5.4.3 Microorganisms Role in Recycling Minerals

Nitrogen Cycle

The nitrogen cycle relies on various microorganisms to convert nitrogen between different forms:

1. Nitrogen Fixation:

• Nitrogen-fixing bacteria (e.g., Rhizobium in legume root nodules and Azotobacter in soil) convert atmospheric nitrogen (N₂) into ammonia (NH₃).
• Symbiotic Relationship: In legumes, Rhizobium receives carbohydrates from the plant and supplies nitrogen compounds in return.

2. Ammonification:

• Saprobionts decompose organic matter (e.g., dead organisms, faeces) into ammonium ions (NH₄⁺).

3. Nitrification:

• Nitrifying bacteria (e.g., Nitrosomonas and Nitrobacter) convert ammonium into nitrates:
• Nitrosomonas: Ammonium ions → Nitrites (NO₂⁻).
• Nitrobacter: Nitrites → Nitrates (NO₃⁻).
• Nitrates are absorbed by plants for growth.

4. Denitrification:

• Denitrifying bacteria (e.g., Pseudomonas) convert nitrates (NO₃⁻) back into nitrogen gas (N₂) under anaerobic conditions (e.g., waterlogged soils).
• This reduces soil fertility.

Phosphorus Cycle

Microorganisms also recycle phosphorus, essential for ATP, DNA, and membranes:

5. Weathering of Rocks:

• Phosphate ions $(PO₄³⁻)$ are released from rocks by weathering.
• Phosphates are absorbed by plants via their roots.

6. Decomposition:

• Saprobionts break down dead plants, animals, and waste, releasing phosphate ions back into the soil or water.

7. Sedimentation:

• In aquatic ecosystems, excess phosphates can settle as sediments, forming new phosphate-rich rocks over time.

Importance of Microorganisms

• Recycling nutrients allows ecosystems to remain productive.
• Decomposition prevents the build-up of organic matter and releases essential minerals.
• Symbiosis (e.g., Rhizobium in legumes) boosts agricultural productivity naturally.