Given a scientific question, generate a correct answer to it.

Ex Input:
The nitrogen that enters living systems by nitrogen fixation is successively converted from organic nitrogen back into nitrogen gas by what?

Ex Output:
bacteria

Ex Input:
What do atoms form by sharing valence electrons?

Ex Output:
covalent bonds

Ex Input:
What is unique and found in all living things because it can form up to four covalent bonds between atoms or molecules?

Ex Output:
carbon