Nitrogen Cycle
The element nitrogen, in different forms, cycles through the earth’s
atmosphere, the land and aquatic systems. Within an aquarium
ecosystem, nitrogen in can be both helpful to some organisms and
harmful to others. In this exercise you will model the change in the
nitrogen chemical formula and structure as it flows through the
aquatic system.
 As a natural part of metabolism, fish excrete waste in a
solid and a liquid form. This waste, mostly liquid, is in the
form of ammonia (NH3). Ammonification also occurs
at this time. Bacteria that is naturally occurring in the
water, breaks down uneaten food, dead and decaying
animals and plants into ammonia.
 When this ammonia is mixed with the water, it grabs
another hydrogen from water molecules and forms
ammonium (NH4+). The chemical equation for this is;
NH3 + H2O <-> NH4++ OH-. Ammonia is extremely toxic
to fish where ammonium is relatively harmless. In a small
aquarium, the toxicity level of ammonia can occur quickly.
The temperature and pH of the water determines which
form the nitrogen will be in. The higher the temperature
and pH, more of the nitrogen will be in the toxic ammonia
form. It is at this part of the cycle that aquatic plants are
most useful. These plants prefer the ammonium form of
nitrogen as a food source.
 If plants are not part of the ecosystem or there are not
enough plants, then nitrification takes place. In this
process, bacterium uses the ammonia as an energy
source. One type of bacteria turns the ammonia into
nitrite (NO2), which is toxic to other organisms. Another
type of bacteria turns the nitrite into nitrate (NO3) a
non-toxic form.
 Unless the aquarium is completely closed off from the air,
atmospheric nitrogen also enters the cycle in the form of
N2. Denitrification converts nitrate back to N2 gas that
is released back to the atmosphere.
 Don’t forget, fish like to eat plants.
A. First, list the chemical formula for each of the following. Next,
using the molecular model kits, construct each formula and
then draw the structure.
Name
Chemical Formula
Chemical Structure
Ammonia
Ammonium
Nitrite
Nitrate
B. Use the information about nitrogen to create (as a group) the
nitrogen cycle within an aquarium environment. Make a poster,
or other form of presentation instructed by your teacher, to
share with the class.