"Marine virology is a new field, and when it comes to the poles in particular we know even less," Claire Evans of the Royal Netherlands Institute for Sea Research told environmentalresearchweb from onboard the Amundsen, a Canadian icebreaker taking part in a research project for International Polar Year. "Grazing [of phytoplankton] has been studied for a long, long time but we don’t know much about viruses which might be equally significant for controlling population and the flow of carbon."
Evans is counting the number of viruses in the Arctic Ocean near Banks Island, as well as examining their diversity and how they are infecting phytoplankton and bacteria.
"When a virus infects a cell – we’re interested in the lytic cycle here – it basically converts that cell to new viruses and cellular debris because it causes the cell to burst open, or lyse," she explained. "So it releases all its content to the water column. What happens then is that that matter and energy – that carbon – isn’t available to creatures to graze, which is how it gets passed up the food web."
Instead, in an effect known as the "viral shunt" the carbon from infected cells remains present as dissolved organic matter in the euphotic zone – the waters near the surface of the oocean where enough light penetrates for photosynthesis to take place. From here, bacteria remineralize it into inorganic nutrients that are used by the phytoplankton.
"The process cycles that matter and energy through the small parts of the food web – basically the surface part of the ocean where the phytoplankton are," said Evans. "If you stop the carbon being transferred into higher trophic [food web] levels then that carbon isn’t lost via the biological pump to the deep oceans."
The biological pump is the mechanism through which organic carbon travels from the euphotic zone to the bottom of the ocean, for example when large organisms such as seals and whales die and their bodies sink to the seafloor. The viral shunt is estimated to recycle around 25% of the organic carbon in the euphotic zone, decreasing the efficiency of the biological pump and making the ocean a less effective sink for carbon dioxide.
"At the poles it may be that there isn’t so much viral lysis," said Evans. "With the really significant changes that are going on in this environment right now we are likely to see significant changes in the speciation and types of organisms and we might get more viral lysis or we might get less. If we start to perturb the normal cycle one way or the other it could potentially have a negative impact – or even a positive impact – on global change."
With this in mind, Evans is trying to find out more about the normal cycle before too much change takes place. Viruses are generally very host specific so alterations in the balance of phytoplankton species will also also affect virus diversity and abundance. Once back from the Amundsen, Evans expects to spend the next 18 months analysing her results.
Ocean viruses don’t just affect the carbon cycle – they can also alter the quality of phytoplankton as a food source. Cells infected with a virus tend to contain a smaller proportion of polyunsaturated fatty acids (PUFAs). These acids are used by animals higher up the food chain for growth and reproduction, so infected organisms are a poorer quality source of food. And, for reasons that aren’t yet clear, organisms tend to prefer to graze on infected cells.
"If the virus infects the base of the food web, it leads to less PUFAs and that might make one copepod species more successful than another, say if one needs these PUFAs for its eggs to properly develop," said Evans. "If you don’t get that PUFA and you get another copepod instead, which may have a lower nutritional value, that could decrease the breeding success of a bird higher up the food chain."