Question

What will happen if useful microorganisms did not exits?

Answer 1

Microorganisms, mainly bacteria in the ocean, are responsible for the oxygen in the air. Trees and so on use up the oxygen they produce in their own respiration - “breathing". The photosynthetic bacteria in the ocean do not use up all they produce, because huge numbers are buried in sediments when they die. So, contrary to popular belief, plants are not responsible for the high oxygen level of the atmosphere: microorganisms are.

A high oxygen concentration is needed for macroscopic creatures like plants and animals to live. The reason has something to do with diffusion, I don't know the details. But if the concentration was zero, they certainly could not survive.

The entire planet would be dead in days. Dead matter would not decay, oxygen would barely be recycled from CO2, many (all?) animals could not digest their food and plants could not get nitrogen from the soil.

Answer 2

we can not make curd or any usefully things which we eat and there more than 2300 medicines made from microorganisms after this we will not get them too