Coccolithovirus
| Coccolithovirus | |
|---|---|
| Virus classification | |
| Group: | Group I (dsDNA) |
| Family: | Phycodnaviridae |
| Genus: | Coccolithovirus |
| Type Species/ Strains | |
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Coccolithovirus is a genus of giant double-stranded DNA virus, in the family Phycodnaviridae. Algae, specifically Emiliania huxleyi, a species of coccolithophore, serve as natural hosts. There is currently only one species in this genus: Emiliania huxleyi virus 86.
Group: dsDNA
Coccolithoviruses are enveloped, icosahedral and have a diameter ranging from 100–220 nm. Their genomes are linear, between 410–415kb in length and predict to encode for approximately 472 proteins.
Coccolithoviruses are part of the family of Phycodnaviridae, one of the five families that belong to a large and phylogenetically diverse group of viruses known as nucleocytoplasmic large dsDNA viruses (NCLDVs). These viruses either replicate exclusively in the cytoplasm of the host cell or start their life cycle in the host nucleus but complete it in the cytoplasm. In the case of EhV-86 the infection strategy is not fully understood but Mackinder et al. (2009) have proposed the following model: The virus enters the host cell via endocytosis, followed by fusion of its lipid membrane with the host vacuole membrane and the release of its nucleoprotein core into the cytoplasm. Alternatively the virus membrane could fuse directly with the host plasma membrane. The virus genome is then released from the capsid into the nucleus, where it is replicated by the viral DNA polymerase. The replicated genome is packed into assembled capsids in the cytoplasm and the newly formed (up to 400–1000) virions are thought to be transported to the plasma membrane and released by a controlled budding mechanism, which leads to the cellular breakdown of the host cell.
During G2 and M stage of the life cycle, the coccosphere is incomplete and the exposure of the plasma membrane to the virus is increased. Even with an intact coccosphere infection can occur due to naturally occurring gaps between the coccoliths.
E. huxleyi is known for forming seasonal algal blooms, that can reach 250,000 km2, during which cell density in the upper 200 m increases from 103 to 105 cells per mL seawater. These algae blooms collapse usually after 5–8 days and several studies have shown that bloom termination is intrinsically linked to infection by coccolithoviruses. Transmission of viruses between algal hosts occurs via passive diffusion. Furthermore, EhV DNA was also detected in copepods, leading to the proposal that viruses are further dispersed by virus-carrying zooplankton.
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