Nitrogen-fixing bacteria are microscopic organisms that convert atmospheric nitrogen gas into forms plants can readily use, a process called nitrogen fixation. They are important for nearly all life on Earth, providing a nutrient otherwise inaccessible to most organisms. Without their activity, the production of proteins, DNA, and other essential molecules would be severely limited.
The Nitrogen Cycle’s Role
Nitrogen is a fundamental element, making up about 78% of Earth’s atmosphere, yet atmospheric nitrogen gas (N₂) is chemically inert and cannot be directly utilized by plants or animals. The nitrogen cycle describes how nitrogen moves between the atmosphere, soil, and living organisms.
This transformation is carried out primarily by nitrogen-fixing bacteria, which deposit the newly converted nitrogen compounds into soils and aquatic systems. Once fixed, nitrogen becomes available for plants to absorb through their roots, incorporating it into their tissues. This process allows nitrogen to enter the food web, as animals then obtain nitrogen by consuming plants or other animals. Other bacteria further process nitrogen compounds, eventually returning nitrogen gas to the atmosphere, completing the cycle.
Types of Nitrogen-Fixing Bacteria
Nitrogen-fixing bacteria can be broadly categorized into two main groups based on their lifestyle: symbiotic and free-living. Symbiotic bacteria establish a mutually beneficial relationship with host plants. A well-known example is the genus Rhizobium, which forms associations with leguminous plants like peas, beans, clover, and soybeans.
Free-living bacteria, in contrast, do not require a host plant and perform nitrogen fixation independently in their environment. These bacteria are commonly found in soil and aquatic habitats. Examples of free-living nitrogen-fixers include certain cyanobacteria, such as Anabaena and Nostoc, as well as genera like Azotobacter, Beijerinckia, and Clostridium.
The Science of Nitrogen Fixation
The core process of biological nitrogen fixation involves the conversion of atmospheric nitrogen (N₂) into ammonia (NH₃). This biochemical reaction is catalyzed by a specialized enzyme complex known as nitrogenase. Nitrogenase is exclusively found in prokaryotes and is highly sensitive to molecular oxygen, which can irreversibly inactivate it.
The conversion of one molecule of nitrogen gas into two molecules of ammonia requires energy input, consuming about 16 molecules of ATP for every molecule of nitrogen fixed. To protect the oxygen-sensitive nitrogenase enzyme, nitrogen-fixing bacteria employ various strategies. In symbiotic relationships, such as those involving Rhizobium in legume root nodules, a protein called leghemoglobin helps to scavenge oxygen, maintaining the low-oxygen conditions necessary for nitrogenase activity. Free-living bacteria may reduce oxygen concentration through high respiration rates, create physical barriers, or modify the enzyme itself to be more oxygen-tolerant.
Benefits for Plants and Agriculture
Nitrogen-fixing bacteria improve plant growth and agricultural productivity. In symbiotic relationships, particularly with legumes, these bacteria reside in specialized structures on the plant roots called nodules, converting nitrogen into a usable form for the host plant. This exchange benefits both organisms: the bacteria receive carbohydrates from the plant as an energy source, while the plant gains access to usable nitrogen.
This natural process reduces the reliance on synthetic nitrogen fertilizers, contributing to more sustainable agricultural practices. By incorporating legumes into crop rotation systems, farmers can naturally enrich the soil with nitrogen, improving fertility for subsequent crops without additional chemical inputs. The nitrogen fixed by these bacteria, ranging from 25 to 500 pounds per acre annually depending on the legume and conditions, directly supports higher crop yields and promotes overall ecosystem health.