Study Notes on Nitrogen Fixation

Introduction to Nitrogen Fixation

  • Nitrogen fixation is critical for converting atmospheric nitrogen into forms usable by living organisms.

1. Types of Nitrogen Fixation

  • Symbiotic Biological Nitrogen Fixation

    • Example: Rhizobia in legumes.
    • Important associations such as Frankia with alder trees.

  • Atmospheric Nitrogen Fixation

    • Occurs via lightning, which provides energy to break the nitrogen molecules.
    • Results in deposition of inorganic nitrogen.

  • Industrial Nitrogen Fixation

    • Refers to the production of fertilizers, creating various nitrogen inputs.

2. Comparison of Natural and Agricultural Nitrogen Fixation

  • Natural and agricultural biological nitrogen fixation are approximately equal, contributing significantly to terrestrial reactive nitrogen pools.
  • Fertilizer application can exceed natural nitrogen inputs, yet cultivation of nitrogen-fixing plants also adds nitrogen.

3. Energy Requirements for Nitrogen Fixation

  • Breaking the triple bond of N₂ requires substantial energy.
  • Example: Requires 5.5 barrels of oil or a couple of metric tons of coal to produce 1 metric ton of ammonia.
  • Biological nitrogen fixation derives energy from organic carbon sources, with rhizobium feeding on plant exudates.

4. Nitrogen-Fixing Organisms

  • Types of Nitrogen Fixers

    • Bacteria, Cyanobacteria, & Frankia (Actinobacteria): Only these organisms can fix nitrogen; fungi and algae cannot.
    • Cyanobacteria: Autotrophs; can photosynthesize and fix nitrogen.

  • Associative Nitrogen Fixation

    • Microbes fix nitrogen in loose association with plants without forming nodules.
    • Commonly occurs in the rhizosphere where plants secrete carbon-rich compounds.

5. Mechanisms of Nitrogen Fixation

5.1 Nitrogenase Enzyme
  • Enzyme complex responsible for fixing nitrogen; consists of two proteins:
    • Dinitrogenase reductase (iron protein)
    • Iron-molybdenum complex
  • Functions require micronutrients like molybdenum; deficiency impacts nitrogen fixation.
5.2 Production and Regulation of Nitrogenase

  • Production is energetically expensive; nitrogenase can constitute up to 10-40% of cell proteins.

  • Regulation:

    • The enzyme is produced constitutively but is repressed by the presence of ammonia in the environment (end product inhibition).
    • In high ammonia levels, the organism ceases nitrogenase production.

  • The enzyme shows sensitivity to oxygen:**

    • Isopropyl Molybdenum and iron components have differing stabilities in oxygen.
    • Must protect against oxidation for effective function.

6. Techniques to Protect Nitrogenase

  • Avoidance: Anaerobes or facultative anaerobes cease nitrogen fixation in the presence of oxygen.
  • Respiratory Protection: Increase aerobic respiration to reduce available oxygen.
  • Specialized Cells: Cyanobacteria form heterocysts to isolate nitrogenase from oxygen.
  • Slime Production: Some organisms use extracellular polysaccharides as diffusion barriers.
  • Conformational Protection: Proteins that change nitrogenase orientation help protect against oxygen.

7. Efficiency of Nitrogen Fixation

  • Efficiency varies by method:
    • Free-living nitrogen fixation: 1-10 kg per hectare per year.
    • Associative fixation: up to 33 kg per hectare per year due to additional carbon from plants.
    • Symbiotic fixation: higher rates due to direct plant support, e.g., alfalfa fixing 230 kg per hectare.

8. Role of Cyanobacteria and Algae in Soil Development

  • Cyanobacteria and algae promote soil formation and preservation via nitrogen-fixing capabilities.
  • Photosynthesis and nitrogen storage are essential for nutrient cycling and soil health.
8.1 Characteristics of Cyanobacteria
  • Prokaryotes with chlorophyll a, capable of photosynthesis and nitrogen fixation.
  • Capable of forming heterocysts for nitrogenase protection from oxygen.
  • Environmental tolerance: Sensitive to temperature extremes; die rapidly under drought or extreme heat conditions.