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:
- Dinitrogenase reductase (iron protein)
- 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)