Microorganisms are minute organisms of microscopic dimensions, too small to be seen by the eye alone. To be viewed, microorganisms must be magnified by an optical or electron microscope. The most common types of microorganisms are viruses, bacteria, blue-green bacteria, some algae, some fungi, yeasts, and protozoans.
Viruses, bacteria, and blue-green bacteria are all prokaryotes, meaning that they do not have an organized cell nucleus separated from the protoplasm by a membrane-like envelope. Viruses are the simplest of the prokaryotic life forms. They are little more than simple genetic material, either DNA (deoxyribonucleic acid) or RNA (ribonucleic acid), plus associated proteins of the viral shell (called a capsid) that together comprise an infectious agent of cells. Viruses are not capable of independent reproduction. They reproduce by penetrating a host cell and diverting much of its metabolic and reproductive physiology to the reproduction of copies of the virus.
The largest kingdom of prokaryotes is the Monera. In this group, the genetic material is organized as a single strand of DNA, neither meiosis or mitosis occur, and reproduction is by asexual cellular division. Bacteria (a major division of the Monera) are characterized by rigid or semi-rigid cell walls, propagation by binary division of the cell, and a lack of mitosis. Blue-green bacteria or cyanobacteria (also in the Monera) use chlorophyll dispersed within the cytoplasm as the primary light-capturing pigment for their photosynthesis.
Many microorganisms are eukaryotic organisms, having their nuclear material organized within a nucleus bound by an envelope. Eukaryotes also have paired chromosomes of DNA, which can be seen microscopically during mitosis and meiosis. They also have a number of other discrete cellular organelles.
Protists are a major kingdom of eukaryotes that includes microscopic protozoans, some fungi, and some algae. Protists have flagellated spores, and mitochondria and plastids are often, but not always, present.
Protozoans are single-celled microorganisms that reproduce by binary fission and are often motile, usually using cilia or flagellae for propulsion; some protozoans are colonial.
Fungi are heterotrophic organisms with chitinous cell walls, and they lack flagella. Some fungi are unicellular microorganisms, but others are larger and have thread-like hyphae that form a more complex mycelium, which take the form of mushrooms in the most highly developed species. Yeasts are a group of single-celled fungi that reproduce by budding or by cellular fission.
Algae are photosynthetic, non-vascular organisms, many of which are unicellular, or are found in colonies of several cells; these kinds of algae are microscopic.
In summary, microorganisms comprise a wide range of diverse but unrelated groups of tiny organisms, characterized only by their size. As a group, microorganisms are extremely important ecologically as primary producers, and as agents of decay of dead organisms and recycling of the nutrients contained in their biomass. Some species of microorganisms are also important as parasites and as other disease-causing agents in humans and other organisms.