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The nitrogen cycle: general

Nitrogen (chemical symbol N) is an essential component of life, such as oxygen and carbon. It is This especially in the DNA and amino acids. He moves between mineral forms and organic forms, the main reservoir being the atmosphere, as the air is composed of nitrogen to almost 80%.

Nitrogen follows therefore biogeochemical cycle. This nitrogen cycle is fairly complex, with different sub-cycles, but it can be summarized quite simply by 3 phases:
- fixation: conversion of atmospheric nitrogen (N2) into nitrogen usable by the living.
- nitrification : processing chain nitrogen compounds.
- denitrification : return of nitrogen to the atmosphere.
All these phases are implementing bacteria.

The source of nitrogen for living things evolved primarily the last component of the nitrogen nitrification: nitrates. The plant will feed on these nitrates, plant herbivores, carnivores and herbivores.

In a freshwater aquarium, the nitrogen cycle is almost reduced to nitrification. Indeed, the setting is not advantageous for the bacteria that are already sufficient nitrogen in the tank. And denitrification requires anaerobic bacteria, which therefore do not like oxygen, which is generally very present in the tray.

The simplified nitrogen cycle in a freshwater aquarium is:

• absorption of nitrate (NO3-) by plants.
• ingestion of plants by fish.
• appearance of organic waste: fish waste, dead leaves, dead bodies, etc..
• nitrification which itself is divided into three stages:
  • ammonification : decomposition of organic nitrogen into ammonium ion (NH4 +) and ammonia (NH3) by bacteria of the genera Bacillus , or Bacterium Micrococcus .
  • nitrosation : transformation of ammonium / ammonia nitrite (NO2-) by bacteria of the genus Nitrosomonas (or Nitrosomas ).
  • nitration : conversion of nitrite (NO2-) in nitrate (NO3-) by bacteria of the genus Nitrobacter .
• absorption of nitrate (NO3-) by plants, and so on, full circle. If the aquarium

was closed, the cycle would not affect the long term, the total amount of nitrogen remaining stable.
But fish can not simply plant for food. The introduction of food brings additional nitrogen, in particular proteins. This nitrogen is not removed and it will therefore accumulate at the location where the cycle is the weakest: the plants are not sufficient to convert all nitrate products. Nitrate concentration increases with the total amount of nitrogen.
is one reason why we have to make regular water changes, replacing a portion water from the tank with fresh water whose nitrate concentration is lower.

Note:
- Ammonium, ammonia and nitrite are toxic to fish. But nature is well made, balanced in a tray, the bacteria maintain virtually zero, transforming them as they emerge.
- Nitrates are toxic at high doses. However, your fish prefer water with less than 25 mg / l of nitrate. From 50 mg / l, we must act (the maximum tolerated dose for drinking water).
- Ammonium and ammonia are in continuous equilibrium, the first being very dominant in acidic water, the second (more toxic) in alkaline water.

Two articles:
- Implementation of the nitrogen cycle: the peak of nitrite
- Imbalances in the nitrogen cycle (forthcoming)

Some links:
- The nitrogen cycle by Earth
- Biogeochemical cycles per Wikipedia: the nitrogen cycle
- The nitrogen cycle by Marco Pagni (microbiologist)
- The nitrogen cycle by Association aquarium