Download Microbial Cycling of Nitrogen

Unit
Animal Science
Problem Area
Aquaculture
Lesson
Microbial Cycling of Nitrogen
Student Learning Objectives
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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
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Ammonia
Ammonia toxicity
Ammonification
Ammonium
Denitrification
Nitrate
Nitrification
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Nitrification
Nitrite
Nitrite toxicity
Nitrobacter
Nitrogen gas
Nitrosomonas
Total ammonia
nitrogen
What is the nitrogen cycle in an aquaculture system?
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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.
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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.
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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?
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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.
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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?
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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?
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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
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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?