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Virivore (equivalently virovore) comes from the English prefix viro- meaning virus, derived from the Latin word for poison, and the suffix -vore from the Latin word vorare, meaning to eat, or to devour; therefore, a virivore is an organism that consumes viruses. Virivory is a well-described process in which organisms, primarily heterotrophic , consume viruses, though some are known to do so, as well.
are considered a top predator in marine environments, as they can Lysis microbes and release nutrients (i.e. Viral shunt). Viruses also play an important role in the structuring of microbial trophic relationships and regulation of carbon flow.
General grazing on viruses is widespread throughout the marine environment, with grazing rates as high as 90.3 mL−1 day−1. When both bacteria and viruses are present, viruses can be ingested at rates comparable to bacteria.
Using Oikopleura dioica and Equid alphaherpesvirus 1 (EhV) as a model, scientists estimated the nutritional gain from viruses;
It's suggested that in smaller grazers, viruses could potentially have a more significant impact on host nutrition. For example, in , the estimated contribution is 9% carbon, 14% nitrogen, and 28% phosphorus.
While smaller bacteria are the ideal food source for grazers due to their size and carbon content, viruses are small, non-motile, and extremely abundant for grazers making them an alternative nutritional choice. For general grazers, to obtain the same amount of carbon from viruses that they get from bacteria, they would need to consume 1000 times more viruses. This does not make viruses the ideal carbon source for grazers. However, there are other benefits to consuming viruses besides growth. Studies show that digested viral particles release amino acids that the grazer can then utilize during their own polypeptide synthesis.
The viral sweep could be affected by many factors such as the size and abundance of the viral particles. The size of the virus will effect the elemental content of the virus particles. For example, a virus with a larger capsid will contribute more carbon, and viruses with larger genomes will contribute more nitrogen and phosphorus as a result of the increased nucleic acids. Additionally, the impact of the viral sweep could be more significant if grazers preying on bacteria infected with viruses are also considered. Overall, by consuming bacteria and viruses, grazers play an important role in cycling carbon.
are a key link in marine food webs as they connect primary and secondary production with higher trophic levels. When phytoplankton Emiliania huxleyi were infected with the coccolithovirus EhV-86, ingestion of the infected cells by the calanoid copepod Acartia tonsa was significantly reduced compared to non-infected cells, indicating selective grazing against infected cells. These results suggest that viral infections reduce grazing, and may potentially reduce food web efficiency by keeping the carbon within the viral shunt-microbial loop, and inhibiting the movement of carbon to higher trophic levels. This emphasizes the importance of the viral sweep for cycling carbon into higher trophic levels. Conversely, Oxyrrhis marina had a grazing preference for virally infected Emiliania huxleyi. It's suggested that the preference of infected cells over non-infected cells is due to physiological changes or change in size of the host cell. O. marina prefer to graze on larger cells as they could potentially get a greater nutritional value from them compared to a smaller cell, which would require the same amount of energy to consume. Infected E. huxleyi exhibit increased cell size compared to non-infected, making them an ideal prey for O. marina. Infected E. huxleyi may also be selected for their palatability as a result of physiological changes during infection. For example, infected cells will have higher nucleic acid content compared to non-infected cells which could improve the nutritional gain to the grazers. Additionally, grazing activity of O. marina has been linked to prey with lower dimethylsulfoniopropionate lyase (DMSP lyase) activity, as they would produce less of the potentially toxic compound acrylate. Virally infected E. huxleyi show reduced levels of DMSP lyase activity, which makes them appealing to O. marina by reducing their exposure to harmful compounds. Lastly, chemical cues such as the release of dimethyl sulfide and hydrogen peroxide during infection likely generate a gradient, making it easier for O. marina to locate the infected E. huxleyi. Preferential grazing on infected cells would make the carbon available to higher trophic levels by sequestering it in particulate form.
Overall, grazing on virus particles and virally infected cells are subject to selective grazing.
EhV particles can be consumed by copepods either as individual virion particles or via host cell infection (in this case, infected Emiliania huxleyi). When infected E. huxleyi was co-incubated with copepods, the fecal pellets produced by the copepods contained an average of 4500 EhVs per pellet. These virion containing pellets were then co-incubated with a fresh culture of E. huxleyi, and rapid viral-mediated lysis of the host cells was observed. When EhV particles alone were co-incubated with copepods, i.e. no E. huxleyi, the fecal particles collected did not contain any virion particles. However, when they fed copepods EhV and Thalassiosira weissflogii, a diatom outside the host range of EhV, the fecal pellets collected contained 200 EhVs per pellet. These pellets when co-incubated with a fresh E. huxleyi culture were highly infectious and completely killed the culture. The absence of virion particles in the fecal pellets produced from sole EhV incubation supports the idea that grazers exhibit selective grazing for viruses. EhV can still be taken up by copepods through host cell infection and when in the presence of an ideal food source. Since viral abundance follows bacterial abundance, it is unlikely that there will be a marine environment where viruses will be the sole nutrient source for grazers.
The results of this experiment have significant ecological impacts. Copepods are capable of moving up and down the water column, and migrating short distances between feeding zones. Specifically, for copepods and EhV, the movement of copepods can transport viruses into new and non-infected populations of E. huxleyi, promoting bloom demise. Additionally, fecal pellets can sink from the mixed layer into deeper parts of the ocean, where they can be assimilated multiple times. These two scenarios represent potential mechanisms in which viruses can be introduced into new marine environments.
The method in which non-host organisms disrupt the viral-host contact is known as transmission interference. Non-host organisms can either have a direct impact by removing the host-organisms, or an indirect one by removing the viruses. These mechanisms cause a reduction in the virus-host contact rates which could significantly impact local microbial population dynamics.
Non-host organisms are capable of removing viruses at rates comparable to natural food particles, bacterial cells, and algal cells, which is higher when compared to grazers that have a viral clearance rate around 4%. In regions of high sponge densities, such as coastal and tropical regions, it is likely that the virus removal rate has been underestimated. The effective removal of viruses likely has global ecological impacts that have gone unrecognized.
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