Microorganisms and Macroorganisms: Differences and Similarities

As noted at the outset, to even a casual observer the great diversity among organisms in size, form, locomotion, and color is evident. Mayr (1997, p. 124) says simply … “The most impressive aspect of the living world is its diversity.”

A common, informal subdivision of this diversity is at about the level of resolution of the human eye into microorganism or microbe (from micro = small and bios = life) and macroorganisms. The division is also a practical one because microorganisms, if they are to be understood as organisms and not as gene sequences, have to be studied in large part by use of various kinds of microscopes. Macroscopically visible green, red, and brown algae, as well as the plants, and most of the animals, constitute the latter category.

Microorganisms include most of the protists (Adl et al. 2005; e.g., the flagellates, amoebas and relatives, sporozoans, ciliates, many of the green algae), the bacteria (as noted earlier, with a small ‘b’ taken to be synonymous with prokaryote and to include both Bacteria and Archaea), the fungi, and certain microscopic invertebrates such as many of the nematodes.
Microorganisms include most of the protists (Adl et al. 2005; e.g., the flagellates, amoebas and relatives, sporozoans, ciliates, many of the green algae), the bacteria (as noted earlier, with a small ‘b’ taken to be synonymous with prokaryote and to include both Bacteria and Archaea), the fungi, and certain microscopic invertebrates such as many of the nematodes. 

Based on their size, viruses obviously are microscopic; they are considered by some scientists to be microorganisms and are often included in texts on microbiology.

They are not discussed other than in passing in this plengdut post. In practice, most biologists probably do not consider very small animals such as the nematodes and rotifers to be microorganisms, although strictly speaking, they fall within the microscopic realm and ‘see’ the world to some extent as do the fungi and bacteria. Thus, obviously, microbiology includes much more than just bacteria, though this is frequently overlooked.

Not surprisingly, exceptions to the above generalities exist, and the distinction between ‘microorganism’ and ‘macroorganisms is clearly an arbitrary and occasionally hazy one. For example, the individual cells of some species of bacteria, such as Epulopiscium fishelsoni, are macroscopically visible, being about a million times the size of typical E. coli cells (Angert et al. 1993).

Of course the eukaryotic microorganisms are much larger than the prokaryotes. Interestingly, some portion of the life cycle of every creature is microscopic; conversely, many microbes produce macroscopic structures and stages.

Whatever the terms may mean to different people, a major subdiscipline of biology and entire university departments have become devoted to the study of bacteriology, or in some cases more broadly, ‘microbiology’. Operationally, second only to the need for microscopy to understand microbes, the major general difference between microorganism and macroorganisms is that, in general, members of the former group have to be cultured in order for most of their properties to be studied.

Although the culturing requirement for identification purposes has diminished with the availability of molecular methods, study of microorganisms under controlled, laboratory conditions will remain a distinctive attribute of microbial ecology.

Of course, microorganisms as a group differ from macroorganisms other than in size alone. The key characteristics that distinguish microbes quantitatively or qualitatively from macroorganisms appear below and are discussed at length in subsequent post (for detailed comparisons between prokaryote and eukaryote cell biology, see Neidhardt et al. 1990; Madigan et al. 2015). In keeping with the earlier comments regarding the taxa that constitute ‘microorganisms’, note that the following synopsis applies to microorganisms in general, of which bacteria are simply the most extreme example:
  • Capacity for dormancy, or occasionally, an extended quiescent or slow growth state. Most plants and some animals such as the insects share this trait at least to a degree. Bacteria excel at negotiating a ‘feast-or-famine’ existence.
  • Highest metabolic rates (bacteria) and potentially high population growth rates (bacteria and some fungi) and thus high numbers of separate genomes all produced with relatively little biomass. Doubling times for some bacteria as short as 12 min under optimal conditions. For unicellular microorganisms, because there can be and typically are many cells, and because each is potentially capable of forming more cells, beneficial mutations and accessory genetic elements such as plasmids (bacteria) can be rapidly established in a population by natural selection.
  • Active growth (population increase) typically within a relatively narrow range of environmental conditions, which are usually distributed discontinuously, but very widely and often globally.
  • High metabolic dexterity, including the capacity for rapid physiological adjustment, frequently involving entire biochemical pathways (bacteria microorganisms). Fast response (‘adaptability’) to changing environments is thus possible, as is clonal propagation (bacteria) of effectively a single genotype, both in the laboratory and to a greater or lesser extent in nature.
  • Direct exposure, frequently of individual cells, to the environment, rather than enclosure within a multicellular, homeostatic soma. Biochemical orderliness but organizational simplicity, reflected by fewer cell types and interactions; true division of labor nonexistent or rudimentary.
  • The foregoing attributes imply a usually extremely high number of ‘individuals’ (as used in this case to mean physiologically independent, functional units, or ramets) per genetic individual and, in terms of local population, per unit of actively occupied or colonized area.
  • Predominantly haploid condition microorganisms (bacteria) in the vegetative part of the life cycle. Consistent with their small size, bacteria have the smallest genomes of any cell.
  • Smaller role in bacteria microorganisms for conventional sexuality as a genetic recombination mechanism relative to its role in macroorganisms; gene transfer and assortment by various ‘unconventional’ means such as parasexuality in the fungi, or in fragmentary fashion such as by transformation, conjugation, and transduction in the bacteria. Bacteria can also exchange large blocks of genes by horizontal transmission from phylogenetically distant sources (species). The bacterial genome is unsurpassed among organisms in its plasticity.
Given these appreciable differences, one might well ask what attributes micro and macroorganisms share. The most general intrinsic commonality is that every living thing is an island of order in a sea of entropy or disarray.

The individual bacterial cell of a single cell type and the blue whale of about 120 cell types (Bonner 1988, p. 122) are constructed and operate in orderly fashion. Cell structure differs in detail among life forms, but the cells of essentially all organisms consist of a peripheral lipid bilayer membrane (the Archaea being the sole known exception) surrounding a cytoplasm.

All cells take up chemicals from the environment, transform them, and release waste products. All can conserve and transfer energy, direct information flow, and differentiate to some degree. ATP is essentially the currency of biologically usable energy in all organisms.

As a first approximation and notwithstanding exceptions, biosynthetic pathways are fundamentally the same in all cells, though organisms vary in their complement of the pathways (see also Neidhardt et al. 1990). The metabolic machinery, including the enzymes, pathways, program for cell division, and sequence of the reactions is remarkably similar (although the subcellular sites and controlling mechanisms differ).

For example, the assembly of a rod-shaped virus particle in an infected cell proceeds through much the same stepwise process as does a microtubule in that cell.

The genetic code and its associated parts such as various RNAs and translation machinery, are fundamentally the same, even down to the four nucleotide building blocks and the specific triplets which code for the same amino acids in a human and a bacterium. (Actually, the code is not quite universal, because most but not all the code words across taxa are the same; for some exceptions and mechanisms of code flexibility, see Ivanova et al. 2014; Ling et al. 2015.)

For all life forms the genetic code is distributed such that, with minor exceptions, every cell of the organism has a complete copy of the DNA recipe. The flow of genetic information from DNA to RNA to protein is universal (the well-rehearsed Central Dogma), even though there are variations in how this information flows. All cells can communicate by chemical signals and all respond to some extent to environmental signals by switching genes on and off.

Although the switching mechanisms vary, and are much more complex in eukaryotes than in prokaryotes, all organisms are able to receive and to react to stimuli. Moreover, as discussed later in the plengdut post, the general features of gene regulation are quite similar in bacteria and higher organisms.

The seemingly universal imprint of biochemical building block molecules and cell features noted above, together with a common function adhering to the same general principles, are what Lehninger (1970, pp. 3–13) referred to as “the molecular logic of the living state.” This remarkable conformity is unlikely to be just one big coincidence; rather, logically, it can be taken as implying that all known life emerged from a common ancestor.

The fundamentally identical cellular biochemistry among organisms led Bonner (1965, pp. 129–130) to state that this characteristic is non-selectable in the sense that, being essential to all known life, it has not been altered by selection since at least Precambrian times. In a similar vein, Monod (quoted by Koch 1976, p. 47) has said that “what is true of E. coli is also true of the elephant, only more so.”
These molecular commonalities brought interdisciplinary research teams together at the start of the DNA era in the late l950s. Does a comparable ecological commonality exist? The thesis of this post is that it does.