Understanding Nitrogen Fixing Bacteria

Nitrogen fixation is a crucial biological process where atmospheric nitrogen (\(\text{N}_2\)), which plants cannot directly use, is converted into ammonia (\(\text{NH}_3\)) by certain microorganisms. This ammonia is then converted into other nitrogenous compounds that plants can absorb from the soil. Nitrogen-fixing microorganisms can be either free-living in the soil or water, or they can live symbiotically in association with plants.

Free-Living Nitrogen Fixation Explained

Free-living nitrogen-fixing bacteria are microorganisms that fix atmospheric nitrogen while living independently in the soil or water, without forming a symbiotic relationship with a specific plant host for the purpose of nitrogen fixation. They obtain energy and nutrients from their environment.

Analyzing the Given Options

Let's examine each microorganism listed in the options:

• A. Rhizobium: Rhizobium is a genus of bacteria known for forming a symbiotic relationship with leguminous plants. They infect the roots and form root nodules, where they fix atmospheric nitrogen for the plant in exchange for carbohydrates. Rhizobium is a symbiotic nitrogen fixer, not free-living.
• B. Mycorrhiza: Mycorrhiza is a symbiotic association between a fungus and the roots of a vascular plant. Mycorrhizal fungi help plants absorb nutrients like phosphorus and water from the soil. While they enhance plant health and can be associated with nitrogen-fixing bacteria, Mycorrhiza itself is a fungal association, not a nitrogen-fixing bacterium, and not a free-living bacterium.
• C. Azospirillum: Azospirillum is a genus of bacteria found in soil and in association with the roots of various grasses and cereals (often termed associative symbiosis or rhizosphere bacteria). They are capable of fixing atmospheric nitrogen, and many species are considered free-living or associative nitrogen fixers.
• D. Azotobacter: Azotobacter is a genus of free-living bacteria found in soil and water. They are aerobic bacteria capable of fixing large amounts of atmospheric nitrogen. Azotobacter is a well-known example of a free-living nitrogen-fixing bacterium.

Identifying the Free-Living Nitrogen Fixers

Based on the analysis:

• Rhizobium is symbiotic.
• Mycorrhiza is a fungal association, not a bacterium, and its primary role is not nitrogen fixation.
• Azospirillum is free-living or associative nitrogen-fixing bacteria.
• Azotobacter is free-living nitrogen-fixing bacteria.

Therefore, the free-living nitrogen-fixing bacteria from the given options are Azospirillum (C) and Azotobacter (D).

Conclusion on the Correct Option

The question asks to identify the free-living nitrogen-fixing bacteria from the list. Options C (Azospirillum) and D (Azotobacter) fit this description. The correct combination is C and D only.

This corresponds to the option stating "C and D only".

Revision Table: Nitrogen Fixing Microbes

Additional Information on Nitrogen Fixation and Microbes

Nitrogen fixation is a vital part of the nitrogen cycle, making atmospheric nitrogen available to living organisms. The enzyme responsible for this process is nitrogenase, which is highly sensitive to oxygen.

Symbiotic Nitrogen Fixation (e.g., Rhizobium):

• Rhizobium bacteria infect legume roots, forming nodules.
• Inside the nodule, the plant provides a low-oxygen environment (using leghemoglobin) and carbohydrates to the bacteria.
• The bacteria fix nitrogen, providing ammonia to the plant. This is a mutually beneficial relationship.

Free-Living Nitrogen Fixation (e.g., Azotobacter, Azospirillum):

• These bacteria live independently in soil or water.
• Azotobacter is an aerobic bacterium but protects its nitrogenase enzyme from oxygen through high respiration rates and protective compounds.
• Azospirillum can be found near plant roots (rhizosphere) or inside root tissues, benefiting from root exudates for energy. They contribute to nitrogen fixation and can also produce plant growth hormones.

Understanding the different types of nitrogen-fixing microbes and their lifestyles is important for studying soil fertility, plant nutrition, and microbial ecology.