Introduction

The ocean covers approximately 71% of the world’s surface. This means that the ocean is very important for the biosphere and the reason we should care about viruses. The oceans are estimated to absorb 3 gigatonnes of carbon dioxide a year, meaning life as we know it could not function without a highly productive ocean playing its part in the carbon cycle. In terms of biomass bacteria are ranked number one for ocean organisms, it has been estimated that the weight of all the bacteria in the ocean is equivalent to 50 million blue whales, or a gigaton to quote units. When looking at the most numerous organisms however the viruses are a clear winner, they are at least an order of magnitude greater than even the bacteria. This boils down to there being on average 10 million viruses in a single teaspoon of seawater. This may seem utterly horrific when thinking about all the terrible human diseases that are caused by viral infections, but thankfully the large majority of these viruses pose no threat to mankind. A large proportion of the viruses in the ocean are known as bacteriophages, this means that they only infect bacteria cells. Bacteriophages can be very useful however, they are put to good use in a number of ways. They were originally discovered in 1915 and even since then they have been in used in medical science and food preservation. They act as bacterial assassins, infecting and killing their highly specific targets. The best part is due to them not being able to infect human cells their use is one of the safest antibacterial methods available. This had led to their widespread use with cheese for example as they are sprayed onto the cheese before it’s packaged helping to preserve it from bacterial breakdown.

When a virus finds an appropriate cell to infect it attaches to the cells surface and injects its own DNA into the host cell. The cell’s own metabolic systems are then hijacked as it’s forced to create replicates of the virus. In a process known as cell lysis, the host cell then bursts releasing the newly created viruses, as well as the former cells contents. The video below is how some viruses get there genetic material into host cells.

The most recent calculations put the number of viruses in the world’s ocean in the region of 4 x 1030. If all these viruses were lined up single file, then they would stretch out 1×1020 km or 10 million light years. Viruses play a crucial role in keeping the number of prokaryotic organisms in the ocean under control. It has been estimated that every day a quarter of the bacteria in the ocean are killed by viral infections, this means that close to 250,000,000 tonnes of the bacterial matter is put back into the water column.

Viral impacts

As said in the title, viruses are administered of death and it is this quality that makes them such a crucial part of ocean ecosystems. Viruses can impact the populations of nearly every organism in the ocean from the largest whales, right down to the smallest phytoplankton. The number of organisms killed by viral infections in a host population is not as a steady state. As the host organisms’ population increases and decreases so does the infecting virus population. This means that viruses help to keep populations of ocean organisms and in balance and may possibly dampen the effect of rapid blooms of certain species.

A second effect that ocean viruses have is their reduction of pathogenic bacteria. With bacteriophage viruses reducing the population of bacteria, it means that instances of bacterial disease will be lowered in the ocean and may actually mean that the populations of some organisms are higher than they would be without viruses.

viral-shunt
Diagram showing viral shunt and how organisms are transferred in dissolved organic matter and particulate organic matter. the proportion of the three elements are not kept in the same proportions as in the hose cell. the particulate matter that has a high proportion of carbon is more likely to sink down the water column where as the dissolved organic matter is more likely to stay in the photic zone. Original source: Suttle, C.A., 2007. Marine viruses—major players in the global ecosystem. Nature Reviews Microbiology, 5(10), pp.801-812.

The most significant effect that viruses have in the ocean is called “viral shunt”. Viral shunt is the process of viruses infecting a prokaryotic cell and then causing cell lysis where the infected cell bursts not only releasing newly made copies of the virus but also all of the former cells condense into the surrounding ocean. It is thought that 65-100% of the available nitrogen and phosphorous in the water column has come from viral lysis. This liberation of biologically available nutrients is what makes viruses critically import to biochemical cycles such as the ocean’s absorption of carbon dioxide, and is what means that life as we know it is possible on earth.

How viruses increase co2 absorbance in the ocean

Even though our understanding of viruses is far from complete, it is thought that they increase the amount of carbon dioxide absorbed by the ocean in two ways. The first method is that they increase the level of surface organisms sinking to the ocean floor. It has been noted in studies that flagellated cell when infected by viruses have lost their ability to swim. Without the ability to swim the cell obviously, begins to sink. The time taken from viral infection to the cell bursting is not a set time and is depend on a number of factors such the physiological condition of the infected cell. If the cell lysis process takes a significant amount of time then it will sink out of the surface water all the nutrients from when the cell bursts will be transferred into the ocean sediment.

virus-carbon-flow
A hypothetical food web showing the movement of carbon with and without viruses. The numbers represent units of carbon moving between the different component organisms. The resulting matter from cell lysis is expected all become dissolved organic matter. Original source: Marine viruses and their biogeochemical and ecological effects, Jed A. Fuhrman, Nature 399, 541-548(10 June 1999), doi:10.1038/21119

The second method in which viruses increase the amount of carbon dioxide absorbed by the ocean is the previously mentioned process of the viral shunt. Levels of nutrients in the water column can often be a limiting factor for phytoplankton growth. Viral shunt released organic compounds that can be quickly absorbed and used by the surrounding uninfected organisms. Without viral shunt is has to be predicted that a lot more of the nutrients incorporated into these cell membranes would sink to the ocean floor and thus reduced the levels of these nutrients in the photic zone. Without these nutrients being retained and released back into the photic zone photosynthesis would operate at a reduced level than it does currently.

Why they are extreme?

Without viruses creating increased photosynthesis in the ocean and having a regulatory effect on many of the organisms in the ocean the world would be a very different place and this is what qualifies them to be extreme organisms.

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