Nitrogen is an essential element for living organisms. It is needed to make proteins, nucleic acids and ATP. The nitrogen cycle shows how this element is transferred between different parts of an ecosystem. In this article we will look at the four major steps and the microorganisms involved in the processes.
The nitrogen cycle
The four main steps in the nitrogen cycle are as follows:
- Nitrogen fixation – bacteria found in the root nodules of leguminous plants are capable of fixing nitrogen gas from the atmosphere into ammonia. When ammonia dissolves in water it forms ammonium ions which can be taken up by plants. Plants synthesise nitrogen-containing compounds e.g. proteins.
- Ammonification – saprobionts (see below) decompose dead plants and animals, as well as waste products from animals (faeces and urine). They produce ammonia, which again dissolves to ammonium ions.
- Nitrification – nitrifying bacteria in the soil convert the ammonium ions into nitrites, then convert the nitrites into nitrates. These can be taken back up by plants.
- Denitrification – under anaerobic conditions such as waterlogged soil, denitrifying bacteria are able to carry out a form of respiration that converts nitrates into nitrogen gas.
Saprobionts and mutualistic symbiotic relationships
Saprobionts are decomposers involved in the ammonification step. They are often fungi, but can also be bacteria. They decompose the proteins, nucleic acids and urea found in dead and waste material by secreting enzymes onto it then absorbing the products. This is a form of extracellular digestion.
The bacteria in the root nodules of leguminous plants have a mutualistic symbiotic relationship with the plants. The bacteria are able to get compounds such as carbohydrates and amino acids from the plants, and the plants are able to get nitrogen compounds from the bacteria. Both are benefiting from the association.
- Nitrogen is needed to synthesise proteins, nucleic acids and ATP.
- There are four main steps in the nitrogen cycle: nitrogen fixation, ammonification, nitrification, and denitrification.
- Saprobionts are decomposers involved in the ammonification step.