Nitrogen is a fundamental building block for all life on Earth, forming components of proteins, nucleic acids like DNA and RNA, and other biomolecules. Despite its abundance, making up about 78% of the atmosphere, this gaseous form is largely unusable by most organisms. Life requires nitrogen in a more chemically reactive form, such as ammonia or nitrates, to synthesize its complex structures. This conversion process is performed by a select group of organisms, bridging the gap between atmospheric nitrogen and biological needs.
The Process of Nitrogen Fixation
Nitrogen fixation is the natural or industrial process that converts inert atmospheric nitrogen gas (N2) into more reactive nitrogen compounds, primarily ammonia (NH3). The atmospheric nitrogen molecule consists of two nitrogen atoms joined by a very strong triple covalent bond, which makes it chemically inert. Breaking this bond requires a significant amount of energy, approximately 941 kJ/mol, rendering N2 largely unreactive under ordinary environmental conditions.
Biological nitrogen fixation, or diazotrophy, is accomplished by specialized enzymes called nitrogenases, exclusively found in certain prokaryotes. These enzyme complexes catalyze the reduction of N2 to ammonia. This process consumes a substantial amount of energy, requiring about 16 ATP molecules for each N2 molecule converted to ammonia. This conversion is the primary way new, usable nitrogen enters both terrestrial and aquatic ecosystems, making it available for other forms of life to assimilate and build organic molecules. Without this process, the nitrogen cycle would halt, limiting the growth of most living organisms.
Key Nitrogen-Fixing Organisms
Only certain prokaryotic microorganisms can perform nitrogen fixation. These specialized organisms are categorized into two groups based on their lifestyle: free-living and symbiotic nitrogen fixers. Free-living diazotrophs, such as the aerobic Azotobacter in soil, and cyanobacteria like Anabaena and Nostoc in aquatic environments, operate independently. They convert atmospheric nitrogen into usable forms without associating with another organism, contributing fixed nitrogen directly to their surroundings.
Symbiotic nitrogen-fixing bacteria form mutually beneficial relationships with plants. A prominent example is the Rhizobium genus, which associates with leguminous plants, including peas, beans, alfalfa, and clover. These bacteria invade the root hairs of host plants, stimulating the formation of specialized structures called root nodules. Within these nodules, the bacteria convert gaseous nitrogen into ammonia, which the plant then utilizes for its growth. In return, the host plant provides the bacteria with carbohydrates, products of photosynthesis, which serve as an energy source for the fixation process.
This partnership allows legumes to thrive in nitrogen-poor soils and enriches the soil with fixed nitrogen when the plants decompose. Other symbiotic examples include Frankia associated with non-leguminous actinorhizal plants.
Why Most Organisms Cannot Fix Nitrogen
Most living organisms, including all animals, plants, and fungi, cannot fix atmospheric nitrogen. This limitation stems from their lack of the nitrogenase enzyme complex. This metalloenzyme is exclusively found in certain bacteria and archaea. The genes for nitrogenase are not present in the genomes of animals, plants, or fungi.
The nitrogenase enzyme is sensitive to oxygen, which irreversibly inactivates it. Aerobic organisms, including humans and most plants, constantly operate in oxygen-rich environments, making it challenging to maintain the strictly anaerobic conditions required for nitrogenase activity. While some nitrogen-fixing bacteria have evolved mechanisms to protect nitrogenase from oxygen, most organisms lack these adaptations.
Consequently, animals obtain their nitrogen by consuming plants or other animals that have already assimilated fixed nitrogen. Most plants rely on nitrogen compounds already present in the soil, supplied by nitrogen-fixing microorganisms. Organisms like humans, cows, oak trees, and mushrooms cannot perform nitrogen fixation.