What Is the Process of Nitrogen Fixation by Bacteria?

Stanislav M.
Mar 5
3 min read

Updated: Mar 10

What Is the Process of Nitrogen Fixation by Bacteria?

Nitrogen fixation by bacteria is a remarkable biological process that transforms inert atmospheric nitrogen gas (N₂) into bioavailable ammonia (NH₃), fueling plant growth and the global food chain. Discovered over a century ago, this process—performed exclusively by certain prokaryotes—provides an estimated 40% of the world's crop nitrogen needs without synthetic inputs.

Without bacterial fixation, modern agriculture would collapse under fertilizer dependency. This natural "factory" not only supports legumes and cereals but also enhances soil fertility for sustainable farming.

The Nitrogen Challenge: Why Fixation Is Essential

Earth's atmosphere contains about 78% nitrogen, yet N₂'s triple bond (bond energy ~945 kJ/mol) makes it inaccessible to most organisms. Plants require reactive forms like nitrate (NO₃⁻), ammonium (NH₄⁺), or organic N from soil.

Synthetic fertilizers from the Haber-Bosch process mimic fixation but consume massive energy (1-2% of global supply) and cause pollution. Bacterial fixation offers a green alternative, recycling atmospheric N₂ efficiently.

Key Players: Types of Nitrogen-Fixing Bacteria

Diazotrophs (N₂-fixing microbes) include over 100 species, classified by habitat and symbiosis.

Symbiotic Diazotrophs

These form mutualistic relationships, primarily with legumes but also sugarcane and cereals.

Rhizobia (Rhizobium, Bradyrhizobium, Sinorhizobium): Nodulate roots of peas, soybeans, alfalfa. Fix 100-300 kg N/ha/year.
Frankia: Tree symbionts (e.g., alders) for actinorhizal plants in poor soils.
Anabaena: Cyanobacteria in Azolla for rice paddies.

Free-Living and Associative Diazotrophs

Independent or loosely associated with roots.

Azotobacter/Azomonas: Aerobic soil bacteria; Azotobacter vinelandii protects nitrogenase with high respiration.
Clostridium: Anaerobic soil fixers.
Azospirillum: Rhizosphere colonizers for maize/wheat; fix 20-50 kg N/ha + hormones.
Derxia/Beijerinckia: Acid-tolerant for tropical soils.

The Biochemical Heart: Nitrogenase Complex

Nitrogenase is a metalloenzyme unique to diazotrophs, absent in eukaryotes. It evolved ~2.5 billion years ago, enabling life on a N₂-rich planet.

Structure

Component I (MoFe protein): α₂β₂ tetramer with P-cluster (Fe₈S₇), FeMo-co (MoFe₇S₉C-homocitrate), and M-cluster for N₂ binding.
Component II (Fe protein): γ dimer transfers 8 electrons stepwise.

Variants exist: vanadium (VFe) or iron-only nitrogenases for low-Mo conditions.

Overall Reaction

\ceN2+8H++8e−+16ATP−>2NH3+H2+16ADP+16Pi

One H₂ is obligatory "waste," lowering efficiency to ~60%.

Detailed Step-by-Step Process

Step 1: Substrate Access and Protection

N₂ diffuses into cells/microzones. Nitrogenase demands anaerobiosis; symbionts use leghemoglobin (pink nodules), free-livers respire rapidly or form cysts.

Step 2: Activation and Electron Transfer

Fe protein docks to MoFe, hydrolyzing 2 ATP per electron. Cycle: Fe protein (reduced) → MoFe → Fe protein (oxidized).

Electrons from ferredoxin/flavodoxin via nitrogen fixation regulatory proteins (Nif). Metals (Mo, Fe, S) shuttle reductions.

Step 3: N₂ Reduction Pathway

N₂ binds FeMo-co end-on. Lowe-Thorneley model: 8 e⁻/8 H⁺ + H₂ release → intermediates (N₂Hₙ) → 2NH₃.

Recent cryo-EM reveals hybrid steps, resolving decades-old debates.

Step 4: Product Assimilation

NH₃ + glutamate → glutamine (GS/GOGAT cycle). Exported to plant or stored as poly-β-hydroxybutyrate in bacteria.

Symbiotic Process: Legume-Rhizobium Partnership

Flavonoid signaling: Root exudates activate bacterial nod genes → Nod factors (chitin oligomers + acyl chain).
Infection thread: Curling root hairs → bacteria invade cortex.
Nodule organogenesis: Cortical divisions form nodule; bacteria become bacteroids in symbiosomes.
Fixation zone: Leghemoglobin maintains 10-40 nM O₂; plant malate fuels ATP.
N feedback: High plant N shuts down fixation (N feedback autoregulation).

Yields: Soybeans fix 150-250 kg N/ha; residues enrich rotations.

Free-Living Process: Soil and Rhizosphere Dynamics

Colonization: Motile bacteria reach roots via chemotaxis.
Microaerobic niches: Biofilms or aggregates exclude O₂.
Carbon fueling: Exudates (10-20% photosynthate) drive high respiration.
Release: 30-50% fixed N exuded as NH₄⁺; rest for bacterial growth.

Azospirillum boosts maize yields 10-30% via N + IAA/phytohormones.

Regulation and Limitations

Nif genes: 20+ clustered; Ntr system senses N status.
O₂ sensitivity: Leghemoglobin, respiratory protection, conformational protection.
Mo requirement: Uptake genes essential.
Energy cost: Limits to 1-5% total N in non-legumes.

Climate/stress reduces rates; inoculants help.

Agronomic Applications and Innovations

Inoculants: Peat/sticker formulations; e.g., Rhizobium for pulses, Azospirillum for millets.
Co-inoculation: N-fixer + P-solubilizer boosts 20-50% yields.
Engineering: Extend to cereals via nif genes (e.g., Symbiotic Engineering).
Benefits: Cut N fertilizer 25-100%, lower GHG, improve soil microbiome.