Unit Animal Science

Problem Area Aquaculture

Lesson Microbial Cycling of Nitrogen

Student Learning Objectives 1. Describe the steps of the nitrogen cycle in an aquaculture system. 2. Identify forms of nitrogen that are harmful to aquaculture species. 3. Identify factors which influence the toxicity of nitrogen in an aquaculture system. 4. Explain procedures for controlling harmful levels of nitrogen in an aquaculture system.

Terms Ammonia Ammonia toxicity Ammonification Ammonium Denitrification Nitrate Nitrification Nitrite Nitrite toxicity Nitrobacter Nitrogen gas Nitrosomonas Total ammonia nitrogen

What is the nitrogen cycle in an aquaculture system? Nitrogen is cycled through the biosphere constantly. A. In aquaculture facilities four forms of nitrogen are likely to be found – ammonia (NH3 ), ammonium (NH4 +), nitrite (NO2 ) and nitrate (NO3 ). B. Ammonia compounds in water are produced as a result of decomposing organic matter through the ammonification process. Sources of organic matter in an aquaculture system include uneaten feed, fish waste, decaying plants and bacteria. Ammonia is converted to nitrite by nitrosomonas bacteria by the process of nitrification. Gaseous ammonia can be lost into the atmosphere by diffusion.

C. Nitrite is converted to nitrate by the nitrobacter bacteria through an oxidation reaction. Nitrates are converted to nitrogen gas (N2 ) in anaerobic conditions and diffused into the surrounding atmosphere. This process is known as denitrification. D. Nitrates that are not denitrified are used by phytoplankton, higher plants and bacteria as a nutrient source. Fish can then consume these organisms beginning the cycle again.

What forms of nitrogen are harmful to aquaculture species? A. Total ammonia nitrogen (TAN) levels are considered when determining ammonia toxicity. TAN includes unionized ammonia (NH3 ) and ammonium (NH4 +), the ionized form. Ammonia is 75–100 times less toxic than ammonium. Death can occur when ammonia levels reach 2 ppm and losses increase when above 7 ppm. Brain damage, increase susceptibility to disease, and slower growth are often seen when ammonia toxicity occurs.

B. Nitrite causes the hemoglobin in blood to oxidize rendering it unable to absorb and carry oxygen. This is a similar to the Blue Baby Syndrome in humans. Nitrite toxicity occurs at much lower levels than ammonia. Nitrite levels greater than 1 ppm can cause fish death.

What factors influence the toxicity of nitrogen in an aquaculture system? Temperature, pH and chloride concentrations are factors that influence the toxicity of nitrogen in an aquaculture system. A. Unionized ammonia is more toxic at warmer water temperatures and at high pH. Ionized ammonia and unionized ammonia exist in equilibrium. As pH increases more unionized ammonia is present. B. Nitrite toxicity is reduced by the presence of chlorides in water. The addition of sodium chloride or calcium chloride can reduce the nitrite poisoning.

What procedures are used to control harmful levels of nitrogen in an aquaculture system? Methods of controlling nitrogen levels include cultural and chemical means. A. Cultural methods of controlling nitrogen levels include the population density control, proper feeding, removing excessive organic material, adding fresh water, and monitoring pH levels. B. Chemical means includes the addition of acidic compounds to maintain the proper pH and the addition of chloride compounds to decrease the toxicity of nitrite.

Review/Summary What is the nitrogen cycle in an aquaculture system? What forms of nitrogen are harmful to aquaculture species? What factors influence the toxicity of nitrogen in an aquaculture system? What procedures are used to control harmful levels of nitrogen in an aquaculture system?