Final answer:
Nitrogen enters the living components of ecosystems through the process of nitrogen fixation, in which bacteria convert atmospheric nitrogen into ammonia and then ammonium, which plants use to make organic molecules.
Step-by-step explanation:
Nitrogen enters the living components of ecosystems predominantly through the process of nitrogen fixation. During nitrogen fixation, certain bacteria convert atmospheric nitrogen gas (N₂) into ammonia (NH₃), which can spontaneously form ammonium (NH₄⁺). Ammonium is further processed by other bacteria into nitrates and nitrites, which plants can then use to synthesize organic molecules such as DNA and proteins, making nitrogen available to higher trophic levels in the ecosystem.
Therefore, the answer to the question 'By what process does nitrogen enter the living components of ecosystems?' is A) Nitrogen fixation. Nitrogen fixation is crucial for incorporating atmospheric nitrogen into organic forms that are accessible to living organisms, playing a vital role in the nitrogen cycle and supporting ecosystem dynamics.