Abstract
The most striking feature about an assemblage of different organisms is the distinction among species in size and architecture. Excluding entities such as viruses that reproduce but are generally regarded as nonliving, the size range spans at least 21 orders of magnitude from wall-less bacteria known as mycoplasmas at about 10−13 g to blue whales, which exceed 108 g. The blue whale, incidentally, is the largest animal ever known.
What is a microorganism? There is no simple answer to this question. The word ‘microorganism’ is not the name of a group of related organisms, as are the words ‘plants’ or ‘invertebrates’ or ‘frogs’. The use of the word does, however, indicate that there is something special about small organisms; we use no special word to denote large animals or medium-sized ones.
W. R. Sistrom, 1969, p. 1
Not just any organism is possible.
F. Jacob, 1982, p. 21
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Notes
- 1.
Cyanobacteria or blue-green algae comprise a large, ecologically and morphologically diverse group of oxygenic, phototrophic prokaryotes containing chlorophyll a and phycobilins, from which chloroplasts evolved in plants. They were primarily responsible for the oxygenation of early Earth beginning in the mid-late Archaean Eon (Schirrmeister et al. 2011; Madigan et al. 2015).
- 2.
With some exceptions, small organisms and the propagules of most organisms assume simple, often spherical or sub-spherical, shapes. Such forms are exceedingly rare in the external morphology of macroorganisms. One reason that a curved (oblate) shape is a good design is that the container, whether a cell wall or the steel skin of storage tank, must withstand tensile but not bending stresses. Furthermore, because the covering is under the same tension per unit length throughout, there is no region more likely to rupture than any other (Chap. 3 in Thompson 1961; Chap. 6 in McMahon and Bonner 1983).
Not far removed from the sphere, an example of a shape universal in both the abiotic and biotic worlds is cubic symmetry. A commonly replicated form is the icosahedron. Icosahedra are regular polyhedra with 20 faces, 12 vertices, and 30 edges. Built upon the fundamental unit of triangles (the only two-dimensional structures that do not deform under pressure), the icosahedron is robust, encloses considerable volume, and can be iterated almost without limit. There are no known icosahedral bacteria, but the shape is found in objects as small as the outer shell of many viruses, ranging up to the complex architectures of plants and animals. As early as the 1960s, Caspar and Klug (1962), in what was destined to be a landmark paper, described the role of icosahedra in viral form; Allen and Hoekstra (1992, pp. 174–188; see also Vogel 1988; Stewart 1998) recount how they are a fundamental building block in the assembly of macroorganisms.
Suggested Additional Reading
Alegado, R.A. and N. King (Organizers). 2014. The Origin and Evolution of Eukaryotes. Cold Spring Harbor Perspect. Biol. Doi:10.1101/cshperspect.a016162. Collected papers on the evolutionary transitions to multicellularity and complexity.
Bonner, J.T. 1965. Size and Cycle: An Essay on the Structure of Biology. Princeton Univ. Press, Princeton, NJ. A fascinating, eloquent account, timeless in its relevance, of how size affects all creatures with emphasis on how size of the organism changes during the course of the life cycle.
Bonner, J.T. 1988. The Evolution of Complexity by Means of Natural Selection. Princeton Univ. Press, Princeton, NJ. An excellent, stimulating synthesis on why there has been a progressive increase in size and complexity from bacteria to plants and animals.
Carroll, S.B. 2001. Chance and necessity: the evolution of morphological complexity and diversity. Science 409: 1102–1109. An insightful and authoritative synthesis on the evolution of life, including the evolutionary increase in size.
Hedges, S.B. 2002. The origin and evolution of model organisms. Nature Rev. Genet.3: 838–849. An interesting and informative synopsis of times of divergence of the prokaryotes, protists, plants, fungi, and animals, subject to the caveat that all such estimates are works in progress.
Knoll, A.H., D.E. Canfield, and K.O. Konhauser (eds.). 2012. Fundamentals of Geobiology. Oxford Univ. Press, UK. An excellent, well-illustrated synthesis by multiple authors on the early history of life on Earth.
Thompson, D’A. W. 1961. On Growth and Form. (Abridged edition edited by J.T. Bonner of the original 1942 text.) Cambridge University Press, Cambridge, England. This classic book remains a benchmark of excellence on the analysis of form.
Vogel, S. 2003. Comparative Biomechanics: Life’s Physical World. Princeton Univ. Press. An interesting, readable, witty, and authoritative explanation pitched at the undergraduate level with innumerable fascinating examples.
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Andrews, J.H. (2017).
Size.
In: Comparative Ecology of Microorganisms and Macroorganisms. Springer, New York, NY. https://doi.org/10.1007/978-1-4939-6897-8_4
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DOI: https://doi.org/10.1007/978-1-4939-6897-8_4
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