Viruses are everywhere. They are on surfaces; they are in the air and in the water. They appear in vast numbers and variations, assays have shown about 100 billion virus particles in one litre of seawater and another study found 174 different species in the lungs of only 10 people, 90% of these viruses had been unknown to science at the time.
The origins of viruses are not entirely clear. One theory claims that viruses appeared before the first cell and present a separate evolutionary line. Another theory suggests that viruses are remnants of early cells that shed all the luxury components of a living cell and kept only the bare minimum of molecules to survive and reproduce. The question if viruses are alive or not however hasn’t been answered so far. Over the decades the science community saw viruses as a poison (the word virus comes from the Latin for poison), then as proper life-forms and later as a biological chemical. Today the virus sits in a grey area between alive and not alive. A virus doesn’t feed, it doesn’t cycle energy and nutrients and it doesn’t carry out metabolic activities. It does however it reproduce, and it does so in an ingenious way. To reproduce a virus needs a host cell. A virus isn’t much more than its DNA (or RNA) wrapped in a coat of proteins. Some more sophisticated viruses also own a lipid membrane and a few accessory proteins that help the virus getting access to its host. Viruses are species specific which means a virus is only able to gain access to cells of a certain species and within this species only a certain kind of cell. The Human immunodeficiency virus (HIV) for example only affects certain cells of the human immune system and the Corvid-19 virus can’t be transferred from human to dogs and cats. Viruses can however change their genetic profile to gain compatibility with another species as SARS and Covid-19, which are both thought to have originated in animals, have shown.
Once a virus has landed on a suitable host cell, it penetrates the cell membrane, empties its DNA (or RNA) into the cell and transforms the involuntary host into a virus factory. The virus uses certain components of the cell to copy its own nucleic acid (DNA or RNA) and built new protein coats for a new generation of viruses. Once this process is finished the new viruses are assembled and released which often means the death of the host cell. The new viruses repeat this process over and over. In this way one virus can multiply into millions and more in a rather short period of time. It is the death of the cell at the end of the reproduction process that the host experiences as an illness. Coronaviruses like the ones that cause the common cold, the flu and the now infamous Corvid-19 reproduce in this way.
The rapid reproduction however comes with a problem. Imagine you are given 12 hours to copy a book multiple times by hand. You will make mistakes, a spelling error here, a missing word there. Something similar happens when the virus DNA (or RNA) is being copied and the result is one or more viruses with a slightly different DNA (or RNA) as the original, the virus has mutated. Viruses however don’t only change their genetic makeup by chance, they also do so to adapt to changes in the environment.
In addition to the just described lytic reproduction cycle some viruses take a slightly different route known as the lysogenic cycle. These viruses combine their DNA (or RNA) with the host chromosome and multiply through the normal cell division. The virus DNA (or RNA) however changes certain aspects of the cell division process and this is why some viruses can cause cancers.
We are however not the only species affected by viruses. In 1988 the phocine distemper virus killed 18.000 common seals and 300 grey seals and a second outbreak in 2002 caused the death of 21.700 common seals along the North Sea, about 50% of the population. The cause for these outbreaks was very likely environmental changes caused by human activity. Pollutants might have interfered with the ability of the affected animals to mount a defence against the infection. Climate change and overfishing may have forced aquatic species, that naturally harbour the virus, into new areas and so exposing the resident animals to an unknown pathogen. Some theories suggest that the Corvid-19 pandemic is also a result of the impact we have inflicted on the environment over the past decades and it is being predicted that the Corvid-19 pandemic is only the first of many. New viruses may evolve, and very old viruses may rise from the thawing permafrost.
While viruses cause disease and death in their hosts, they also play an important role in ecosystems. Especially in the ocean viruses, most of them bacteriophages that only attack single celled organisms, play a vital role by controlling the population dynamics of marine microbes like single celled algae. Microorganisms constitute more than 90% of the biomass in the sea and it is estimated that viruses kill around 20% of this biomass each day. By doing so viruses prevent a single species of microbe from dominating their environment and as a consequence maintain diversity at the very bottom of the food chain and also increase the supply of nutrients to the pelagic food web. In addition the dead microbes release dimethyl sulphide, a gas that, after being oxidized into various sulphur containing compounds, induces cloud formation and so affects the carbon and oxygen cycle. This process is known as viral shunt and what has been discovered about the viral shunt mechanism so far is very likely only the tip of the proverbial iceberg. Viral shunt has also been found in freshwater environments and it is thought that something similar happens in the soil as well.
Scientists are just starting to understand the influence of viruses on ecosystems and the interrelation between viruses, plants, animals and us but it seems clear that viruses play a big part in keeping the balance in the natural web.
Carsten Krieger, March 2020
Image by Fusion Medical Animation