Microbial Diversity

Abstract

Prokaryotic and eukaryotic microorganisms dominate life on Earth with respect to the numbers of individuals and biomass. Yet, the number of species of macroorganisms described as well as the estimated numbers of yet to be described species exceeds those of the microorganisms by an order of magnitude. Such numbers give an incomplete picture of the diversity in the microbial world. Only two modes of life are found in higher plants and animals: oxygenic photosynthesis and aerobic respiration. In the microbial world a great variety of processes occur, aerobic as well as anaerobic, phototrophic (oxygenic and anoxygenic), respiratory (with oxygen or other electron acceptors), fermentative and chemoautotrophic (using reduced inorganic compounds as energy sources). Also phylogenetically the microbial world is by far the most diverse. Most branches in the small ribosomal subunit ribonucleic acid (RNA)‐based phylogenetic tree contain only microorganisms, whereas higher plants and animals only form short lineages in the eukaryotic branch.

Key concepts

  • The microbial world encompasses most of the phylogenetic diversity on Earth, as all Bacteria, all Archaea and most lineages of the Eukarya are microorganisms.

  • The morphological diversity in the microbial world is much smaller than that of the higher plants and animals.

  • With respect to numbers of individual microorganisms and biomass the microbial world is far more important than the world of macroorganisms, but the numbers of species of microorganisms described to date and the predicted numbers of yet undescribed species are about an order of magnitude lower than those of the macroorganisms.

  • Based on their modes of energy generation and the variety of organic and inorganic compounds used as energy source and/or nutrients, the microbial world and especially the world of the prokaryotes, has many more possibilities than are realized by macroscopic animals and plants.

  • Sequencing of microbial DNA isolated from natural environments (environmental genomics, metagenomics) has revealed the existence of a tremendous variety of yet uncultured microorganisms, showing that the true microbial diversity in nature is much higher than currently recognized on the basis of organisms studied in culture.

Keywords: prokaryotes; Archaea; Bacteria; eukaryotes; phylogeny; metabolic diversity

Figure 1.

The universal small‐subunit rRNA sequence‐based phylogenetic tree. Branches that contain macroorganisms (fungi, algae, higher plants and animals) are highlighted orange; all other branches consist entirely of microorganisms.

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Further Reading

Dworkin M, Falkow S, Rosenberg E, Schleifer KH and Stackebrandt E (eds) (2006) The Prokaryotes. A Handbook on the Biology of Bacteria: Ecophysiology and Biochemistry, 3rd edn. New York: Springer.

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Oren, Aharon(Dec 2009) Microbial Diversity. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020376]