Nitrogen Cycle

Biology: AQA A Level, topic 5.4.1

Saprobionts

Saprobionts are microorganisms (e.g. bacteria and fungi) that feed on dead or decaying organic matter. Saprobionts play an important role in decomposition and in recycling nutrients.

Importance of nutrient recycling

Nutrient recycling is an important process in ecosystems because it allows the nutrients to be continuously reused.
Nutrients are recycled in the nitrogen cycle by saprobionts.

Saprobiont nutrition

Saprobionts secrete enzymes onto the dead organic matter. This is called extracellular digestion.
The enzymes break down large, insoluble molecules into smaller, soluble molecules.
These smaller molecules can then be absorbed by the saprobiont.
The saprobiont will then either store the molecules or use them in respiration.

Role in nitrogen cycle

Saprobionts are important in the ammonification stage of the nitrogen cycle.
Through extracellular digestion, saprobionts break down organic matter that contains nitrogen (e.g. urea, proteins, nucleic acids).
Ammonium ions are released as a result of extracellular digestion.
These ions then move onto the next stage of the nitrogen cycle: nitrification.

Nitrogen Cycle

The nitrogen cycle shows how nitrogen is transferred between organisms and their ecosystem. There are four steps in the nitrogen cycle, each of which involves bacteria:

1) Nitrogen fixation

Atmospheric nitrogen (N₂) is converted to ammonia (NH₃) by nitrogen-fixing bacteria in the soil.
Bacteria (e.g. Rhizobia) form a mutualistic relationship with leguminous plants by inhabiting their root nodules.
Leguminous plants (e.g. alfalfa, peas, beans) are a family of plants.
The bacteria provide a source of ammonium ions for the plants and the plant provides sugars.

2) Ammonification

When an organism dies or produces waste (e.g. faeces), it is decomposed by saprobionts.
This releases the nutrients contained inside.
Saprobionts decompose the organisms' biomass by extracellular digestion and inorganic ammonium ions are released into the soil.
Ammonification is a by-product of saprobiont nutrition.

3) Nitrification

Ammonium ions that are released from ammonification are converted into nitrates by bacteria in the soil.
Nitrifying bacteria (e.g. Nitrosomonas) firstly convert NH₄⁺ ions to nitrites.
Another bacteria (e.g. Nitrobacter) then convert nitrites to nitrates.

4) Denitrification

Denitrifying bacteria convert nitrates in the soil back into atmospheric nitrogen (N₂).
This takes place in anaerobic conditions (e.g waterlogged soils).

Nitrogen cycle

Test yourself on Nitrogen Cycle

Which of these are NOT activities of saprobionts?
Which of the following statements about saprobionts are true?
Stages of the Nitrogen Cycle
Which of the following is true of the nitrogen cycle?
Which stage in the nitrogen cycle converts atmospheric nitrogen into ammonia?

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