Nitrogen is in the air in the form N,, which cannot be incorporated into nitrogen-containing molecules. However, some organisms can fix atmospheric nitrogen, making the nitrogen available for use in the biosynthesis of nitrogen-containing molecules. Identify the correct statements about nitrogen fixation. a. Atmospheric nitrogen is reduced to the biologically useful form NH, (or NH). b. Nitrogen fixation is energetically neutral, using a negligible amount of ATP. c. The enzyme nitrogenase, which takes part in nitrogen fixation, is inactivated by oxygen. d. Nitrogen fixation in nature and in the lab requires a metal cofactor or catalyst. e. The availability of fixed nitrogen limits biological productivity. f. Ammonium from the atmosphere is fixed to more usable forms of nitrogen, such as nitrate (NO, ).