I believe we first have to determine where the line of both sentience and intelligence are with regards to non-terrestrial species. We have a bias towards intelligence and sentience basing them on ourselves but in truth, we can be completely off.

Frankly I believe if microscopic life was to become intelligent and sentient, it may be done through becoming a hive-mind. Each individual microorganism contributes to the greater processing capability of the whole, not too unlike the vex from destiny. The whole forming a collective consciousness within a colony like some pyrosome in earths ocean.