Microorganisms exist as single cells or unspecialized groups of cells and are often difficult to see without the aid of some form of technology, such as a magnifying glass or microscope. Microorganisms are found in all five of the kingdoms. Bacteria, placed into the kingdom of monera , are the simplest of the traditional microbes and include single-celled organisms that do not have a nucleus. The kingdom protista includes the more complex, single-celled organisms that have a nucleus, such as paramecia and amoeba. The kingdom fungi contains multi cellular organisms such as mushrooms as well as single-celled, nucleated organisms such as yeast.
Fungi have cells with cell walls that lack chlorophyll.
Many of the simple animals can be considered microorganisms because of their size. Rotifers , the up clean organisms in many aquatic ecosystems such as an aquarium, are composed of less than 50 cells but are considered animals because of their other characteristics. Microscopic plants are typically classified as algae . Algae have many of the characteristics of more complex plants and are found most often in water, though some algae live in wet soil. Bacteria , in kingdom monera, have a cell wall and a cell membrane but no distinct nucleus.