The key difference between Azotobacter and Rhizobium is that Azotobacter is a free-living nitrogen-fixing bacterium present in the soil, while Rhizobium is a symbiotic nitrogen-fixing bacteria that form a mutually beneficial association with legume plants.
Nitrogen fixation is the process that converts free atmospheric nitrogen into readily available more reactive nitrogen compounds such as ammonia, nitrates, or nitrites in the soil. Soil microorganisms, especially soil bacteria, carry out nitrogen fixation. Nitrogen-fixing microorganisms are mainly two types as free-living (non-symbiotic) and mutualistic (symbiotic) microorganisms. Azotobacter and Rhizobium are two types of nitrogen-fixing bacteria. Azotobactor is a free-living nitrogen-fixing bacterium, while Rhizobium is a symbiotic nitrogen-fixing bacterium.
1. Overview and Key Difference
2. What is Azotobacter
3. What is Rhizobium
4. Similarities Between Azotobacter and Rhizobium
5. Side by Side Comparison – Azotobacter vs Rhizobium in Tabular Form
What is Azotobacter?
Azotobacter is a free-living nitrogen-fixing bacterium found in the soil. Dutch microbiologist and botanist Martinus Beijerinck discovered and described the first bacterium Azotobacter chroococcum of this genus. They are usually motile and oval or spherical-shaped. They also form thick-walled cysts and may produce large quantities of capsular slime. Azotobacter bacteria are gram-negative and are found in neutral and alkaline soils or water. These bacteria are aerobic and free-living soil microbes. Azotobacter plays an important role in the nitrogen cycle in nature. They fix inaccessible atmospheric N2 into accessible forms for plants and involve in N2 fixation. Moreover, humans use Azotobacter to produce biofertilizers, food additives, and some biopolymers.
Nitrogenase is the most important enzyme in nitrogen fixation. Species of Azotobacter have several types of nitrogenase. The basic one is molybdenum-iron nitrogenase. The alternative types contain vanadium and iron. Vanadium nitrogenase is more active than Mo-Fe nitrogenase at low temperatures. The importance of these bacteria is they are not only playing an important in N2 fixation; they also synthesize biologically active substances. These active substances include phytohormones such as auxins that stimulate plant growth.
What is Rhizobium?
Rhizobium is a symbiotic nitrogen-fixing bacterium that forms a mutually beneficial symbiotic association with legume plants. Rhizobium bacteria belong to the genus Rhizobium. They are gram-negative, rod-shaped soil bacteria that fix atmospheric nitrogen. The Dutch microbiologist Martinus Beijerinck was the first to isolate and cultivate this microorganism from the nodules of legumes in 1888.
The Rhizobium species form an endosymbiotic nitrogen-fixing association with roots of legumes and Parasponia. These bacteria colonize plant cells and form root nodules. They convert atmospheric nitrogen into ammonia by using an enzyme called nitrogenase. This entire process provides organic nitrogenous compounds such as glutamine or ureides to the plant. The plant, in turn, provides the bacteria with organic compounds made by photosynthesis. Furthermore, Rhizobium is capable of solubilizing phosphorous.
What are the Similarities Between Azotobacter and Rhizobium?
- Azotobacter and Rhizobium are two important soil bacteria.
- Both belong to the phylum Proteobacteria.
- They are N2 fixing bacteria.
- Both have nitrogenase enzyme.
- They are both motile.
- Both can be used as biofertilizers.
What is the Difference Between Azotobacter and Rhizobium?
Azotobacter is a free-living nitrogen-fixing bacterium inhabiting the soil. On the other hand, Rhizobium is a symbiotic nitrogen-fixing bacterium that forms a mutually beneficial association with legume plants. So, this is the key difference between Azotobacter and Rhizobium. Moreover, Azotobacter is oval or spherical shaped. In contrast, Rhizobium is rod-shaped. Thus, this is another significant difference between Azotobacter and Rhizobium. Furthermore, Azotobacter belongs to the class Gammaproteobacteria, while Rhizobium belongs to the class Alphaproteobacteria.
The below infographic lists the differences between Azotobacter and Rhizobium in tabular form.
Summary – Azotobacter vs Rhizobium
N2 fixation is the process by which atmospheric nitrogen is converted by either a natural or an industrial means to form nitrogenous compounds such as ammonia, nitrates, or nitrites. Biological N2 fixation is carried out by specialized prokaryotes such as soil bacteria. It was first discovered by Dutch microbiologist Martinus Beijerinck in 1901. Azotobacter and Rhizobium are two types of nitrogen-fixing bacteria. Azotobacter is a free-living nitrogen-fixing bacterium, while Rhizobium is a symbiotic nitrogen-fixing bacterium. Rhizobium forms a mutually beneficial association with legume plants. Thus, this is the summary of the difference between Azotobacter and Rhizobium.
1. “Azotobacter cells” By DAN H. JONES – JONES D. H. FURTHER STUDIES ON THE GROWTH CYCLE OF AZOTOBACTER / JOURNAL OF BACTERIOLOGY, 1920, VOL. 5, NO. 4 Р. 325-341  (Public Domain) via Commons Wikimedia