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