Nitrification

The bacterial process that converts toxic ammonia to nitrites and then to nitrates. It is the crucial half of the nitrogen cycle in any aquarium.

Detailed explanation

Nitrification is an aerobic process (requires oxygen) performed by chemoautotrophic bacteria. These bacteria obtain energy from the oxidation of inorganic nitrogen compounds: they do not need organic carbon as an energy source.

The first step is the oxidation of ammonia (NH3/NH4+) into nitrites (NO2-). The main culprits are bacteria of the genus Nitrosomonas, Nitrosospira and some archaea (Nitrosopumilus). This step releases energy that the bacteria use to live and reproduce.

The second step is the oxidation of nitrites (NO2-) into nitrates (NO3-). Bacteria of the genus Nitrospira are the main players. Contrary to what was believed for decades, Nitrospira is more important than Nitrobacter in most aquariums.

Nitrification is a slow process. Nitrifying bacteria reproduce every 12-24 hours, much more slowly than heterotrophic bacteria. This is why the maturation cycle of an aquarium takes weeks and not days.

The factors that influence nitrification are: temperature (optimal 25-30°C), pH (optimal 7.5-8.0, below 6.5 it slows down a lot), dissolved oxygen (a lot is needed), surface available for colonisation.

“Bacteria in a bottle” products (such as Seachem Stability, Dr. Tim's, Microbe-Lift Nite-Out) contain cultures of nitrifying bacteria that can speed maturation. Not all are equally effective: refrigerated ones tend to work better.

Aggressive filter washing, antibiotic treatment, or a long blackout can decimate the nitrifying population causing a “mini-cycle” with spikes in ammonia or nitrite.

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