viruses and polyhedron symmetry for round viruses. Because they have small genomes, viral genes must repeat protein subunits. Each subunit has identical bonding contacts with the neighbors. Repeated interaction with chemically complementary surfaces at the subunit interfaces, naturally leads to a symmetric arrangement (3D patterns). The bonding contacts are usually noncovalent. This ensures error-free self-assembly and reversibility. Thus, if the gene responsible for the viral protein is inserted into another cell for expression, that cell will produce viral proteins that will self-assemble into shells – fake viruses with no genome inside. Depending on the species, the 3D shape and the repetitive interactions of viral proteins allow for different types of bonding patterns, which in turn lead to different configurations and capsid sizes (Table 1.3.).
Thus, some viruses can be comparable in size with certain life forms (Table 1.4.). For a degree of comparison, M. gallicepticum is 3 up to 10 times smaller than the diameter of the largest giant viruses. Giant viruses that infect single-celled eukaryotes like amoebas (i.e. Acanthamoeba castellanii), such as Pithovirus sibericum, Pandoravirus salinus, or Pandoravirus dulcis, are about 1–1.5 μm (1000–1500 nm) in length [91, 92]. Other more well-known giant viruses are Megavirus chilensis (400 nm) or Acanthamoeba polyphaga Mimivirus (390 nm), each with considerable dimensions over the size of certain prokaryotes [91]. In terms of physical size and genome complexity, giant viruses closed a significant gap between the realms of viruses and the prokaryotic/eukaryotic unicellular organisms [91].
Table 1.3 Extreme sizes in viruses.
| Viruses | Eukaryotic viruses (μm) | Prokaryotic viruses (μm) |
|---|---|---|
| Min | 0.017 | 0.03 |
| Max | 1.5 | 0.2 |
The table shows the minimum and maximum physical dimensions of viruses found in both eukaryotes and prokaryotes. The values represent averages of the measurements published in the scientific literature and are presented in micrometers.
Table 1.4 Single-celled organisms vs. viruses.
| Eukaryotes | Prokaryotes | Eukaryotic viruses | Prokaryotic viruses | |||||
|---|---|---|---|---|---|---|---|---|
| Species | Val (μm) | Species | Val (μm) | Species | Val (μm) | Species | Val (μm) | |
| Min | Prasinophyte algae | 0.8 | Mycoplasma genitalium | 0.15 | Porcine circovirus | 0.017 | Phages | 0.03 |
| Max | Caulerpa taxifolia | 300 000 | Thiomargarita namibiensis | 1400 | Pithovirus sibericum | 1.5 | Phages | 0.2 |
The table shows a comparison between extreme microscopic sizes of viruses and unicellular organisms, that covers both eukaryotes and prokaryotes.
On the other scale, Porcine circovirus is the smallest virus (17 nm) and is found in multicellular eukaryotes [93, 94]. Almost all isolated viruses from prokaryotes show ranges between 30 and 60 nm. Giant prokaryotic viruses with capsids diameters ranging from 200 to more than 700 nm have been reported [95]. Nevertheless, these comparisons between virus sizes in prokaryotes and eukaryotes can be misleading as more specialized life forms can lead to more extreme variations in size, complexity, and methods of infection.
1.10.1 Viruses vs. the Spark of Metabolism
How can P. sibericum be so big yet lifeless? There are several reasons for which viruses are not considered alive nor do they become alive from our perspective. More robust viral species of considerable size have a reasonable probability to incorporate parts of biochemical mechanisms from the infected cells (inside their capsid). Thus, although giants viruses may incorporate functional metabolic pathways of a cell, those functional parts will have nothing to consume since the capsid does not allow the proper exchange of molecules between the interior of the capsid and the outside environment. Those metabolic pathways that can consume component parts inside the capsid may inactivate the virus. Even assuming that there can be a possibility for a primitive metabolism, capsid proteins hinder replication of a possible “new life form.” This is the likely reason why a virus of considerable size lacks the spark of metabolism. But are viruses alive? The virus environment is the cell. Without this environment, viruses become inactive until different stochastic processes lead to reactivation. For cells, the environment is represented by molecules that can be metabolized. Without these substances, cells either decay in simpler macromolecules or enter a hibernation state like viruses do. Therefore, the answer is relative and dependent on our reference system.
1.11 The Diffusion Coefficient
But why a discussion about the size of organisms? Mass diffusivity (diffusion coefficient) is a physical constant that impacts the way an organism can evolve. The cell volume must be balanced with the cell surface; otherwise, the exchange with the external environment becomes inefficient. This exchange consists of metabolites that must exit the cell per unit time or nutrients that must enter the cell per unit time. Multicellularity allows an organism to exceed the size limits normally imposed by diffusion. On the other hand, unicellular organisms with increased size have a decreased surface-to-volume ratio and may have difficulty in absorbing and transporting sufficient nutrients throughout the cell. As a counterbalance, unicellular eukaryotic organisms have among the most varied shapes and sizes observed in nature. Both unicellular and multicellular organisms can achieve a high surface-to-volume ratio by favoring DNA mutations that lead to a convoluted surface. For instance, to increase their surface area, choanocyte organisms can take many forms, such as C. taxifolia, which resembles a kind of “pine leaf” or S. fragilissima, which has a convoluted surface.
1.12 The Origins of Eukaryotic Cells
All eukaryotic cells contain membrane-bound organelles (e.g. the nucleus, mitochondria, chloroplasts, and so on). The complexity of the eukaryotic cell is given by the presence and the interaction of organelles. The origin of organelles has always been a mystery difficult to explain. However, the endosymbiotic theory is the leading evolutionary theory for the origin of eukaryotic cells. The idea of endosymbiosis was first proposed by Konstantin Mereschkowski in 1905 [96, 97]. According to the theory of endosymbiosis, the eukaryotic cell is like a Matryoshka (Russian doll). A symbiotic relationship where one organism lives inside the other is known as endosymbiosis. The term “primary