Question

Which of the following is nitrogen-fixing algae?
(a) Nostoc, Anabaena, Oscillatoria
(b) Azolla, Anabaena, Azotobacter
(c) Oscillatoria, Anabaena, Azolla
(d) Azolla, Nostoc, Oscillatoria

Answer 1

Hint: All the three are nitrogen-fixing algae also termed as cyanobacteria. They live in water and can manufacture their food through the process of photosynthesis and releases oxygen. They do not have membrane-bound organelles.

Complete step by step answer:
Nostoc, Anabaena, and Oscillatoria are nitrogen- fixing algae. All these are blue- green algae of group prokaryotes. Nowadays blue- green algae are called cyanobacteria because they are a type of bacteria that normally look green and sometimes turn bluish when scums are drying. They are capable of fixing atmospheric nitrogen as free-living forms and also in mutual association with the roots of plants. Other than nitrogen cyanobacteria can also fix carbon from carbon dioxide during photosynthesis. If the blue- green algae multiply to high numbers, toxins may also produce health problems for people or domestic animals who come in contact with the algae.
Azotobacter is a free-living nitrogen-fixing bacteria fixing molecular nitrogen from the atmosphere without a symbiotic relationship with plants.
The Azolla is a pteridophyte plant that floats on the surface of the water through numerous small closely overlapping scale-like leaves and their roots are hanging in the water. They form
a symbiotic association with cyanobacteria Anabaena for Nitrogen fixation.and does fulfill the need for nitrogen by the plant itself.
So, the answer is 'Nostoc, Anabaena, Oscillatoria'.

Note:
- The cyanobacteria are free- living blue- green algae present in the water of rice paddies.
- They are growing as epiphytes on the surface of the green algae.
- They can fix not only atmospheric nitrogen but also carbon from the carbon dioxide during photosynthesis.