microorganism

Microorganisms, or microbes, are organisms that are too small to be seen with the naked eye. They include bacteria, archaea, fungi, algae, and protozoa, and they inhabit every environment that supports life, from soil and water to the bodies of plants and animals, and extreme habitats such as acidic hot springs or deep-sea vents.

Classification is based on cellular organization. Prokaryotes comprise bacteria and archaea, lacking a defined nucleus. Eukaryotic microorganisms include fungi, algae, and protozoa, which have cells with a nucleus. Viruses are acellular particles that replicate only inside host cells and are often discussed separately from true microorganisms because they are not cells.

Most microorganisms are unicellular, though some form simple colonies or multicellular structures. They vary in metabolism and can grow in aerobic or anaerobic conditions, using sunlight, chemical energy, or organic compounds. Reproduction occurs by division, budding, spores, or viral replication (in viruses).

Ecological and practical roles: microbes drive nutrient cycles, decay organic matter, fix nitrogen, support photosynthesis in aquatic systems, and form biofilms. In medicine and industry, they are used in fermentation, biotechnology, and production of enzymes and antibiotics; some cause diseases in humans, animals, and plants. Probiotics and symbiotic relationships are notable examples of beneficial microbes.

Study and methods: Microbiology uses culture techniques to grow microbes, microscopy, and molecular methods such as DNA sequencing to identify and classify them. Many environmental microbes are not readily cultured in the lab, which has driven culture-independent approaches.