The question has multiple parts about determining concentrations of different chemicals in making seawater culture solutions but the final part asks why there's no nitrogen added to the media. I assume it's because it's dissolved though the atmosphere but I'm not positive. Question is as follows:

I am culturing a N2 fixing cyanobacterium and have the following media recipe for growing them: To make 1 L of salt water add the following to fresh water: (list of chemicals) Now, make up 1L stock solutions of each of the following (we’ll skip the vitamins) : (more chemicals). Now add 1 mL of each of the first 4 stock solutions (from part 2) to the seawater solution (from part 1). Now add 100 µL of the Zn, Mn, and Mo stock solutions (from part 2) to the seawater solution. Now add 10 µL of the Co and Cu stock solutions (from part 2) to the seawater solution.

Why is there no nitrogen in my media?