Microbiology and Nitrogen Cycle

All of the mentioned organisms can bring about biological nitrogen fixation.

Nitrogen fixing bacteria are present in the root nodules of leguminous plants.

Rhizobium lives in a symbiotic relationship with the root nodules of leguminous plants.

Blue green algae are able to fix nitrogen from atmosphere and enrich soil with nitrogen. 

Legumes like gram (chickpea) form symbiotic relationships with Rhizobium bacteria, creating nitrogen-fixing nodules on their roots. These bacteria convert atmospheric nitrogen into ammonia that plants can use. Mustard, rice, and cotton are non-leguminous plants and do not form such nitrogen-fixing nodules.

Nitrogenase is the key enzyme complex responsible for biological nitrogen fixation - it converts atmospheric N2 into ammonia (NH3). This enzyme is found in nitrogen-fixing bacteria like Rhizobium. Nitrogen deaminase and Nitrodioxidase are not real enzymes involved in this process, and amino acid decarboxylase is involved in amino acid metabolism, not nitrogen fixation.

Rhizobium (misspelled as 'Rhozobium' in the option) is a well-known nitrogen-fixing bacterium that forms symbiotic relationships with legume roots. Acetobacter is acetic acid bacteria, Mycoplasma lacks a cell wall and is not a nitrogen fixer, and Chlamydia is an obligate intracellular pathogen - none of these fix nitrogen.

Nitrification is the two-step process where ammonia (NH3) is first converted to nitrite (NO2-) by bacteria like Nitrosomonas, then nitrite is converted to nitrate (NO3-) by Nitrobacter. The specific conversion mentioned (ammonia to nitrite) is the first step of nitrification. Nitrogen fixation converts N2 to NH3, denitrification converts nitrates back to N2, and eutrophication is nutrient pollution causing algal blooms.

Rhizobia is “the group of soil bacteria that infect the roots of legumes to form root nodules”. Rhizobia are found in the soil and after infection, they produce nodules in the legume where they fix nitrogen gas (N₂) from the atmosphere, turning it into a more readily useful form of nitrogen (N). From here, the nitrogen is exported from the nodules and used for growth in the legume. Once the legume dies, the nodule breaks down and releases the rhizobia back into the soil where they can live individually or reinfect a new legume host.

Pea plant can fix nitrogen from air.

Rhizobia are soil bacteria that fix nitrogen (diazotrophs) after becoming established inside the root nodules of legumes.

This is the correct option as azotobacter plays the role of nitrogen fixation in nitrogen cycle.

Conversion of nitrate into nitrogen is denitrification; conversion of ammonia into nitrites and then into nitrates is nitrification; conversion of atmospheric nitrogen into ammonia is nitrogen fixation; and conversion of organic nitrogen within the remains of plants or animals back into ammonium by bacteria or fungi is ammonification.

Bacillus stearo thermophilus is thermophilic and its spores are killed by an exposure of compressed steam for 12 minutes at 121⁰C; therefore, its spores are used to test the efficacy of moist heat sterilization (autoclave). Other means to test efficacy of moist heat sterilization are chemical indicators, autoclave tapes and thermocouples.

Psychrophilic bacteria-bacteria that can grow best at temperatures below 20^oC, e.g., soil and water saprophytes and may cause spoilage of refrigerated foods.