Question

Rhizobium and Frankia
(a) Are free- living in soil, but as a symbiont for atmospheric nitrogen fixation
(b) Produce nodules on the roots of leguminous plants
(c) Are strictly anaerobic
(d) More than one option is correct

Answer 1

Hint: These come under the category of mutualistic (symbiotic bacteria) . The symbiotic bacteria enter into the root hairs of the plant and multiply leading to the formation of root nodules. They are also responsible for the fixation of an atmospheric gas which is the most abundant.

Complete answer:
Nitrogen- fixing bacteria are the microorganisms that are capable of transforming atmospheric nitrogen into fixed nitrogen usable by plants into inorganic compounds. More than 90 percent of all nitrogen is fixed by these organisms which makes them the key member of the nitrogen cycle
Frankia fixes nitrogen, both under symbiotic and free- living aerobic conditions associated with actinorhizal plants but Rhizobium cannot fix aerobically associated with leguminous plants like a pea. Both form root nodules but in different plants
Nitrogen- fixing bacteria can be classified into two types:
- Free- living (non- symbiotic) bacteria
- Mutualistic (symbiotic bacteria)
The first kind, free- living bacteria include cyanobacteria, Nostoc, Anabaena, genera like Azotobacter, Beijerinkia, and Clostridium. The Second kind comprises the symbiotic bacteria includes Rhizobium (associated with leguminous plants like a pea) and Frankia (associated with actinorhizal plants)
The symbiotic bacteria enter into the root hairs of the plant and multiply leading to the formation of root nodules where they convert free nitrogen to ammonia, which is utilized by the host plant
So, the correct answer is 'Are free- living in soil, but as symbionts for atmospheric nitrogen fixation'.

Note: Nitrogen is an important element for plant growth and production. It is a major component of chlorophyll, which is the most important pigment needed for photosynthesis, as well as amino acids, which are the key building blocks of proteins.
Plants instead depend upon combined, or fixed, forms of nitrogen. Most of this nitrogen is provided to cropping systems in the form of fertilizers. The use of these fertilizers has led to ecological problems. Biological nitrogen fixation is a natural means of providing nitrogen for plants