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Phaeoviruses ( Phycodnaviridae ) are large icosahedral viruses in the phylum Nucleocytoviricota with dsDNA genomes ranging from 160 to 560 kb, infecting multicellular brown algae (Phaeophyceae). The phaeoviral host range is broader than expected, not only infecting algae from the Ectocarpales but also from the Laminariales order. However, despite phaeoviral infections being reported globally, Norwegian kelp species have not been screened. A molecular analysis of cultured and wild samples of two economically important kelp species in Norway ( Saccharina latissima and Laminaria hyperborea ) revealed that phaeoviruses are recurrently present along the Norwegian coast. We found the viral prevalence in S. latissima to be significantly higher at the present time compared to four years ago. We also observed regional differences within older samples, in which infections were significantly lower in northern areas than in the south or the fjords. Moreover, up to three different viral sequences were found in the same algal individual, one of which does not belong to the Phaeovirus genus and has never been reported before. This master variant therefore represents a putative new member of an unclassified phycodnavirus genus.

Viruses influence the ecology and diversity of phytoplankton in the ocean. Most studies of phytoplankton host-virus interactions have focused on bloom-forming species like Emiliania huxleyi or Phaeocystis spp. The role of viruses infecting phytoplankton that do not form conspicuous blooms have received less attention. Here we explore the dynamics of phytoplankton and algal viruses over several sequential seasons, with a focus on the ubiquitous and diverse phytoplankton division Haptophyta, and their double-stranded DNA viruses, potentially with the capacity to infect the haptophytes. Viral and phytoplankton abundance and diversity showed recurrent seasonal changes, mainly explained by hydrographic conditions. By 454 tag-sequencing we revealed 93 unique haptophyte operational taxonomic units (OTUs), with seasonal changes in abundance. Sixty-one unique viral OTUs, representing Megaviridae and Phycodnaviridae , showed only distant relationship with currently isolated algal viruses. Haptophyte and virus community composition and diversity varied substantially throughout the year, but in an uncoordinated manner. A minority of the viral OTUs were highly abundant at specific time-points, indicating a boom-bust relationship with their host. Most of the viral OTUs were very persistent, which may represent viruses that coexist with their hosts, or able to exploit several host species.

Viruses are thought to be fundamental in driving microbial diversity in the oceanic planktonic realm. That role and associated emerging infection patterns remain particularly elusive for eukaryotic phytoplankton and their viruses. Here we used a vast number of strains from the model system Emiliania huxleyi /Emiliania huxleyi Virus to quantify parameters such as growth rate (µ), resistance (R), and viral production (Vp) capacities. Algal and viral abundances were monitored by flow cytometry during 72-h incubation experiments. The results pointed out higher viral production capacity in generalist EhV strains, and the virus-host infection network showed a strong co-evolution pattern between E. huxleyi and EhV populations. The existence of a trade-off between resistance and growth capacities was not confirmed.

We have isolated three novel lytic dsDNA-viruses from Raunefjorden (Norway) that are putative members of the Mimiviridae family, namely Haptolina ericina virus RF02 (HeV RF02), Prymnesium kappa virus RF01 (PkV RF01), and Prymnesium kappa virus RF02 (PkV RF02). Each of the novel haptophyte viruses challenges the common conceptions of algal viruses with respect to host range, phylogenetic affiliation and size. PkV RF01 has a capsid of ~310 nm and is the largest algal virus particle ever reported while PkV RF01 and HeV RF02 were able to infect different species, even belonging to different genera. Moreover, PkV RF01 and HeV RF02 infected the same hosts, but phylogenetic analysis placed them in different groups. Our results reveal large variation among viruses infecting closely related microalgae, and challenge the common conception that algal viruses have narrow host range, and phylogeny reflecting their host affiliation., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Marine viruses are major evolutionary and biogeochemical drivers in marine microbial foodwebs. However, an in-depth understanding of the cellular mechanisms and the signal transduction pathways mediating host-virus interactions during natural bloom dynamics has remained elusive. We used field-based mesocosms to examine the "arms race" between natural populations of the coccolithophore Emiliania huxleyi and its double-stranded DNA-containing coccolithoviruses (EhVs). Specifically, we examined the dynamics of EhV infection and its regulation of cell fate over the course of bloom development and demise using a diverse suite of molecular tools and in situ fluorescent staining to target different levels of subcellular resolution. We demonstrate the concomitant induction of reactive oxygen species, caspase-specific activity, metacaspase expression, and programmed cell death in response to the accumulation of virus-derived glycosphingolipids upon infection of natural E. huxleyi populations. These subcellular responses to viral infection simultaneously resulted in the enhanced production of transparent exopolymer particles, which can facilitate aggregation and stimulate carbon flux. Our results not only corroborate the critical role for glycosphingolipids and programmed cell death in regulating E. huxleyi-EhV interactions, but also elucidate promising molecular biomarkers and lipid-based proxies for phytoplankton host-virus interactions in natural systems.

The mechanisms by which phytoplankton cope with stressors in the marine environment are neither fully characterized nor understood. As viruses are the most abundant entities in the global ocean and represent a strong top-down regulator of phytoplankton abundance and diversity, we sought to characterize the cellular response of two marine haptophytes to virus infection in order to gain more knowledge about the nature and diversity of microalgal responses to this chronic biotic stressor. We infected laboratory cultures of the haptophytes Haptolina ericina and Phaeocystis pouchetii with CeV-01B or PpV-01B dsDNA viruses, respectively, and assessed the extent to which host cellular responses resemble programmed cell death (PCD) through the activation of diagnostic molecular and biochemical markers. Pronounced DNA fragmentation and activation of cysteine aspartate-specific proteases (caspases) were only detected in virus-infected cultures of these phytoplankton. Inhibition of host caspase activity by addition of the pan-caspase inhibitor z-VAD-fmk did not impair virus production in either host–virus system, differentiating it from the Emiliania huxleyi-Coccolithovirus model of haptophyte–virus interactions. Nonetheless, our findings point to a general conservation of PCD-like activation during virus infection in ecologically diverse haptophytes, with the subtle heterogeneity of cell death biochemical responses possibly exerting differential regulation on phytoplankton abundance and diversity. [ABSTRACT FROM PUBLISHER]

The isolation and characterization of a virus (designated EhV) that infects the marine coccolithophorid Emiliania huxleyi (Lohmann) Hay & Mohler are described. Three independent clones of EhV were isolated from Norwegian coastal waters in years 1999 and 2000. EhV is a double-stranded DNA-containing virus with a genome size of ∼415 kilo-base pairs. The viral particle is an icosahedron with a diameter of 160–180 nm. The virus particle contains at least nine proteins ranging from 10 to 140 kDa; the major capsid protein weighs ∼54 kDa. EhV has a latent period of 12–14 h and a burst size of 400–1000 (mean, 620) viral particles per cell. A phylogenetic tree based on DNA polymerase amino acid sequences indicates EhV should be assigned to the Phycodnaviridae virus family and that the virus is most closely related to viruses that infect Micromonas pusilla and certain Chlorella species. [ABSTRACT FROM AUTHOR]

The isolation and characterization of a virus (designated EhV) that infects the marine coccolithophorid Emiliania huxleyi (Lohmann) Hay & Mohler are described. Three independent clones of EhV were isolated from Norwegian coastal waters in years 1999 and 2000. EhV is a double-stranded DNA-containing virus with a genome size of ∼415 kilo-base pairs. The viral particle is an icosahedron with a diameter of 160–180 nm. The virus particle contains at least nine proteins ranging from 10 to 140 kDa; the major capsid protein weighs ∼54 kDa. EhV has a latent period of 12–14 h and a burst size of 400–1000 (mean, 620) viral particles per cell. A phylogenetic tree based on DNA polymerase amino acid sequences indicates EhV should be assigned to the Phycodnaviridae virus family and that the virus is most closely related to viruses that infect Micromonas pusilla and certain Chlorella species. [ABSTRACT FROM AUTHOR]