Diazotrophs are microscopic organisms, primarily bacteria and archaea, that convert inert atmospheric nitrogen into a form usable by other living things. This unique ability is fundamental to sustaining life on Earth. Without them, a crucial nutrient for all life would be severely limited, impacting ecosystems globally.
The Nitrogen Fixation Process
Atmospheric nitrogen (N2) is abundant, making up about 78% of the air we breathe. However, its strong triple bond makes it unusable by most organisms. Diazotrophs perform nitrogen fixation, converting N2 into ammonia (NH3) through biological nitrogen fixation (BNF).
This conversion relies on the enzyme nitrogenase, found only in diazotrophs. Nitrogenase is sensitive to oxygen, which can damage it. Diazotrophs protect this enzyme through strategies like living in low-oxygen environments, high respiration rates, or specialized structures like heterocysts in cyanobacteria. The N2 to ammonia conversion is energy-intensive, requiring ATP and electrons. Nitrogenase enzymes contain iron-molybdenum or iron-vanadium cofactors for activity.
Diverse Habitats and Partnerships
Diazotrophs are widespread, inhabiting diverse environments and forming various associations. Some are free-living, existing independently. Examples include anaerobic Clostridium in oxygen-poor soils and aerobic Azotobacter vinelandii in oxygen-rich environments. Photosynthetic cyanobacteria, such as Anabaena cylindrica and Nostoc commune, are also free-living in aquatic and terrestrial ecosystems.
Other diazotrophs form mutualistic partnerships. A key example is Rhizobia bacteria, which form symbiotic relationships with leguminous plants like peas, beans, and clover. These bacteria live in root nodules, where the plant provides a low-oxygen environment and nutrients, and the bacteria supply fixed nitrogen. Leghemoglobin in nodules helps regulate oxygen.
Frankia species form similar nodules on non-leguminous woody plants, including Alnus and Casuarina. Some diazotrophs also associate with grasses, residing on or within their roots, as seen with Azospirillum in maize and sugarcane.
Their Vital Role in Ecosystems
Diazotrophs are important to the health and productivity of nearly all ecosystems due to their role in the global nitrogen cycle. Nitrogen is a fundamental building block for proteins, nucleic acids, and other biomolecules. Its scarcity in a usable form often limits biological growth. By converting atmospheric nitrogen into ammonia, diazotrophs supply this limiting nutrient, supporting plant growth and the food web.
In agriculture, diazotrophs are important as a natural, sustainable nitrogen source. Their activity reduces the need for synthetic fertilizers, which are energy-intensive and can cause pollution. The fixed nitrogen enhances crop yield and promotes plant development. Many diazotrophs also produce plant growth-promoting substances like phytohormones and help solubilize other nutrients like phosphorus. This improves nutrient uptake, enhances plant resilience, and supports sustainable agricultural practices.