Nitrogen fixation is a natural biological process that converts inert atmospheric nitrogen gas (\(N_2\)) into chemically reactive forms, most notably ammonia (\(NH_3\)). Although nitrogen makes up nearly 78% of the Earth’s atmosphere, this gaseous form is inaccessible to most life due to the strength of its molecular bond. This conversion effectively “fixes” the abundant atmospheric nitrogen into a compound that plants and other organisms can absorb. This transformation is fundamental to sustaining life, providing the nitrogen building blocks required for complex biological molecules.
Converting Atmospheric Nitrogen
The difficulty in making atmospheric nitrogen usable stems from the triple covalent bond that joins the two nitrogen atoms in the \(N_2\) molecule. This bond is one of the strongest in nature, requiring immense energy to break. Biological nitrogen fixation overcomes this barrier using a highly specialized enzyme complex called nitrogenase.
The nitrogenase enzyme is responsible for catalyzing the reduction of dinitrogen gas to ammonia. This complex is a two-component metalloenzyme, most commonly containing molybdenum and iron at its active site, known as the FeMo-cofactor. The overall reaction is energetically demanding, requiring a substantial input of energy, typically the hydrolysis of 16 molecules of adenosine triphosphate (ATP) for every molecule of \(N_2\) converted.
The nitrogenase enzyme is extremely sensitive to molecular oxygen, which rapidly and irreversibly inactivates it. Therefore, the process must occur in an anaerobic environment. Nitrogen-fixing organisms have evolved various strategies to manage this paradox, as many require oxygen for their own energy generation. The final product of this microbial reduction is ammonia (\(NH_3\)), which quickly protonates to ammonium (\(NH_4^+\)) in soil or cells.
The Biological Fixers
Nitrogen fixation is restricted to a select group of prokaryotic microorganisms, including certain bacteria and archaea, collectively known as diazotrophs. These are the only organisms that possess the genetic machinery to synthesize the oxygen-sensitive nitrogenase enzyme. These fixers are categorized based on their lifestyle and relationship with plants.
One major category is symbiotic nitrogen fixation, which involves a mutually beneficial relationship, most famously between Rhizobium bacteria and plants in the legume family. The bacteria live inside specialized structures on the plant roots called root nodules. Within these nodules, the plant hosts a protein called leghemoglobin, which functions to tightly bind and regulate the oxygen concentration, maintaining the low-oxygen conditions necessary to protect the nitrogenase enzyme.
The other type is free-living nitrogen fixation, carried out by diazotrophs that exist independently in the soil or aquatic environments. Examples of these free-living fixers include aerobic bacteria like Azotobacter and anaerobic bacteria such as Clostridium. Cyanobacteria are also important free-living fixers, and some species develop specialized, thick-walled cells called heterocysts to create an anaerobic environment for nitrogenase.
Essential Role in Ecosystems
Nitrogen fixation is the primary natural pathway by which new, bioavailable nitrogen enters the biosphere. This process is the initial step in the global Nitrogen Cycle, fundamentally influencing the productivity of both terrestrial and aquatic ecosystems. The fixed nitrogen, in the form of ammonium, is then incorporated by plants and microorganisms to synthesize the complex molecules necessary for life.
Fixed nitrogen is a core component of amino acids, the building blocks of proteins and enzymes. It is also indispensable for the construction of nucleic acids (DNA and RNA), which carry genetic instructions. Without the continuous supply of fixed nitrogen from diazotrophs, plant growth would be severely limited, leading to a collapse of food webs.
Biological nitrogen fixation contributes approximately half of the bioavailable nitrogen on Earth. For contrast, the industrial Haber-Bosch process, which synthesizes ammonia for fertilizer production, requires extreme conditions of high heat (around 500 °C) and pressure (200-400 atmospheres) to force the inert nitrogen to react. Biological nitrogen fixation achieves the same chemical result under ambient temperatures and pressures, albeit at a high cellular energy cost, highlighting the sophistication of the microbial enzyme system.