Biotechnology and Bioinformatics (1+1)

6.1.3.3. Nitrogen removal

Ammonia is the principal excretory product of most aquatic organisms. Inputs of ammonia cannot be eliminated from the water body. But ammonia is toxic, acutely and chronically, to fish and invertebrates. Ammonia should be maintained below 0.1 mg/l (total ammonia). The most efficient way to do this is by the establishment of a biological filter. A biological filter is a collection of naturally occurring bacteria, which oxidize ammonia to nitrite, and other bacteria, which then convert nitrite to nitrate.

Nitrite is formed either by the oxidation of ammonia (nitrification) or the reduction of nitrate (denitrification).

Nitrite is toxic to fish and some invertebrates and should be maintained below 0.1 mg/l.

Nitrate is the end product of nitrification. The vast majority of aquaculture ponds accumulate nitrate as they do not contain a denitrifying filter. In gene ral, nitrate should be maintained below 50 mg/l (measured as NO₃-N) but it is not a critical water quality factor. The most common ways to reduce nitrate are water changes and growing live plants.

• The enzymes namely Ammonia monooxygenase (AMO) and Hydroxylamine oxidoreductase (HAO) are involved in the oxidation of ammonia to nitrite.
• Nitrobacte r sp. is facultatively mixotrophic and capable of growing anaerobically with nitrate as electron acceptor, producing nitrite, nitric oxide and nitrous oxide and then to nitrogen gas.