ShareLife on Earth - Ecosystems At Risk
Interactions Within Ecosystems:Bacteria and the Nitrogen Cycle
Background:
All living things require nitrogen to make the building blocks for proteins
and other important molecules. Our atmosphere is mostly made of nitrogen
but most living things are unable to make use of the nitrogen in our atmosphere.
Lightning and some bacteria convert nitrogen gas (N2) into
forms such as nitrate (NO3) that can be used by plants. Once
the nitrogen enters living food chains it is passed from producers to
consumers and ultimately to decomposers that break down nitrogen compounds.
Either the nitrogen is then returned to the atmosphere or the bacteria
make compounds that plants can use again.
This multi-day activity has several levels of skills you will use.
- You will practice the skills needed to measure nitrogen in several forms.
- You will observe the interactions between organisms critical to the latter part of the nitrogen cycle where nitrogen is returned to the atmosphere.
Materials:
- Test equipment for ammonia (NH4), nitrite (NO2) and nitrate (NO3) levels commonly found in aquaria. Test kits such as FastTestTM or SeaTestTM work well and require a minimum of preparation. These kits can be aquired with a quick search of the Internet or a trip to your local aquarium supplier. Teachers may wish to supply samples of water to students.
- Instant OceanTM or another salt mix suitable for preparing a marine environment.
- A 10-gallon fish tank. (A sawed-off milk jug or pop bottle will work in a pinch.)
- A quantity of aragonite sand sufficient to make a layer 2 to 3 inches deep in the bottom of each aquarium.
- A piece of uncured live rock (see your local marine aquarium supplier) or a sample of sand and gunk from an established marine aquarium to provide decaying matter and a supply of the proper organisms. If you use the sand for this step you'll need to add about 10 grams of fresh meat or a dead shrimp for every 10 gallons of water in your final setup to provide some extra organic matter.
Skill Building:
Read the instructions and practice using the use of the kits for analyzing
NH4 , NO2 , and NO3 levels in water.
Make sure you practice enough to ensure that your measurements during
the experiment will be accurate.
Nitrogen in Ecosystems:
Now that you have the ability to measure the nitrogen compounds in water
you are ready to see what happens when different organisms work together
to break down nitrogen compounds in a new aquarium.
Set up your aquarium with a thick layer of aragonite sand at the bottom and about one pound of uncured live rock or used aquarium sand for every gallon of water. If you added the used aquarium sand instead of the live rock, you'll need to add about 10 grams of fresh meat or a dead shrimp for each 10 gallons of water.
If you monitor the tank continually, you will notice that ammonia levels begin to drop about the time that nitrite becomes measurable. This signifies that bacteria are present that are capable of converting ammonia (NH4 from animal wastes and decaying things) into nitrite (NO2). As nitrite levels rise, ammonia levels will drop. The next stage begins as different bacteria convert nitrite to nitrate (NO3). Again you will see a drop in one compound as the other rises. Finally, denitrifying bacteria will begin to process nitrate and release the nitrogen into the atmosphere as nitrogen gas (N2). Nitrate levels will drop and remain relatively constant in a healthy marine ecosystem. In a small tank the nitrate levels tend to rise slowly over time.
If you take careful measurements several times a day for a week or to you will be able to create a plot similar to that shown in Figure 1 below.
Analysis:
- What factors seemed to change the most in this experiment?
- How would you propose to conduct the experiment to make the nitrogen compound levels remain constant, instead of fluctuating?
