Nitrogen fixation is a crucial process that transforms atmospheric nitrogen into forms plants can use. Carried out by specialized microorganisms, it's the primary natural source of new nitrogen in ecosystems, contributing 200-300 million metric tons annually.

Symbiotic fixation involves partnerships between microbes and plants, while non-symbiotic fixation is performed by free-living organisms. The nitrogenase enzyme complex is key to this process, catalyzing the energy-intensive conversion of N₂ to NH₃ under specific environmental conditions.

Biological Nitrogen Fixation

Biological nitrogen fixation significance

• Process converting atmospheric nitrogen (N₂) to biologically available forms carried out by specialized microorganisms called diazotrophs transforms N₂ to ammonia (NH₃)
• Primary natural source of new nitrogen in terrestrial ecosystems contributes approximately 200-300 million metric tons of nitrogen annually
• Reduces dependence on synthetic nitrogen fertilizers improves soil fertility and crop yields (soybeans, alfalfa)
• Supports ecosystem productivity enables colonization of nitrogen-poor environments (arctic tundra, desert soils)

Symbiotic vs non-symbiotic fixation

• Symbiotic nitrogen fixation involves mutualistic relationship between diazotrophs and host plants occurs in specialized structures (nodules) on plant roots (Rhizobium-legume, Frankia-actinorhizal)
• Provides direct nitrogen supply to host plant generally more efficient than non-symbiotic fixation
• Non-symbiotic fixation performed by free-living diazotrophs occurs in soil, water, and on plant surfaces (Azotobacter, Clostridium, cyanobacteria)
• Fixed nitrogen released into environment less efficient but more widely distributed

Nitrogenase enzyme in fixation

• Key enzyme complex catalyzes reduction of N₂ to NH₃ composed of two protein components:
  1. Dinitrogenase reductase (iron protein)
  2. Dinitrogenase (molybdenum-iron protein)
• Reaction catalyzed: $N₂ + 8H⁺ + 8e⁻ + 16ATP → 2NH₃ + H₂ + 16ADP + 16P_i$
• Highly sensitive to oxygen requires anaerobic conditions or protective mechanisms
• High energy demand (ATP consumption)
• Regulation through transcriptional control and post-translational modification

Environmental factors of fixation

• Temperature increases with temperature up to an optimum varies by organism (25-35℃ for many soil bacteria)
• Soil moisture affects oxygen availability and microbial activity waterlogging can inhibit fixation
• Oxygen concentration inhibits nitrogenase activity aerobic diazotrophs employ protective mechanisms (slime layers, heterocysts)
• Soil pH influences microbial community composition most nitrogen-fixing bacteria prefer neutral to slightly acidic soils (pH 6-7)
• Nutrient availability phosphorus and molybdenum essential for nitrogenase function iron required for electron transport
• Light intensity affects energy availability for fixation in cyanobacteria (marine Trichodesmium)
• Carbon availability provides energy source influences microbial activity and population dynamics (root exudates, soil organic matter)