I'll try to answer:
Imagine that all microorganisms suddenly appeared from the earth, why do you think that animals and plants would eventually disappeared from Earth.
Firstly, most animals and plants needs microorganisms to live. For example, at humans some bacteria produce essential vitamis. Humans could not live without bacteria in their body.
But lets assume that some animals and plants could live without microorganisms in their body.
Microorganisms (specificly
decomposers) are also very important because they
"recycle" organic matter and brake it down into simpler forms of matter, which are
essential by producers. (Some kind of Worms are also considered decomposers, but in fact, some bacteria inside the warm decompose the organic matter.) So,
without microorganisms, the producers would eventually ran out of the inorganic matter they need, and die. In that case, the rest of the higher-organisms would also die.If by contrast, all higher organisms suddenly disappeared from the Earth, why similar fate would not befall microorganisms?
Microorganisms could survive without higher-organisms present.
Firstly, there are
producers microorganisms, which would
provide energy to the microorganism ecosystem. Their role would be similar with the role of the plants for our ecosystem.
Secondly, there will be the
decomposers who would continue to decompose organic matter
providing the producers the inorganic matter they need.
These microorganisms would be enough for the ecosystem to survive.
Some other microorganisms (consumers) would also survive (acquiring energy that is provided to the ecosystem by producers).