Which of the following is not a free living nitrogen fixing bacteria ?

Question

Accepted Answer

The correct option is (1): Frankia

Frankia is an actinomycete, a genus of filamentous, Gram-positive bacteria, primarily renowned for its **symbiotic** relationship with a diverse group of non-leguminous plants, which are collectively known as actinorhizal plants (e.g., alder trees, Casuarina). Unlike the other organisms in the list, which are classic examples of bacteria known *exclusively* for their free-living nitrogen fixation, *Frankia*'s dominant ecological role is to establish specialized structures called **actinorhizal root nodules** on the host plant. Inside these nodules, it fixes atmospheric nitrogen for the plant, forming a mutually beneficial partnership. While some *Frankia* strains technically possess the *nif* genes and can fix nitrogen in the *free-living* state under specific, highly protected laboratory conditions by forming specialized vesicles to exclude oxygen, its biological significance and classification are overwhelmingly tied to its symbiotic function. Therefore, in the context of this question, which asks for the organism that is *not* a free-living fixer among a list of prominent free-living examples, *Frankia*'s dominant symbiotic nature makes it the most appropriate answer.

The incorrect option is (2): Klebsiella

Klebsiella is a well-established genus of bacteria that includes species capable of free-living, **non-symbiotic** nitrogen fixation. Klebsiella is typically classified as a facultative anaerobe, meaning it possesses the metabolic flexibility to live and fix nitrogen both in the presence and absence of oxygen. Certain species, such as *Klebsiella pneumoniae*, are commonly found in diverse environments like soil and water, and they contribute to the nitrogen content of the soil entirely independently of a host plant. Since they do not form specialized symbiotic structures like root nodules for their fixation, they are unequivocally categorized as free-living nitrogen fixers.

The incorrect option is (3): Clostridium

Clostridium is a genus of bacteria that is strictly classified as an obligate anaerobe, meaning its metabolism and survival depend on an environment completely devoid of oxygen. Many species within this genus, with *Clostridium pasteurianum* being a notable example, are considered classic and well-documented instances of **anaerobic, free-living** nitrogen-fixing bacteria. These organisms inhabit waterlogged soils or deep soil layers where oxygen concentration is negligible. This anaerobic nature naturally protects their highly oxygen-sensitive nitrogenase enzyme, allowing them to carry out the process of fixing atmospheric nitrogen without any symbiotic plant relationship.

The incorrect option is (4): Beijerinckia

Beijerinckia is correctly grouped with other genera, most notably *Azotobacter*, as a prominent example of an **aerobic, free-living** nitrogen-fixing bacterium. This genus is known for living independently in the soil, utilizing atmospheric oxygen for its respiration. Simultaneously, it employs specific metabolic strategies, such as having a very high respiration rate and producing copious amounts of slime, to protect its nitrogenase enzyme from inactivation by oxygen. Its ecological function is to fix atmospheric nitrogen directly into the soil, operating entirely outside of any specialized host plant relationship, which solidifies its classification as a non-symbiotic, free-living fixer.