Question

Nitrogen-fixing organisms which can be free as well as symbiotic is
A) Anabaena
B) Azotobacter
C) Liverworts
D) Mosses

Answer 1

Hint:The atmospheric nitrogen is not available to plants. There are some nitrogen-fixing bacteria that convert atmospheric nitrogen into ammonia which is readily available to plants.

Complete answer:
The nitrogen cycle recycles certain phases that include nitrogen fixation, ammonification, nitrification, and denitrification. Denitrification is the process through which the nitrates and nitrites are converted back to atmospheric nitrogen. This process is performed by the anaerobic bacteria.Rhizobium is symbiotic nitrogen-fixing bacteria that live in symbiotic association with the leguminous plants in the root nodules of plants. They provide nitrogen to the plant and in return get shelter from the plant. Azotobacter is a free-living nitrogen-fixing bacterium. Anabaena is a free-living nitrogen-fixing bacterium. However, it also forms a symbiotic association with plants like mosquito fern. While these exist as plankton, it is known to form symbiotic relationships with the water fern Azollaand is found growing together at the surface of streams and ponds on temperate and tropical regions.

Hence, they are symbiotic bacteria as well. Symbiosis is any of several living arrangements between members of two different species, including mutualism, commensalism, and parasitism. Both positive (beneficial) and negative (unfavorable to harmful) associations are therefore included, and the members are called symbionts. All association between the two species that live together is symbiotic, whether the species benefit, harm, or have no effect on one another.

Thus, the correct answer is option A: 'Anabaena.'

Notes: The microbes which carry out biological nitrogen fixation are commonly called biological nitrogen fixers. for eg: Rhizobium and Azotobacter.