What kind of micro-organisms are there in nitrogen fixing bacteria?

• The question asks about the kind of micro-organisms that these bacteria are.
• The other given options are just examples of nitrogen-fixing bacteria.
• The correct answer will be 'prokaryotic' as it describes the type of organism that the nitrogen-fixing bacteria are.

Prokaryotic bacteria

• An organism without a well-defined nucleus is termed a prokaryote.
• The word 'prokaryote' is derived from the Greek words 'pro' which means 'before' and 'karyon' which 'nut' or 'kernel'.
• Prokaryotes lack membrane-bound organelles found in eukaryotes.
• Prokaryotes reproduce by an asexual form of reproduction.
• Prokaryotes include bacteria and cyanobacteria (blue-green algae).
• Nitrogen Fixation - Conversion of atmospheric nitrogen into nitrates so that it can be utilized by plants is termed nitrogen fixation.
• The ability of nitrogen fixation is possessed only by certain prokaryotes containing an enzyme complex called dinitrogenase that catalyses the reduction of dinitrogen to ammonia. e.g.: Rhizobium, Azotobacter, Clostridium, Beijerinckia, etc.
• The prokaryotes either fix nitrogen symbiotically (by living in the cells of higher plants) or non-symbiotically (by living freely in the soil).

Additional Information

Rhizobium

• It is a symbiotic nitrogen-fixing bacteria.
• Rhizobium is found in the nodules of legumes and is responsible for nitrogen fixation.

Azotobacter

• It is a non-symbiotic nitrogen-fixing bacteria.
• These are gram-negative, aerobic, free-living soil microbes that play an important role in the nitrogen cycle.

Cyanobacteria

• The common name of cyanobacteria is 'blue-green algae'.
• These are photosynthetic bacteria containing flattened sacs called thylakoids.
• Some cyanobacteria possess the ability of nitrogen-fixation in anaerobic conditions by means of specialized cells called heterocyst.