Nitrogen is one of the major building blocks of life. It makes up 80 percent of Earth's atmosphere and it is described as one of the most important growth-limiting nutrients for both land and marine plants. This is because the most abundant form of nitrogen—dinitrogen, or N2—is particularly stable and resistant to chemical reactions, meaning that it cannot be assimilated by most organisms. Given the important role nitrogen plays in all living matter, microbes that can 'fix' atmospheric N2 into more accessible forms for organisms are crucial components of Earth's ecosystems.