Describe two strategies that nitrogen­fixing bacteria use

Question

Problem:

Question 1: Why is nitrogen fixation (conversion of atmospheric nitrogen into organic nitrogen) relatively difficult for microbes to achieve?

Question 2: Describe the two different strategies that nitrogen­fixing bacteria use to prevent their nitrogenases coming into contact with oxygen.

Question 3: How did the discovery of anammox (ANaerobic AMMonium Oxidation) in 1999 change scientists' understanding of the global nitrogen cycle?

Could someone help to explain the answer of these questions.