which living organisms is crucial to ensuring the nitrogen gas from the air becomes available as a nutrient for living things?

Bacterias

Explanation:

Rhibozium is a famous bacteria which is usually associate with the bean. That's why beans have so much protein in their composition, altough they are plants:  the Rhibozium fixes Nytrogen which is used for assembling proteins.

There are also other bacterias which are important for the process, we call them nitrobacter. They're function is, bassicaly, to converts the fixed Nytrogen (in form of NH4+) into a more available form for plants.

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