Members of the Microbial World
Most microbes are too small to be seen by the naked human eye. The average diameter of a bacterial coccus is 1 μm which is equivalent to 10^(-6) m. However some are large enough to see without a microscope (bread molds) but can be distinguished from other macroscopic plant or animal cells by their lack of highly distinguished tissues.
Taxonomical Hierarchy of Life
All forms of life can be classified under one of the 3 domains (Eukarya, Archaea, and Bacteria) and can be further distinguished into one of the five kingdoms (Protista,Fungi,Monera,Animalia,Plantae)
Bacteria and Archaea are strictly microbial and the only kingdom that falls under the two are Monera; some organisms are bacteria and others are archaea. That leaves the remaining four kingdoms under the domain Eukarya; which are Protista, Fungi, Plantae, and Animalia.
Although Archaea and Bacteria only have one kingdom to share between between the two, they are too different to be combined into one category. The three domains are differentiated by their rRNA sequences tracing back all the way to the last universal common ancestor (LUCA). When all similar rRNA sequences were placed together in a phylogenic tree, Bacteria had branched off completely opposite from Eukarya and Archaea which eventually split up.
Although bacteria and archaea are both prokaryotic and single-celled, they have many differences. For instance, cells of Archaea lack peptidoglycan in their cell wall and instead have Unique Membrane Lipids. Bacterial cells usually have peptidoglycan in the cell wall and also includes Cyanobacteria which is important for oxygen production in the environment.
The microorganisms which fall under the domain, Eukarya are Protista and Fungi.
Protists are unicellular and typically larger than cells of archaea and bacteria. Algae, which are photosynthetic, and Protozoa, motile hunter-grazers, are both Protists. There are also molds which fall under the kingdom, Protists. Slime molds are one of them and they have 2 stages in their life cycle with the first stage of life as protists and the second stage as fungi. Water Molds are devastating plant diseases which was responsible for the Irish potato famine.
Fungi includes molds, mushrooms, and yeast. Molds and mushrooms are multicellular and form hyphae. Yeasts are unicellular.