Question

Which of the following is a free-living aerobic non-photosynthetic nitrogen-fixing bacterium?
A. Rhizobium
B. Azotobacter
C. Nostoc
D. Azospirillum

Answer 1

Hint: Azotobacter is a genus of usually motile, oval or spherical bacteria. It forms thick-walled cysts and may produce large quantities of capsular slime.

Complete answer: Rhizobium: The rhizobium has the photosynthetic properties of aerobic anoxygenic phototrophs. It can perform photosynthetic electron transport only under aerobic conditions. It is mutualistic or symbiotic which means that they depend on others for their survival. It plays an important role in nitrogen fixation.
Azotobacter: Azotobacter is a free-living or non- symbiotic bacteria.
Azotobacter is non-photosynthetic. It is aerobic bacteria and plays an important role in nitrogen fixation. It fixes atmospheric nitrogen to ammonia.
Nostoc: Nostoc is anaerobic. Its species have filaments with normal photosynthetic cells and they can fix nitrogen by N2-fixing heterocysts. It is free-living or non-symbiotic.
Azospirillum: Azospirilla is Gram-negative free-living nitrogen-fixing rhizosphere bacteria. It is non- photosynthetic. They can grow in aerobic, microaerobic, and anaerobic conditions.

Additional information: They are usually oval but may take various forms from rods to spheres.
1. In microscopic preparations, the cells can be dispersed or form irregular clusters or can also form chains of varying lengths.
2. In fresh cultures, cells are mobile due to the numerous flagella.
3. The cells lose their mobility later, become almost spherical, and produce a thick layer of mucus, forming the cell capsule.
4. The shape of the cell is affected by the amino acid glycine, which is present in the nutrient medium peptone.

So, the answer is B. Azotobacter

Note: Azotobacter is used by humans for the production of biofertilizers, food additives, and some biopolymer.