Which component is fixed by $Rhizobium$ bacteria?

The most famous nitrogen-fixing bacterium/biofertilizer is:

Analogy type questions:
$(1)$ Reduction of nitrogen to ammonia : Biological nitrogen fixation : Nitrogenase : ........
$(2)$ Free-living bacteria : Rhodospirillum : Free-living blue-green algae : ........

How many electrons and protons are required to fix a molecule of dinitrogen $(N_2)$ (in each)?

The enzyme nitrogenase is protected from the inhibitory effects of oxygen in root nodules by .............

For nitrogen fixation,the useful pigment is