Mineral Nutrition

Biological nitrogen fixation is catalysed by the enzyme nitrogenase, which reduces $N_2$ to ammonia.
Nitrogenase is a Mo–Fe protein: molybdenum is a component of the enzyme.
Molybdenum is also a component of nitrate reductase, both of which take part in nitrogen metabolism.
Zinc activates carboxylases and is needed for auxin synthesis, copper is associated with redox enzymes, and manganese is needed for the photolysis of water.
Hence the element with an important role in nitrogen fixation is molybdenum.

Free-living nitrogen-fixing microbes may be aerobic or anaerobic.
$Azotobacter$ and $Beijerinckia$ are free-living aerobic nitrogen fixers.
$Rhodospirillum$ is a free-living anaerobic nitrogen fixer; $Bacillus$ is also free-living.
$Rhizobium$ is a symbiotic nitrogen fixer that lives in the root nodules of legumes.
Hence the free-living anaerobic nitrogen fixer is $Rhodospirillum$.

Macronutrients are needed in large amounts (more than 10 mmole per kg of dry matter): C, H, O, N, P, K, S, Ca and Mg.
Micronutrients or trace elements are needed in very small amounts (less than 10 mmole per kg of dry matter): Fe, Mn, Cu, Mo, Zn, B, Cl and Ni.
Boron, molybdenum and zinc are micronutrients.
Magnesium is a macronutrient; it is a constituent of the ring structure of chlorophyll and activates enzymes of respiration and photosynthesis.
Hence magnesium is not a micronutrient.