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Microbes are a large, diverse group of organisms that exist as single cells or as colonies (clusters of cells of the same species) that are able to grow, generate energy, and reproduce independently of other cells. Microbes may be eukaryotic or prokaryotic. The eukaryotic microbes include algae, fungi, slime molds, and protozoa, and the prokaryotic microbes are the bacteria, archaebacteria, and cyanobacteria. (Because viruses cannot metabolize and reproduce without hijacking the metabolic processes of a host cell, viruses are not true cells. Thus, debate exists about whether viruses are living organisms and thus technically “microbes.”)

Microbes colonize virtually all terrestrial and aquatic habitats on Earth, from polar glaciers to deep-ocean hydrothermal vents. Microbes contain the greatest species diversity of any group of organisms on Earth. Bacteria ...

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