nitrogen com- 1
mation of NH 4 and maintain a high diffusion gradient for
pounds excreted from other aquatic animals or present in
NH 3 (Bucking, 2017). Increasing pH of the ambient water
runoff and intake waters. The organisms regulating the
decreases excretion of NH 3 as does increasing the concen-
different steps in the aquatic nitrogen cycle are remark-
tration of NH 3 and a reversed inward flux of NH 3 can
ably independent, and products from different steps can
occur. Ammonium ions can be transported in the potas-
accumulate in the system. Species of Nitrosomonas and sium ion (K )/sodium ion (Na ) by the substitution
1
1
2
Nitrobacter oxidize ammonia to NO . Following this, of NH 1 for K 1 in the Na /K 1 exchange mechanism to
1
2
oxidation of NO 2 to NO 2 occurs, and species of 4 1 1
2 3 give Na in/NH out exchange occurring in the basolat-
Nitrobacter are the primary organisms that accomplish 4
eral region of the brachial epithelium. The activity of
this. Species of Nitrosomonas and Nitrobacter are dissim- 1 1
Na /K -ATPase is upregulated when NH 3 is high in
ilarly affected by oxygen tension, pH, temperature, etc.
ambient water along with the expression for cortisol recep-
Species of Nitrobacter are the most sensitive, and when tors (Sinha et al., 2012). Increased NH 3 in fish increases
they are disadvantaged, the level of NO 2 increases. The
2 glycolysis in cytosol and decreases the TCA cycle in the
end step in the nitrogen cycle is reduction, with produc-
mitochondria and the brain is sensitive to these effects.
tion and release of N 2 into the atmosphere. The most toxic 1
The NH 4 ion at high endogenous concentrations can
compounds in the aquatic nitrogen cycle are NH 3 and substitute for K 1 in ion transporters and disrupt electro-
2
NO . Changes in the nitrogen cycle can also be seasonal chemical physiology. Physical activity, feeding, and
2
and/or related to weather conditions or changes in the stress-linked increase in cortisol levels increase blood NH 3
indoor controlled environment. For example, NO 2 poi- by amino acid catabolism (Randall and Tsui, 2002).
2
soning (brown blood disease) in the southern United
Increased blood NH 3 reduces physical activity. There
States is more common in the fall and spring (Durborow
appears to be metabolic dysfunction in the liver and links
et al., 1997). When the concentration is less than 1 μmol
2 to disruption of neurotransmitters and electrochemistry in
NO , the water is generally considered unpolluted
3 skeletal muscle (Randall and Tsui, 2002). Increasing water
(Jensen, 2003).
NH 3 decreases swimming ability (Wicks et al., 2002). The
decrease in swimming ability is linked to depolarization of
21
white muscles, and increasing water calcium ions (Ca )
Ammonia
ameliorated NH 3 toxicity. Feeding fish appears to provide
Recirculation systems, for economic reasons, can have some protection from toxicity by ambient NH 3 likely by
densities as high as 0.3 kg of fish/kg of water that can decreasing gluconeogenesis (Wicks and Randall, 2002).
result in high NH 3 loads. Ammonia is very toxic to fish. Immune suppression can occur. Daoust and Ferguson
Total NH 3 -nitrogen requires monitoring because NH 3 is (1984) used ambient conditions of pH 7.93 and NH 3
the primary and most toxic nitrogen waste excreted by concentrations of 0.0, 0.215, and 0.423 ppm, respectively,
freshwater fish. The best predictor of water safety is total for 90 days to study the toxicity of NH 3 in rainbow trout
NH 3 -nitrogen (TA-N), which is the sum of NH 3 1 NH 1 (Oncorhynchus mykiss) fingerlings. Signs of central
4
forms. Ammonium ion has a low order of toxicity. nervous system disfunction were observed, no histopathol-
Estimated NH 3 production by fish is 0.02 kg of NH 3 - ogy was observed in the gills and the occurrences of acido-
nitrogen produced for each kilogram of feed fed and the philic droplets in epithelial cells of renal proximal
nitrogen is excreted within 24 h (Masser et al., 1999; Ip convoluted tubules were increased in fish exposed to NH 3 .