Phaeoviruses Present in Cultured and Natural Kelp Species, Saccharina latissima and Laminaria hyperborea (Phaeophyceae, Laminariales) , in Norway

Abstract

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.

Keywords: MCP; Nucleocytoviricota; Phycodnaviridae; kelp; phaeovirus; phylogeny; prevalence.

Figure 1

Sampling stations. Full map to the left, and detailed sampling areas to the right. 1—Sognefjorden area, 2—Korsfjorden and Hardangerfjorden areas, and 3—Rogaland area.

Figure 2

Number of possible viral master variants per kelp sample and per location (A–C) or per area (D) on average. (A) L. hyperborea 2021–2022; (B) S. latissima 2021–2022; (C,D) S. latissima 2016–2018 (see Table S3).

Figure 3

Percentage of each possible viral master variant for both kelp species, year, and location (A–C), or per area (D), on average. (A) L. hyperborea 2021–2022; (B) S. latissima 2021–2022; (C,D) S. latissima 2016–2018.

Figure 4

Maximum likelihood phylogenetic tree of the amplified phaeoviral MCP fragment from our three viral master variant protein sequences (I, II, and III, magenta), aligned with other known giant viruses (Nucleocytoviricota) (see Table S4 for accession numbers). Model of substitution: LG + G₄. Nodes are bootstrap values (only bootstraps > 70 are shown), and branch lengths represent evolutionary distances. The tree is rooted with out-group Fowlpox virus.

Figure 5

Theoretical model for virus prevalence under 10 °C (blue line) or 20 °C (orange line) over time. Dashed lines represent the actual viral prevalence reduction due to viral genome segregation after meiosis for each temperature.