2a. Nitrogen fixing bacteria: They convert atmospheric nitrogen into ammonia, which becomes ammonium when mixed with water, which can be used by plants. They are helped by an enzyme called nitrogenase that is the only substance that can break the triple covalent bond.
2b. Nitrifying bacteria: Converts the ammonia into nitrates and nitrites.
2c. Denitrifying bacteria: They metabolize the nitrogen oxides and turn them back into nitrogen gas.
3. Animals get their nitrogen compounds from eating substances that grow in the soil or have eaten substances that grow in the soil.
4. Nitrogenase and lightening are similar because they both convert atmospheric nitrogen into ammonia/ammonium which make it usable. Without nigrogenase and lightening, there would be no usable nitrogen and far less living things on the planet.