Site hosted by Angelfire.com: Build your free website today!

Nitrogen Cycle

The nitrogen cycle is one of several cycles of matter that occur on earth. The element nitrogen, N, exists mainly in three forms, ammonia, NH3 , nitrite, NO2, and nitrate, NO3. Other important forms of nitrogen include amine, NH2, ammonium, NH4 and elemental diatomic nitrogen, N2. In aquatic systems, the cycling of nitrogen between these forms is an important aspect of the health of the system.

In aquaculture systems the cycle begins when fish are introduced into a tank and begin feeding. Fish food is normally high in protein which is is a family of compounds composed of very large molecules containing up to hundreds of thousands of atoms. These molecules are digested by the fish into amino acid molecules which contain nitrogen atoms. The fish then metabolize the amino acids to release energy. In the metabolic process, amine, NH2, is removed from the amino acid molecules. The amine is then converted to ammonia NH3, and released into the water through the fish’s gills. Accumulatiion of ammonia in the water can be disasterous to fish. For example, levels of between 0.2 to 0.5 mg/L (milligrams of ammonia per liter of water) are normally lethal to koi, therefore something must be done to remove this waste from the water.

Ammonia can be reduced by altering the physical conditions of pH, temperature, and salinity. The physical condition of water known as pH is a measurement of the concentration of H. The pH scale, however, is exponential and runs backward. In other words, a pH of 5 has 10 times more H than a pH of 6. Lower pH favors higher concentration of ammonium. This is because ammonium and ammonia exist in a dynamic equilibrium;

NH4+ <===> NH3 + H
Lower pH means a higher concentration of H which shifts this reversible reaction to the left producing more of the less lethal ammonium (NH4). Lower temperature will also shift the reaction to the left, causing more of the nitrogen to be in the less lethal form. Higher salinity also favors the ammonium. Different fish species vary in their needs but in terms of reducing ammonia use lower pH, cooler water, and higher salinity.

Ammonia can also be removed through the use of a biological fitration system. The system uses aerobic bacteria to cycle the nitrogen from the ammonia to the less toxic nitrite and finally to nitrate which is the least toxic form of nitrogen. The filter system includes a container filled with some inert material with a large surface area for the bacteria to attach. Water from the tank cycles through the filter. Since the process is aerobic, oxygen must be abundant. An air stone in the bottom of the filter will oxygenate the water to support the growth of only aerobic bacteria . A polyethylene drum or trash can makes a good container and the filter material could be lava rocks, bird netting, bulk filter material of various composition such as polypropylene, PVC blocks or ribbon, Bio Balls, Bio Barrels, or many others.

One version of the biological filtration system uses 40 pounds of lava rock, a 55 gallon plasitic drum, PVC pipe and valve, one inch plastic netting or mesh, a submersible pump, and water plants. The bottom of the drum has 4 six inch pieces of four inch diameter PVC pipe set upright to hold up plastic mesh. The PVC supports have holes drilled to prevent water from becoming stagnant within them. The forty pounds of lava rock are on top of the plastic mesh and water plants such as water hyachant is grown on the top. The water is pumped, using a submersible pump, in the bottom, filters up through the lava rock, and exits out the top. Air stones can also be placed in the bottom to increase oxygenation. A valve is placed at the inflow pipe that can be closed whenever the submersible pump is turned off to prevent backflow into the tank.

The first step in the cycle is the conversion from ammonia (NH3) to nitrite (NO2). This is accomplished by a common soil bacterium, Nitrosomonas.

2NH3 + 3 1/2 O2 Nitrosomonas > 2 NO2 + 3 H2O
Nitrite while being less toxic to fish.is still a problem. This is because nitrite disrupts the hemoglobin’s ability to carry oxygen in the blood. If the nitite were allowed to accumulate in the water it could cause listlessness and oxygen starvation in the fish. For instance a concentration of only 0.15 mg/L can kill koi under 15 cm in length. If fish seem listless, do not eat, and stay at the bottom of the tank, there may be a nitrite problem. The remedy is to replace 20% of the tank water with fresh water. Usually, the second stage of the nitrogen cycle takes care of excess nitrite.

Nitrobacter, another common bacterium, metabolizes nitrite (NO2) to nitrate (NO3).

2 NO2 + O2 Nitrobacter > 2NO3
Fortunately, nitrate can be tolerated by all fish up to concentrations of 500 mg/L. If plants are included in the tank, they will uptake the nitrate as fertilizer and use it in the synthesis of proteins, thus completing the cycle. Water plants or diverting the water to a side hydroponic system can help use up the nitrate but if an algae bloom occurs in the tank making the water pea green oxygen will be depleted, causing oxygen starvation in the fish.

If ammonia, nitrite, and nitrate levels are monitored over time as a new tank is established, the concentration of each will increase, then decrease respectively until a balance occurs. This balance is achieved when the bacteria population grows to a point where it is capable of metabolizing all of the waste products of the fish.

Should the fish biomass increase either due to fish growth or introducing more fish to the tank, “new tank syndrome” may result. The fish will become lethargic and not as responsive as before the addition. The “syndrome” results from the buildup of excess ammonia and nitrite. Until the bacterial population grows to meet the new needs of the tank the system will be out of balance. If ammonia and nitrite continue to build up and the fish health does not improve, it may be that the fish load is too great for the tank size. To remedy this situation the newly added fish can be removed, another biological filter can be added, or 20% of the water can be replaced with fresh water.

As previously mentioned, the nitrogen cycle is an aerobic, meaning that is requires oxygen. The biological filter should be well aerated using an air pump, tubing and airstones. When dissolved oxygen levels fall to 0.6 - 0.7 mg/L, nitrification becomes inhibited and anaerobic conditions may develop. One anaerobic bacterium that may begin to grow is Pseudomonas, a denitrifier. It converts nitrite to nitrogen gas;

2NO3 -----> 2NO2 ----> 2NO ----> N2
This may seem good since the end product is nitrogen gas which will eventually escape into the atmosphere, but other anaerobic processes may also occur which break down organic materials into dihydrogen sulfide (H2S) which can be lethal in concentrations less than 1 ppm. The gas, which smells like rotten eggs, prevents the hemoglobin in the fish’s blood from carrying oxygen.

An aquaculture system must have adequate physical conditions of pH, temperature, and salinity. It also must have an adequate biofiltration system including large enough surface area, correct flow of water, good aeration, and occasional water changes. If thees conditions are met, the toxins produced by the fish will be converted to less harmfull substances by the bacteria and plants and a healthy fish population will result.