Question

Why is Rhizobium categorised as a symbiotic bacterium? How does it act as a bio fertilizer?

Answer 1

Rhizobium is the bacteria found in the soil that plays an important role in nitrogen fixation particularly in leguminous plants. These are gram negative bacteria that are present in close association with root nodules. Symbiosis is the type of interaction where both the organisms are benefited in some or the other ways.

Complete answer:
By now, we are clear with the term symbiosis and also got to know about Rhizobium. Getting into the details, many leguminous plants have root nodules in their roots. In these nodules, the rhizobia bacteria lives. The main role of rhizobia is to take atmospheric nitrogen which is in the form of ammonia and then convert it into nitrites or nitrates, so that the plant can easily absorb them. Here the plant is benefited because the bacteria makes the nutrients available and the plants in return provide shelter to the bacteria. As both plants and bacteria are benefitted, they are categorised as symbiotic bacteria. Biofertilizers are the organisms which supply nutrients to the plants. In this case, as Rhizobium is supplying nitrates and nitrites to the plants, it is said to act as a bio fertilizer. These nitrates are an important source of nitrogen and reduce the cost of fertilizer being used. Hence the above explanation justifies the given question.

Note:
The leguminous plants have root nodules because the Rhizobium infects the plants and as a result of this infection, the plants produce bulged structures which are being called as nodules. This is a soil bacteria and cannot fix nitrogen in free condition. It can fix the nitrogen only when it enters the nodules of the plant.