What is a virus?
Viruses are infectious agents that replicate exclusively within a living cell. Viruses can infect all types of organisms, from animals and plants to bacteria and archaea. They consist of a viral particle that protects the viral genome from degradation in the extracellular environment and allows the infection of another cell. The viral particle often referred to as virion is composed of genetic material, a protein coat and, in some cases, a lipid envelope. Viruses are highly diverse genetically and structurally, and many different virus families have been described to date. Viral genomes are formed by either single strand (ss) or double strand (ds) DNA or RNA. Single stranded genomes are divided into positive or negative strand types, depending on whether or not they are complementary to the viral messenger RNA. The mechanism of replication is diverse and depends on the nature of the viral genome, requiring RNA or DNA polymerases of viral or host origin, and the mechanism of messenger RNA production (Baltimore classification of viruses). Viral particles are also diverse, with a size ranging from 20 to 300 nm in diameter, a helical or icosahedral morphology, and an additional lipid envelope in some cases
Replication
Viruses cannot replicate on their own and need to infect a host cell for replication. Upon virus attachment and entry, viruses take full control of the host cell and use the host cells machinery and cellular compartments to produce copies of the viral genome and synthesize new virus particles. We divide the virus infection in two different cycles. The lytic cycle leads to the death of the infected cell and the release of high numbers of infectious virus particles into the environment that are ready to infect neighbouring cells. Alternatively, viruses may introduce their viral genome into the host cell and remain silent within the infected cell in a latent state, where the viral genome is integrated into the host genome or maintained as an episome (a genetic element inside a bacterial cell). This type of infection is known as lysogenic infection for bacterial viruses (bacteriophages). Viruses that establish lysogeny or latency may later become activated to induce a lytic replication cycle that will destroy the infected cell and produce new virus particles that will infect new cells, ensuring the transmission and persistence of the virus in the host population.
Viruses - more abundant than you might think
In the past, only viruses known to be pathogenic were isolated and studied. More recently, with the advancement of methodology, it has been realized that viruses constitute the most abundant and diverse biological entities on Earth (Suttle 2005; Rohwer 2009), and more focus has been directed towards them. Viral abundances in aquatic ecosystems are usually quantified using epifluorescent microscopy counting stained virus-like particles (VLPs), but virus particles have also been visualized by electron microscopy (Kepner 1998; Wilson 2000) and flow cytometry analyses. Environmental samples contain 1000-10000 million VLPs per litre of water in the oceans, 100-1000 VLP in aquatic habitats and 1000 million VLPs per cubic cm of soil (Sawstrom 2007, Sawstrom 2008, Suttle 2005). This is typically about 10 times the number of bacteria for aquatic habitats and it is estimates that they outnumber bacteria in the soil 5- to 1000-fold (William et al 2013). It has been estimated that there are 4 x 1030 virus particles in the oceans, equivalent to the carbon mass of 75 millions of blue whales (Suttle 2005). If all these viral particles were aligned in a single column, they would extend 10 million light years, 100 times the distance across our galaxy. This extraordinary abundance of viruses has changed our view of the role of viruses in ecosystems.
Viruses in the polar regions
Viruses has been found to be highly abundant in Arctic ecosystem, and they play an important role in the control of microbial communities, in these ecosystems, with low nutrients and under extreme environmental conditions (Rastojo and Alcami 2018).
Still, little is currently known about viral diversity, ecology and function in the Arctic. The limited number of studies on Arctic viruses have mainly identified viruses from the sea ice and cryoconite holes, at the surface of Arctic glaciers (De Corte 2011; Anesio 2011). In comparison, a high number of studies focus on Antarctic viruses, which can be expected to provide information on the viral communities in polar regions that are adapted to extreme environmental conditions similar to those found in the Arctic. In order to highlight the importance and function of polar viruses in the environment, examples from aquatic environments as well as Antarctica are included here.
Figure illustrating the structural differences between RNA and DNA.
Figure: Sponk, Creative Commons 3.0
SARS-Coronavirus (Coronaviruses), intracellular and on the surface of vero cells. Electron microscopy, ultrathin section. Bar = 100 nm
Source: Hans R. Gelderblom, Freya Kaulbars/RKI
Influenza A virus A/California/7/2009 (H1N1), colouring, negative staining, Transmission electron microscopy (TEM). Initial magnification x 85000
Source: Gudrun Holland, N. Bannert/RKI
Photo of cryoconite holes. These form when dark particles are blow on top of glaciers, then when being heated by the sun melts holes into the glacier. They then work as small oases for microbes.
Photo: Alan Grinberg. Wrangell-St. Elias National Park & Preserve, Alaska, USA. Attribution-NonCommercial-NoDerivs 2.0 Generic (CC BY-NC-ND 2.0)
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