Here's why:

* Carbon dioxide (CO2): Plants and other photosynthetic organisms use CO2 during photosynthesis to create sugars and other organic compounds. This process forms the basis of the food chain. Animals obtain carbon by consuming plants or other animals.

* Nitrogen (N2): Nitrogen is a crucial component of proteins, nucleic acids, and other vital molecules. While the atmosphere is primarily composed of nitrogen gas, most organisms cannot directly use it. Instead, certain bacteria convert nitrogen gas into usable forms like ammonia (NH3) through nitrogen fixation. This ammonia is then taken up by plants and used to build organic compounds.

While many organisms can obtain oxygen (O2) from the atmosphere, they are not able to directly take it in as a nutrient. Oxygen is a byproduct of photosynthesis and is vital for respiration in most organisms.

It's important to note that some organisms, like bacteria, can utilize other elements present in the atmosphere, such as sulfur or phosphorus, for specific metabolic processes.