VIROVAC: Filter-feeding mesozooplankton as uncharted accomplices in marine virus ecology

Viruses fill the world's oceans and are more abundant than any other biological entity on the planet! Even though viruses are not "alive", they are critical regulators of the diversity of organisms and the efficiency of biogeochemical processes that occur in the oceans. Despite this, we know very little about viruses, or about how these tiny entities persist and spread. One group of marine animals, the planktonic tunicates known as appendicularians, are specialists when it comes to feeding on low concentrations of small food particles. In fact, the filtering "houses" that they live inside and use to trap food particles are so efficient that they can even trap large viruses! This suggests that viruses can not only prey upon the host organisms that they infect, but that they themselves can end up on the menu for small-particle specialists like appendicularians. Although being trapped in the filtering house of an appendicularian may mean digestion for a virus particle, the rapidity with which appendicularians renew their houses and empty their guts provides the possibility of virus ?survival? when trapped inside a discarded house or faecal pellet. Both houses and faecal pellets sink rapidly to the ocean floor, ferrying trapped virus particles to an unknown ecological fate. The previously overlooked interaction between appendicularians and viruses thus stands to reshape our understanding, not only of the role of viruses in the ocean, but also of the mechanisms for how viruses may persist and spread both in space and in time. Which viruses can be trapped and ingested by appendicularians? How many viruses sink to the ocean floor trapped inside appendicularian houses and faecal pellets? How does trapping inside houses and faecal pellets affect the infectivity of viruses relative to their free-floating counterparts? In the VIROVAC project, researchers at NORCE Environment and the University of Bergen are attempting to answer these questions using a combination of laboratory experiments and field sampling. They are assisted by experts in virus and phytoplankton ecology in Canada. Through small-scale clearance experiments we have confirmed that the appendicularian Oikopleura dioica efficiently removes different types of marine viruses from seawater, and that these viruses retain infectivity in O. dioica faecal pellets, despite having passed through the animal gut! We have also taken part in a large seawater mesocosm experiment, during which we demonstrated that O. dioica can also efficiently remove EhV from complex natural marine virus assemblages, moving the potential importance of this interaction from the laboratory into the marine environment. During the past two years, we have also gathered sediment and water samples around Bergen to test for the presence of EhV particles throughout the water column and upper sediment layers. Our cruise series demonstrates clearly that EhV particles can sink to the seafloor and collect in sediments, where they retain their infectivity for weeks and possibly months. Using the data from this two-year cruise series, we are in the process of testing the validity of our mechanistic model in which O. dioica abundance in the water column positively correlates with EhV accumulation in underlying sediments. The most important lessons learned during this project are that O. dioica feeding may directly impact both the abundance and ecological fate of different marine viruses.

We have generated new knowledge about the ability of the appendicularian Oikopleura dioica to trap, ingest and redisperse large algal viruses of phylogenetically and ecologically diverse phytoplankton hosts. Virus particle size has only a moderate effect on the efficiency with which viruses are trapped and removed. Trapped and ingested viruses are rapidly passed through the animal gut and dispersed inside discarded houses and fecal pellets, where a fraction of viruses retain infectivity for weeks and possibly months. We have also demonstrated that infectious EhV particles are present in surface and downcore sediments at three different western Norwegian stations at which O. dioica naturally occurs in the water column. These findings in concert suggest that the O. dioica-virus interaction may be one important mechanism by which planktonic marine viruses are ferried to bottom sediments where they can retain their infectivity and seed infection of seasonal phytoplankton blooms.

Viruses fill the world's oceans and are more abundant than any other biological entity on the planet! Even though viruses are not "alive", they are critical regulators of the diversity of organisms and the efficiency of biogeochemical processes that occur in the oceans. Despite this, we know very little about viruses, and in particular about how these tiny entities persist and spread. One group of marine animals, the planktonic tunicates known as appendicularians, are specialists when it comes to feeding on low concentrations of small food particles. In fact, the filtering "houses" that they live inside and use to trap food particles are so efficient that they can even trap large viruses! This suggests that viruses can not only prey upon the host organisms that they infect, but that they themselves can end up on the menu for small-particle specialists like appendicularians. Although being trapped in the filtering house of an appendicularian may mean digestion for a virus particle, the rapidity with which appendicularians renew their houses and empty their guts provides the possibility of virus “survival”, either trapped in a discarded house or inside a faecal pellet, both of which sink rapidly to the ocean floor and an unknown ecological fate. The previously overlooked interaction between appendicularians and viruses thus stands to reshape our understanding, not only of the role of viruses in the ocean, but also of the mechanisms for how viruses may persist and spread both in space and in time. Which viruses can be trapped and ingested by appendicularians? How many viruses sink to the ocean floor trapped inside appendicularian houses and faecal pellets? How does trapping inside houses and faecal pellets affect the infectivity of viruses relative to their free-floating counterparts? In the VIROVAC project, researchers at NORCE Environment and the University of Bergen will attempt to answer these questions using a combination of laboratory experiments and field sampling. They are assisted by experts in virus and phytoplankton ecology in Canada. Through small-scale clearance experiments we have confirmed that the appendicularian O. dioica efficiently removes different types of marine viruses from seawater, and that these viruses retain infectivity in O. dioica faecal pellets, despite having passed through the animal gut! We have also taken part in a large seawater mesocosm experiment, during which we demonstrated that O. dioica can also efficiently remove EhV from complex natural marine virus assemblages, moving the potential importance of this interaction from the laboratory into the marine environment. We have also conducted three one-day cruises near Bergen and gathered sediment and water samples to test for the presence of EhV particles, and have shown that EhV particles can sink to the seafloor and collect in sediments, where they retain their infectivity for weeks and possibly months. The most important lessons learned are that O. dioica feeding may directly impact both the abundance and ecological fate of different marine viruses. Furthermore, our ability to detect infectious virus particles in seafloor sediments allows us to test our mechanistic model that O. dioica abundance in the water column positively correlates with EhV accumulation in underlying sediments.