What are the major group of microorganisms?

Microorganisms are the small organisms that cannot be seen with naked eyes. Major groups of microorganisms are bacteria, virus, protozoa and algae. Bacteria are the prokaryotic, unicellular organisms while viruses are acellular. Protozoans are single celled eukaryotic mostly aquatic organisms. Algae are chlorophyll containing photosynthetic eukaryotic single to multiple celled organisms.