Answer: Antonie van Leeuwenhoek
Antonie van Leeuwenhoek was one of the first people to observe microorganisms, using a microscope of his own design, and made one of the most important contributions to biology. (Robert Hooke was the first to use a microscope to observe living things.)
^Robert Hooke used it to observe living things, I don't believe it was microorganisms though.
Hope this helped!! :)
