Paul A. Gagniuc - Algorithms in Bioinformatics

Marine life shows both the maximum and minimum sizes for unicellular organisms. For instance, a member of the green algae, Caulerpa taxifolia , is a unicellular organism of 30 centimeters in length, or more [78]. The Syringammina fragilissima is another example of a unicellular organism, which reaches ∼20–25 cm in diameter or Ventricaria ventricosa, which is a cell of 2–4 cm in diameter [79, 80]. On the other hand, the smallest unicellular eukaryote appears to be Ostreococcus tauri , a marine green alga with a diameter of about 0.8 μm [81, 82].

Through cooperation, eukaryotic multicellular organisms have been able to evolve large dimensions. In water, buoyancy counterbalances gravity and it allowed for evolution of the largest organisms on the planet. For instance, Balaenoptera musculus (the blue whale) is a marine mammal of 27–30 m and around 170–200 tones [83]. It may be the largest contemporary organism on the planet. On land, Loxodonta africana (the African savanna elephant) is the largest living land animal [84]. Among birds, Struthio camelus (the common ostrich) can reach 2.8 m in height and weigh over 150 kg [85].

On the other hand, prokaryotes from Mycoplasma species show some of the smallest possible dimensions for life (∼100 species). For instance, bacteria Mycoplasma gallicepticum and Mycoplasma genitalium are likely two of the smallest self-replicating forms of life, with a diameter of ∼0.0002 mm (0.2 μm or 200 nm) [86, 87]. This small size is 2 up to four times smaller than the wavelength of a photon of light from the visible spectrum (700–400 nm). However, the largest species of bacterium found among prokaryotes are Thiomargarita namibiensis and Epulopiscium fishelsoni (between 0.5 and 0.7 mm), which are comparable in size to some unicellular eukaryotes [88, 89].

Small viruses are predominant and may be round, or rod shaped, or a combination of the two in the case of prokaryotes. A capsid is the protein shell of a virus, which can remind us of the idea of Platonic solids. For some small viruses, usually the self-assembly process is dictated by their nucleic acids sequence (motif seeds) [90]. This protein shell seals their genetic material from the environment. However, many capsid proteins can self-assemble with no additional help. Viral proteins have structural properties that allow regular and repetitive interactions among them. Identical protein subunits are distributed with helical symmetry for rod-shaped 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.

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.

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.

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.