State the roles of microorganisms in the nitrogen cycle, limited to: decomposition, nitrification, nitrogen fixation and denitrification (generic names of individual bacteria, e.g. Rhizobium, are not required)

Microorganisms play essential roles in the nitrogen cycle, facilitating the transformation of nitrogen through various processes. Here’s an overview of their contributions, focusing on decomposition, nitrification, nitrogen fixation, and denitrification.

1. Decomposition

• Role of Microorganisms: Decomposers, including bacteria and fungi, break down dead plant and animal matter as well as waste products. During this process, they convert organic nitrogen found in proteins and nucleic acids back into ammonium ions (NH₄⁺) through a process known as ammonification. This transformation is crucial as it recycles nitrogen back into the soil, making it available for subsequent processes in the nitrogen cycle.

2. Nitrification

• Role of Microorganisms: Nitrification is a two-step aerobic process carried out by specialized bacteria:
  • First Step: Ammonium ions (NH₄⁺) are oxidized to nitrite (NO₂⁻) by ammonia-oxidizing bacteria. This step is critical as it converts ammonia, which can be toxic at high concentrations, into a less harmful form.
  • Second Step: Nitrite is further oxidized to nitrate (NO₃⁻) by nitrite-oxidizing bacteria. Nitrate is a form of nitrogen that plants can readily absorb and utilize for growth.

These microbial processes are vital for converting ammonia into forms that are usable by plants, thus facilitating the flow of nitrogen through the ecosystem.

3. Nitrogen Fixation

• Role of Microorganisms: Nitrogen fixation involves converting atmospheric nitrogen gas (N₂) into ammonia (NH₃), making it available for biological use. This process is primarily carried out by certain bacteria:
  • Biological Nitrogen Fixation: Some bacteria live symbiotically with plants (especially legumes) and fix nitrogen in root nodules, while others exist freely in the soil or water.
  • Abiotic Nitrogen Fixation: Although not microbial in nature, lightning also contributes to nitrogen fixation by converting N₂ into nitrates during storms.

These processes are crucial because they introduce biologically available nitrogen into ecosystems, supporting plant growth and development.

4. Denitrification

• Role of Microorganisms: Denitrification is the process by which nitrates (NO₃⁻) are converted back into nitrogen gas (N₂), completing the nitrogen cycle. This anaerobic process is carried out by denitrifying bacteria that use nitrates as an electron acceptor during respiration in low-oxygen environments.

Denitrification helps to reduce excess nitrates in the soil and prevents the accumulation of nitrogen compounds that could lead to environmental issues such as eutrophication.