Three novel Giruses

Primary production in the oceans is hugely maintained by photosynthetic microorganisms, forming the basis of oceanic food webs. The diversity abundance and dynamics of these photosynthetic microbes is regulated by not only physico-chemical factors but also grazers and parasites. Viruses play an especially important part in terms of the break-down of the microalgae, particularly in larger blooms where it is thought the large population size maintains a high virus population. Johannessen et al. focus on virus-algae interactions of the Haptophytes, Prymnesiales spp. Currently, most isolated algal-infecting viruses have been shown to have dsDNA genomes, belonging to either Mimiviridae or Phycodnaviridae families. Mimiviridae family belongs to the group Nucleocytoplasmic Large DNA Viruses (NLDV), including many giant viruses with genome sizes up to 1.2 Mbp. This family, along with six other families are suggested to belong to a fourth domain of life; the capsid encoding organisms.

Viral strains were collected from two sites off the coast of Norway, while microalgal cultures obtained from the Universities of Oslo and Bergen. Electron microscopy was used to assess the morphology of viral lysates in terms of shape and size. Genomes sizes were characterised using gel electrophoresis and the nature of the viral genomes (double-or single-stranded, DNA or RNA) was determined using enzyme digestion and further gel electrophoresis. PCR was then used to amplify sequences for phylogenetic analysis using ClustalW.

Ten phytoplankton strains were screened for viral infection, eight of which showing lysis within two weeks. Three of these viral lysates were chosen for purification and were classified as novel species; Haptolina ericina virus RF02(HeVRF02), Prymnesium kappa virus RF01(PkVRF01), and Prymnesium kappa virus RF02(PkVRF02). All three viruses were tailless and have icosahedral capsids, with linear dsDNA genomes. All were capable of infecting multiple hosts suggesting that cross-species infectivity is more widespread that previously thought, perhaps making the current system of viral nomenclature, based on the name of the host it infects, redundant. PkVRF01 and HeVRF02 were able to infect the same host, while phylogenetic analysis placed them in different groups. This cross-species infection may also explain how viruses with relatively low abundances survive in environments of low-host abundance, this versatility allows for survival. They also showed novel characteristics in terms of host range and phylogenetic affiliation, all three show similarities to the NCLDV group. Their size, shape, location of virions, MCP and DNA polB-phylogenies suggest all three to belong to the Mimiviridae family, showing there is still a lot of work needed on this ever expanding family. The strain PkV RF01, showed to be especially novel in terms of size as it is the largest algal virus particle to be reported. Its large capsid size suggests it to have a larger genome than any other cultured algal virus. Its position on the phylogenetic tree also suggests its relation to Megavirus chilensis, a species also capable of infecting multiple hosts.

It has been suggested that giant virus discoveries such as this are a way of bridging the gap between the living and non-living, having possible implication for the origin of life. The debate is still hot, however, I believe studies like these are a way of further explaining such controversies. Giruses are clearly of utmost importance and future study may reveal much about the way we classify life.

Johannessen, T., Bratbak, G., Larsen, A., Ogata, H., Egge, E., Edvardsen, B., Eikrem, W. and Sandaa, R. (2015). Characterisation of three novel giant viruses reveals huge diversity among viruses infecting Prymnesiales (Haptophyta). Virology, 476, pp.180-188.