Denitrification Aerobic vs Anaerobic Bacteria in Aquariums

Aerobic vs Anaerobic Bacteria in Aquariums

Aerobic nitrifying bacteria convert ammonia (NH3) into nitrate (NO3). Anaerobic denitrifying bacteria convert nitrate into nitrogen gas (N2). The key difference is that aerobic bacteria require oxygen while anaerobic bacteria must live in environments with little to no oxygen.

Aerobic and anaerobic bacteria are responsible for the nitrification and denitrification of our aquariums. Each bacteria type removes certain compounds from a tank, which reate suitable environments for our fish.

To explain their differences, it’s important to understand how these bacteria work.

What do the terms aerobic and anaerobic mean?

Bacteria is classified as being “aerobic” or “anaerobic” based on whether or not it lives in the presence of oxygen. Aerobic bacteria uses oxygen while anaerobic bacteria uses other elements, such as nitrogen or sulfur.

Aerobic bacteria use oxygen as its oxidizing agent

Aerobic bacteria uses oxygen to oxidizes ammonia into nitrite and then into nitrate. This is the nitrogen cycle, or nitrification process, that most fishkeepers should be somewhat familiar with.

Aerobic bacteria have the ability to populate in our aquariums because water contains oxygen. The ammonia part of this reaction is produced when our fish poop, plants decay or excess fish food rots.

Anaerobic bacteria use something other than oxygen as its oxidizing agent

Anaerobic bacteria lives in environments wil little to no oxygen. This type of bacteria has evolved to use elements other than oxygen, such as nitrogen, as an oxidizing agent. This allows the bacteria to live in anoxic environments that do not contain oxygen.

In fact, the presence of oxygen prevents important enzymes from allowing denitrification to occur. Meaning, denitrifying bacteria must live in environments with very little or no oxygen in order to function properly.

One method for producing anaerobic bacteria in an aquarium is by having a deep substrate.

This key difference in oxygen causes these two bacteria types to convert and produce fundamentally different chemical compounds as part of their oxidizing processes.

How do aerobic and anaerobic bacteria work?

They use a process called oxidation to acquire energy

There is a process in chemistry called oxidation that allows nitrifying and denitrifying bacteria to consume energy from their food sources.

What happens is an oxidizing agent, oxygen or nitrogen, attracts electrons from a chemical compound, the reducing agent, to form a new chemical compound. This process releases energy stored in the reducing agent, which the bacteria consumes.

For example, aerobic bacteria breathes in oxygen and uses it to convert ammonia (NH3) into nitrite (NO2) and energy. Another aerobic bacteria then does the same thing but converts nitrite (NO2) into nitrate (NO3).

The ammonia in this case is the food source that the bacteria gets it energy from. The oxygen is simply an oxidizing agent that helps the bacteria accomplish this.

Anaerobic bacteria breathes in nitrogen from nitrate (NO3) and uses it to convert carbon food sources into nitrogen gas (N2) and energy.

What is oxidation?

Oxidation is a chemical reaction where electrons are transfered between two chemical compounds. The compounds that loses electrons is called the reducing agent, while the compound that gains electons is the oxidizing agent.

In aquariums, oxidation is responsible for converting ammonia into nitrate during nitrification. And also during the denitrifcation process where nitrate is converted into nitrogen gas.

Oxidation occurs often in nature.

What is an oxidizing agent?

An oxidizing agent is an element that attracts electrons from a compound. Oxygen is the most effective oxidizing agent and will typically be used when present.

However, there are other less effective oxidizing agents that can be used when there is no oxygen. For example, nitrogen is used as the oxidizing agent during the denitrification process.

How aerobic and anaerobic bacteria are different from each other

Aerobic and Anaerobic bacteria use similar processes to generate energy. However, there are some key differences that affect the fishkeeping hobby based on the environments these bacteria live in.

One nitrifies while the other denitrifies

Two types of aerobic bacteria convert ammonia into nitrate. Ammonia-oxidizing bacteria called nitrosomonas oxidize ammonia into nitrite. Then another aerobic bacteria called nitrobacter oxidizes nitrite into nitrate.

This is important to fishkeeping because ammonia and nitrite are toxic to fish is low concentrations (0.25ppm or higher). Nitrate is also toxic but only after it reaches concentrations above 40ppm or so.

For most fishkeepers, they will have to conduct water changes periodcally to lower the concentration of nitrates. However, aquariums that contain anaerobic bacteria maya not need to manually lower nitrate concentrations.

Anaerobic bacteria converts nitrate into nitrogen gas, which then floats to the surface of an aquarium and gasses off into the atmosphere.

What this means is that denitrifying bacteria reduce the need for water changes because they naturally lower nitrate concentrations.

They consume different food sources

Aerobic bacteria use energy from nitrogen as its food source. So during nitrification, the things fishkeepers want to get rid of (ammonia and nitrite) are the food sources for the bacteria.

During denitrification, the food source is primarily organic carbon compounds, such as fish waste, decaying plant material and waste from other microbial life living in the substrate.

They use different oxidizing agents

Aerobic bacteria use oxygen as an oxidizing agent to attract electrons from ammonia and nitrite. Whereas anaerobic bacteria use nitrogen from nitrate (and sometimes nitrite) to attract electrons from carbon food sources.

So during denitrification, the stuff fishkeepers want to get rid of (nitrate) is not the food source but rather the oxidizing agent that allows anaerobic bacteria to receive energy from its food sources.

Ryan Ferguson

Founder, Rooted Tank

Ryan Ferguson, the founder of Rooted Tank, started fishkeeping in 2019. He has continued to level-up his planted aquarium skills and wanted to share his journey and knowledge with other aquarists.


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