Diazotrophs are a specific group of microorganisms, including certain bacteria and archaea, that are capable of converting nitrogen from the atmosphere into a form usable by other living things. The term “diazotroph” refers to organisms that can grow without needing external sources of fixed nitrogen. These microbes are widespread and act as natural nitrogen factories, taking the abundant but inert nitrogen gas (N2) from the air and transforming it into ammonia (NH3), a compound that plants can absorb and use for growth.
The Process of Nitrogen Fixation
The atmosphere is approximately 78% nitrogen, but this nitrogen exists as a gas (N2) held together by a strong triple bond, rendering it inaccessible to most organisms. Diazotrophs perform a process called biological nitrogen fixation to break this bond and convert atmospheric nitrogen into ammonia. This conversion is carried out by a specialized enzyme called nitrogenase, which is highly sensitive to oxygen.
To overcome this challenge, diazotrophs have developed various strategies to protect the nitrogenase enzyme from oxygen’s effects. Some, for instance, live in low-oxygen environments or consume oxygen so rapidly that it doesn’t interfere with the fixation process. This biological process is the primary way that new, usable nitrogen is introduced into ecosystems.
Types and Habitats of Diazotrophs
Diazotrophs are categorized based on their lifestyle. One major group consists of free-living diazotrophs, which function independently in their environment. These include organisms like Azotobacter, which lives in the soil, and various types of cyanobacteria found in aquatic systems. Cyanobacteria, sometimes called blue-green algae, can perform photosynthesis and fix nitrogen, though some species have specialized cells called heterocysts to shield the nitrogenase enzyme from oxygen.
Another significant group is the symbiotic diazotrophs, which form mutually beneficial relationships with host organisms. The most well-known example is the partnership between Rhizobium bacteria and leguminous plants such as peas, beans, and soybeans. These bacteria colonize the plant’s roots and form nodules, creating an oxygen-controlled environment for nitrogen fixation. The plant provides the bacteria with carbohydrates for energy, and in return, the bacteria supply the plant with fixed nitrogen.
Ecological Significance
Diazotrophs are a primary source of new nitrogen in almost every ecosystem, from terrestrial forests to open oceans. In many natural environments, the availability of usable nitrogen is a limiting factor for productivity. By converting atmospheric nitrogen into a bioavailable form, diazotrophs provide the nutrient that fuels the growth of primary producers like plants and algae.
This input of nitrogen supports entire food webs. As plants are consumed by herbivores, the nitrogen is transferred up through the trophic levels. The activity of diazotrophs is a component of the global nitrogen cycle, ensuring that this element is continuously circulated to support life. Their presence in diverse habitats, including salt marshes and coral reefs, highlights their impact on global biodiversity.
Agricultural Applications
The ability of diazotrophs to enrich soil with nitrogen has significant applications in agriculture. For centuries, farmers have utilized practices like crop rotation with legumes to improve soil fertility, unknowingly harnessing symbiotic nitrogen fixation. Today, this process is applied through biofertilizers, which are products containing living microorganisms. These microbial inoculants, containing diazotrophs like Rhizobium and Azospirillum, can be added to soils or coated onto seeds to enhance the nitrogen supply to crops.
The use of diazotroph-based biofertilizers offers a sustainable alternative to synthetic nitrogen fertilizers. The manufacturing of synthetic fertilizers is an energy-intensive process that contributes to greenhouse gas emissions, and their overuse can lead to environmental issues like water pollution. By leveraging biological nitrogen fixation, agricultural systems can reduce their reliance on these synthetic inputs. This approach helps lower economic costs for farmers and promotes healthier soil ecosystems.