What are the major groups of microorganisms?

The major groups of microorganisms include bacteria, viruses, fungi, protozoa, and algae, each with distinct characteristics, structures, and ecological roles. Bacteria are single-celled prokaryotic organisms without a defined nucleus, found virtually everywhere and playing crucial roles in nutrient cycling, digestion, and decomposition, though some cause diseases. Viruses are non-cellular infectious agents that require living host cells to reproduce, causing numerous plant, animal, and human diseases. Fungi include both single-celled yeasts and multicellular molds and mushrooms, functioning primarily as decomposers and also being used in food production and medicine.

Protozoa are single-celled eukaryotic organisms, many living freely in water or soil, while some are parasitic and cause diseases like malaria and amoebic dysentery. Algae are photosynthetic microorganisms ranging from single-celled forms to multicellular seaweeds, producing much of Earth's oxygen and forming the base of aquatic food chains. Understanding these groups is essential because microorganisms collectively perform indispensable functions—decomposing organic matter, fixing atmospheric nitrogen, producing antibiotics, fermenting foods, and maintaining ecological balance—while some also pose health risks requiring careful management and control.