Nitrogen is an important element in DNA and proteins; however, atmospheric nitro

Question

Nitrogen is an important element in DNA and proteins; however, atmospheric nitrogen is the only source of this
element for life on Earth and it cannot be absorbed by eukaryotes. As a result, nitrogen? fixing bacteria species,
such as Rhizobia, live in a mutualistic relationship with plants like legumes. These species of bacteria are
obligate aerobes.

a. Explain what is meant by the term "obligate aerobe" and how that relates to Rhizobia metabolism.

b. What is the role of each organism in the Rhizobia?legume mutualistic relationship?

c. There is a theory that legumes are deliberately driving Rhizobia evolution by cutting off nutrient supplies
for those bacteria that fix less nitrogen. What would you expect to see in a Rhizobiapopulation if this
theory is true and why?

Explanation / Answer

a)

Nitrogen fixation is a process that occurs in an atmosphere where oxygen is not present as the enzyme nitrogenase would be deactivated by the presence of oxygen. Rhizobium is said to be an obligate aerobe, which states that the organism cannot live without the presence of oxygen. This also means that oxygen is the final electron acceptor in Rhizobium, the electron passing through the carrier molecules would be absorbed by oxygen leading to the formation of water.

b)

Each organism found in the nodules of Rhizobium are capable of fixing nitrogen, which the plant utilizes and the plant in turn supplies the food the organism. Each organism found in the nodule of the plant is capable of fixing nitrogen through the nitrogenase enzyme, and at the same time the plant supplies each organism with the requisite food and oxygen.

c)

In a mutualistic behaviour each organism involved in the relationship would be equally benefitted, and if we see one organism superior to the other then a type of parasitic behaviour steps up. Hence, the theory is not right.

If the theory is found to be true then the oranism would not fix nitrogen as fixation of nitrogen though an integral part of the organism, it does it when it is supplied with ample quanta of food. The energy derived through such reactions would enable the fixation of nitrogen as the process of nitrogen fixation is an energy exploiting process.

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