Microbiology and Nitrogen Cycle

Earth’s atmosphere contains 78% nitrogen but it cannot be used by living organisms due to its inert form. This inert nitrogen is fixed into biologically active ammonia by free living or symbiotic bacteria and other microorganisms. The free living bacteria are present in soil where they perform nitrogen fixation; example: Azotobacter, Azospirillum and Clostridium. This makes option B correct. The finger like projections on roots of legumes, termed as root nodules, carries Rhizobium bacteria which in turn are responsible for nitrogen fixation. Legume get ammonia from Rhizobium and provide carbohydrates as an energy source to them; a symbiotic relation. This makes option C incorrect as root nodules harbour symbiotic bacteria. Nitrogen fixation does not occur in air as none of nitrogen fixing microbes are present in air; option A is incorrect. Thus, the correct answer is option B. 

Bacterias like Azotobacter and Azospirillum fix atmospheric Nitrogen while free living in soil, thus enriching Nitrogen content of soil.

Azotobacter is free-living aerobic non-photosynthetic nitrogen fixing bacterium which have wide variety of metabolic abilities, they fix atmospheric nitrogen as they produce nitrogenase enzyme and convert it to ammonia. They are free living in soil and do not associate as symbionts. 

Azospirillum is a nitrogen-fixing bacterium that is commonly associated with the roots of grasses and cereals, including paddy (rice) fields.

The interaction between Agrobacterium tumefaciens and plants is one of the most studied systems in plant-microbe interactions due to its ability to genetically transform plants.

Escherichia coli  is a Gram-negative bacteria. It plays an important role in the field of biotechnology, where it has served as a host organism in recombinant DNA technology. The bacterium can be grown and cultured easily. One of the first useful application of recombinant technology was the manipulation of E.coli to produce human insulin.
Rhizobium meliloti is a Gram-negative, nitrogen-fixing bacterium. It is a symbiotic bacteria lives in the association with legumes. The bacteria possesses nif genes, responsible for the nitrogen fixation.
Bacillus thuringensis is a soil dwelling bacteria. It has been used as a biological pesticide. During spore formation, the bacteria produces crystal proteins called as endotoxins which have insecticidal action.
Crude oil is a complex mixture of hydrocarbons. Several strains of bacterium Pseudomonas putida were known to produce enzymes that degrade these hydrocarbons, i.e., the bacteria ate the hydrocarbons, nourishing themselves and reproducing.

Bacteria like E. coli have very short generation times. Combined with the high copy number of plasmid they produce, they duplicate the plasmid with pretty good fidelity for other uses. Agrobacterium tumefaciens is capable of transferring a particular DNA segment of the tumour-inducing (Ti) plasmid into the nucleus of infected cells where it is subsequently integrated into the host genome. 

The genus Pseudomonas are adapted to very different environments. This capability to thrive in various habitats coincides with an enormous metabolic capacity of this genus to use recalcitrant compounds as carbon source as well as to produce a wide range of secondary metabolites and biopolymers. Environmental applications of various Pseudomonas species in biodegradation of recalcitrant pollutants as well as biocontrol agents in plant growth promotion is very common.

Non-symbiotic bacteria in the soil and symbiotic bacteria that live in the roots of plants are capable of nitrogen fixation. Bacteria in the genera Clostridium and Azotobacter are non-symbiotic nitrogen-fixing bacteria, while those in the genus Rhizobium are symbiotic bacteria. Pseudomonas species fixes nitrogen under microaerobic conditions in the free-living state and colonizes rice endophytically. Nitrogen fixation in phototrophic bacteria has been found in Rhodospirillum rubrum. 

Actinomyces species of bacteria form colonies that have fungus like branched networks of hyphae and are therefore also known as ray fungus. They are gram positive and a few species of nitrogen fixing bacteria live in symbiosis with actinorhizal plants of the genus Alnus. 

Statement 1 is false (no eukaryotes fix nitrogen). Statements 2, 3, and 4 are correct. Since only one statement is false, the answer 'One' is correct.

Pnuematophores are called as breathing roots. These are found in mangrove trees. These are the modifications in root which grows in the same direction as that of shoot. Thus, they are negatively geotropic as well as positively phototrophic.

Pneumatophores are negatively geotropic roots found in mangroves. They have pores for gaseous exchange. These pores are important because the soil is waterlogged and there is deficiency of air in the soil.

Biofertilizers are available commercially in our country for farmers.