Our planet’s atmosphere is rich in nitrogen, an element required for all life. This atmospheric nitrogen exists as a gas (N2) that most living things cannot use directly. A group of microorganisms, known as diazotrophs, can take this unusable atmospheric nitrogen and convert it into ammonia, a form that plants and other organisms can absorb. These organisms are bacteria and archaea that perform a process called biological nitrogen fixation.
This capability makes diazotrophs foundational to nearly every ecosystem. By converting atmospheric nitrogen into a bioavailable form, they supply a nutrient that would otherwise be severely limited. This natural fertilization supports the growth of plants, which form the base of most food webs, impacting the productivity of both natural and agricultural systems.
The Process of Nitrogen Fixation
The conversion of atmospheric nitrogen (N2) into ammonia (NH3) is an energetically demanding task. The two nitrogen atoms in an N2 molecule are held together by a strong triple bond, and breaking this bond requires a substantial amount of energy. Diazotrophs accomplish this using a specialized enzyme complex called nitrogenase.
The nitrogenase enzyme complex works by systematically adding electrons and protons to the N2 molecule until the triple bond is broken and two molecules of ammonia are formed. This reaction is energy-intensive, consuming large quantities of adenosine triphosphate (ATP), the cell’s primary energy currency. A cell must expend a significant portion of its metabolic energy to convert a single molecule of N2.
A requirement for nitrogenase to function is the absence of oxygen, which can irreversibly damage the enzyme. Diazotrophs have evolved various strategies to protect their nitrogenase from oxygen’s harmful effects. Some live in oxygen-poor environments, like soils or sediments, while others create specialized internal conditions to shield the enzyme.
Types of Diazotrophic Organisms
Diazotrophs are categorized into two main groups based on their lifestyle: free-living and symbiotic. Free-living diazotrophs function independently in their environment, fixing nitrogen for their own cellular needs. This group includes bacteria found in diverse habitats, from soil and water to the surfaces of plant roots.
A prominent example of free-living diazotrophs is the genus Azotobacter, which resides in neutral to alkaline soils. In aquatic environments, cyanobacteria are significant nitrogen fixers. These photosynthetic bacteria, also called blue-green algae, are found in both freshwater and marine systems. Other free-living examples include anaerobic bacteria like Clostridium and facultative anaerobes such as Klebsiella.
Symbiotic diazotrophs form mutually beneficial relationships with host organisms, primarily plants. The most well-known example is the partnership between bacteria of the Rhizobium genus and legumes, such as peas, beans, and clover. These bacteria invade the plant’s roots, prompting the formation of specialized structures called root nodules. Inside these nodules, the plant provides the bacteria with energy from photosynthesis and a controlled, low-oxygen environment. In return, the rhizobia supply the plant with a steady source of ammonia.
This relationship is not exclusive to legumes. Bacteria from the genus Frankia form similar nitrogen-fixing nodules on the roots of actinorhizal plants, which includes alder and bayberry trees. Some cyanobacteria form symbiotic associations with a variety of organisms, including lichens, ferns, and cycads.
Ecological and Agricultural Significance
The ecological role of diazotrophs is apparent in environments where nitrogen is scarce. They often act as pioneer organisms, colonizing nitrogen-poor soils and enriching them over time. This enrichment allows other plants to establish themselves, initiating ecological succession. By providing this new source of nitrogen, diazotrophs support the entire food web.
In marine environments, diazotrophic cyanobacteria are important to ocean productivity and the global carbon cycle. They provide nitrogen to the marine food web and contribute to the biological carbon pump. When these organisms die, they sink to the deep ocean, taking their stored carbon with them and sequestering it from the atmosphere. This process has implications for global climate regulation.
From an agricultural perspective, biological nitrogen fixation by diazotrophs contrasts with the industrial Haber-Bosch process. This method is used to produce synthetic nitrogen fertilizers by combining atmospheric nitrogen and hydrogen under immense pressure and high temperatures, consuming vast amounts of fossil fuels. While important for modern food production, it carries a significant energy cost and contributes to environmental issues like greenhouse gas emissions and water pollution from fertilizer runoff.
Farmers can leverage diazotrophs to enhance soil fertility sustainably. Practices like crop rotation, where nitrogen-fixing legumes are planted in alternation with other crops, help replenish soil nitrogen. Farmers can also use biofertilizers, which are inoculants containing live diazotrophic bacteria. Applying these to seeds or soil can boost the land’s nitrogen-fixing capabilities, reducing the need for synthetic fertilizers and minimizing the environmental impact of farming.