Nitrogen Cycle

I believe an understanding of the Nitrogen Cycle is essential before attempting to keep shrimp. Ammonia (NH3) is toxic to shrimp, and so it is therefore important to understand how this compound is formed and removed from aquaria to keep the levels low.

The Nitrogen Cycle of Shrimp Aquaria


  1. Nitrogen enters the nitrogen cycle of shrimp aquaria by the food we add for the shrimp. This is consumed by the shrimp and assimilated into their biomasses or excreted as nitrogenous waste.
  2. The proteins of the nitrogenous waste are decomposed by bacteria to amino acids, which are in turn are ammonified by decomposer bacteria to ammonia and ammonium.
  3. Nitrifying bacteria exists in the filter of the aquarium, which nitrify ammonia/ammonium to nitrite and nitrite to nitrate. Nitrogen leaves the cycle in these forms during water changes.
  4. Nitrates are absorbed and assimilated by algae, plants and biofilm. Shrimp consume these and assimilate the nitrogen, or these die and are decomposed.

The Toxicity of Ammonia – Temperature & pH

The toxicity of ammonia is affected by both of these factors. This can be explained by examining the equilibrium of the protonation of ammonia. When ammonia is protonated it forms relatively harmless ammonium ions (NH4+).

According to Le Châtelier’s principle, if you impose a change on an equilibrium the equilibrium will shift to oppose the change.

Temperature

NH3 + H3O+ ↔ NH4+ + H2O

Firstly, this reaction is exothermic (it gives out heat). It follows that:

  • An increase in temperature will cause the equilibrium to shift to the left-hand side, taking in the heat energy. This increases the amount of ammonia.
  • A decrease in temperature will cause the equilbrium to shift to the right-hand side, giving out heat energy. This decreases the amount of ammonia.

Lowering the temperature therefore lowers the toxicity of ammonia, simply by converting a higher amount to ammonium.

pH

NH3 + H3O+ ↔ NH4+ + H2O

pH is inversely proportional to the amount of H+ in the water, which are present in the form H3O+. We can therefore rewrite this equilibrium as follows:

NH3 + H+ ↔ NH4+

It follows that:

  • An increase in pH (decrease in H+) will cause the equilibrium to shift to the left-hand side, producing H+ ions. This increases the amount of ammonia.
  • A decrease in pH (increase in H+) will cause the equilibrium to shift to the right-hand side, taking in H+ ions. This decreases the amount of ammonia.

Lowering the pH therefore lowers the toxicity of ammonia, simply by converting a higher amount to ammonium.

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