When microbes live independently but cooperate and share nutrients, it is called?

(Microbial) Synergy

Explanation:

Microbial synergy is a 'relationship' in which microbes support each other's growth as they cooperate and share nutrients, but are also able to work independently. However, this relationship produces an much-better effect than the overall process of their individual effects.

The opposite of this phenomena is antagonism in which, in a single culture media, only one of the organisms (only one species) grows.