Microbiology and Nitrogen Cycle

Azolla forms a symbiotic association with Anabaena for nitrogen fixation.

The cyanobacteria (also referred to as blue-green algae) have chlorophyll a similar to green plants and are photosynthetic autotrophs. The cyanobacteria are unicellular, colonial or filamentous, fresh-water marine or terrestrial algae. The colonies are generally surrounded by a gelatinous sheath. They often form blooms in polluted water bodies. Some of these organisms can fix atmospheric nitrogen in specialized cells called heterocysts, e.g, Nostoc and Anabaena.

Nostoc belongs to cyanobacteria genus, and it can be used as biofertilizer. It is also called the blue-green algae. Nostoc converts atmospheric nitrogen into ammonia, which can be further converted to a suitable form for plant growth. Biofertilizers enhance the quality of plants and soil.

Biofertilizers are the suspension of microorganism which mainly increases the growth of the plants. These secrete the growth promoting substances. The microorganisms in bio-fertilizers restore the soil's natural nutrient cycle and build soil organic matter. Bio-fertilizers such as Rhizobium, Azotobacter, Azospirilium and blue-green algae (BGA) are used. Anabaena in association with water fern Azolla contributes nitrogen up to 60 kg/ha/season and also enriches soils with organic matter.

(A)Mesophile is an organism that grows in a moderate temperature from 20-45 degree Celcius.

• Rhizobium is a genus of bacteria associated with the formation of root nodules on plants. These bacteria live in symbiosis with legumes. They take in nitrogen from the atmosphere and pass it on to the plant, allowing it to grow in soil low in nitrogen.
  
• A lichen is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi in a symbiotic relationship. 
  
• Mycorrhizal fungi colonize the plant's root system and develop a symbiotic association called “mycorrhiza” They form a network of fine filaments that associate with plant roots and draw nutrients and water from the soil that the root system would not be able to access otherwise.
  
• Lichens, Rhizobium and Mycorrhizae  have symbiotic relationships and follow same mode of nutrition.
• So the correct answer is " All the above".

Acetobacter suboxydans is a gram-negative bacteria of the order rhodospirilla. It is used in the formation of ascorbic acid. A. suboxydans is widely used commercially in the production of ketonic acids like ascorbic acid (Vitamin C). 

Serratia is a genus of gram negative, facultatively anaerobic, rod-shaped bacteria of the enterobacteriaceae family. The most common species S. marcescens, is normally the only pathogen and usually causes nosocomial infections.

Karl Theodor Ernst von Siebold identified that many microbes, specifically protozoa, are single-celled organisms. This helped clarify the nature of microscopic life.

Viruses are the smallest, typically ranging from 20 to 300 nanometers. Bacteria are larger, usually in the micrometer range, and fungi are generally the largest of the three.

A virus consists of genetic material (DNA or RNA) enclosed in a protein coat called a capsid. They lack the cellular machinery of bacteria or fungi.

Viruses are considered non-living because they cannot reproduce or carry out metabolic processes independently. They require a host cell to replicate.