Final answer:
Antonie van Leeuwenhoek is the Dutch merchant who first saw and described microorganisms under a microscope. He developed a powerful lens and observed single-celled organisms, which we now know as bacteria and protists.
Step-by-step explanation:
The Dutch merchant who was the first to see and describe microorganisms under a microscope was Antonie van Leeuwenhoek. Born in the Dutch Republic in 1632, Van Leeuwenhoek developed a lens powerful enough to view microbes. In 1675, using a simple but powerful microscope, he observed and described single-celled organisms, which we now know as bacteria and protists.