Track 3 · Lesson 18

The nitrogen cycle, step by step

7 minutes · 1 ordering activity

Fish waste can hurt fish. So how does it become plant food?

It changes, one step at a time.

My waste can hurt me. Who makes it safe?

Think of a relay race. One runner carries the baton, then hands it to the next.

Nobody runs the whole race alone.

Fish give off a waste, mostly through their gills. Too much of it can hurt fish.

This waste is called ammoniaA waste fish give off into the water. Too much of it hurts fish.
See it in the glossary.

The Munchers run first. They change ammonia into something called nitriteWhat one kind of bacteria makes from ammonia. It can hurt fish too.
See it in the glossary.

Nitrite can hurt fish too, so the race isn’t over.

The Finishers take the baton next. They change nitrite into something called nitrateWhat a second kind of bacteria makes from nitrite. Plants use it as food.
See it in the glossary.

That is the finish line.

Nitrate is much safer for fish, and plants use it as food.

Remember nitrogenA nutrient that helps a plant grow big, green leaves.
See it in the glossary, which helps leaves grow green? Nitrate carries nitrogen to the roots.

Here is the whole relay. Fish waste goes in, and plant food comes out.

  1. Fish waste
  2. Munchers
  3. Nitrite
  4. Finishers
  5. Nitrate
  6. Plant food
  • A · fish
  • B · ammonia
  • C · nitrite
  • D · nitrate
  • E · plant

Your turn. Run the relay.

Start with the fish’s dinner.

Put the steps in order.

Move the steps up or down. Then check.

  1. Fish give off ammonia. Right spot Not quite
  2. Munchers turn ammonia into nitrite. Right spot Not quite
  3. Roots take up the nitrate. Right spot Not quite
  4. Fish eat their food. Right spot Not quite
  5. Finishers turn nitrite into nitrate. Right spot Not quite

Put the steps in order.

The right order:

  1. Fish eat their food.
  2. Fish give off ammonia.
  3. Munchers turn ammonia into nitrite.
  4. Finishers turn nitrite into nitrate.
  5. Roots take up the nitrate.

Ammonia, then nitrite, then nitrate. The plants get the last handoff.

Plants grow, fish eat, and the crew keeps working. It goes round and round.

This loop is called the nitrogen cycleThe loop that turns fish waste into plant food, step by step.
See it in the glossary.

Check 1 of 3

Which one is safest for fish?

Show the answer

Nitrate. Right. Nitrate is much safer for fish, and plants use it as food.

Check 2 of 3

What do the Munchers make?

Show the answer

Nitrite. Right. Munchers turn ammonia into nitrite.

Check 3 of 3

Why does a system need lots of Finishers?

Show the answer

They turn nitrite into safer nitrate. Right. They finish the job, so nitrite doesn't build up.

You just learned how fish waste becomes plant food in the nitrogen cycle.

Next up: Starting a system: why you wait before adding fish.

Next lessonCourse map
Go deeper: the chemistry

Ammonia is NH3. In water, much of it becomes ammonium, NH4+, which is less harmful. Warm water and a high pH tip the balance back toward ammonia.

Nitrite is NO2−, and nitrate is NO3−. Each step adds oxygen, so the bacteria need oxygen in the water.

Fish give off most of their ammonia through their gills. Leftover food and solid waste also break down and make more.

This work also releases a little acid, so over time the water slowly becomes more acidic. Lesson 21 shows how growers handle that.

Munchers and Finishers are nicknames. Scientists know several kinds of bacteria that do each job.

In 2015, two research teams reported bacteria that do both steps on their own. One team found them in a filter that cleaned water for fish.

Plants can also take up some ammonium directly. In most aquaponics systems, though, nitrate is their main source of nitrogen.

Sources

  • Somerville, C., Cohen, M., Pantanella, E., Stankus, A. and Lovatelli, A. Small-scale aquaponic food production: Integrated fish and plant farming. FAO Fisheries and Aquaculture Technical Paper No. 589. Food and Agriculture Organization of the United Nations, Rome, 2014. Link
  • Rakocy, J. E., Masser, M. P. and Losordo, T. M. Recirculating Aquaculture Tank Production Systems: Aquaponics, Integrating Fish and Plant Culture. SRAC Publication No. 454. Southern Regional Aquaculture Center, November 2006 revision. Link
  • Hargreaves, J. A. and Tucker, C. S. Managing Ammonia in Fish Ponds. SRAC Publication No. 4603. Southern Regional Aquaculture Center, December 2004. Link
  • Durborow, R. M., Crosby, D. M. and Brunson, M. W. Nitrite in Fish Ponds. SRAC Publication No. 462. Southern Regional Aquaculture Center, June 1997 (revised). Link
  • McGee, M. and Cichra, C. Principles of Water Recirculation and Filtration in Aquaculture. FA-12. Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First published October 1988, reviewed April 2000. Link
  • Deer, C., Hu, B., Dunn, B. and Dusci, J. Nitrification and Maintenance in Media Bed Aquaponics. Fact sheet HLA-6729. Oklahoma Cooperative Extension Service, Oklahoma State University, October 2021. Link
  • Daims, H., Lebedeva, E. V., Pjevac, P., Han, P. et al. Complete nitrification by Nitrospira bacteria. Nature 528, 504-509 (2015). Link
  • van Kessel, M. A. H. J., Speth, D. R., Albertsen, M., Nielsen, P. H., Op den Camp, H. J. M., Kartal, B., Jetten, M. S. M. and Lücker, S. Complete nitrification by a single microorganism. Nature 528, 555-559 (2015). Link

All sources · Corrections