Bacteria constitute a large domain of prokaryotic
microorganisms. Typically a few micrometres in length, bacteria
have a number of shapes, ranging from spheres to rods and
spirals.
Virus
A virus is a small infectious agent that replicates only inside the
living cells of other organisms. Viruses can infect all types of life
forms, from animals and plants to microorganisms, including
bacteria and archaea.
Fungi
A fungus is any member of a large group of eukaryotic organisms that
includes microorganisms such as yeasts and molds, as well as the more
familiar mushrooms.
Algae
Algae are a very large and diverse group of eukaryotic
organisms, ranging from unicellular genera such as Chlorella
and the diatoms to multicellular forms such as the giant kelp, a
large brown alga that may grow up to 50 meters in length.
Protozoa
Protozoa are a diverse group of mostly motile unicellular eukaryotic
organisms, but the term is no longer commonly used in modern taxonomy.