Marnaviridae

Marnaviridae is a family of positive-stranded RNA viruses in the order Picornavirales that infect various photosynthetic marine protists. Members of the family have non-enveloped, icosahedral capsids. Replication occurs in the cytoplasm and causes lysis of the host cell. The first species of this family that was isolated is Heterosigma akashiwo RNA virus (HaRNAV) in the genus Marnavirus, which infects the toxic bloom-forming Raphidophyte alga, Heterosigma akashiwo. As of 2021, there are twenty species across seven genera in this family, as well as many other related virus sequences discovered through metagenomic sequencing that are currently unclassified.

Interactions between members of the Marnaviridae family and their hosts have notable significance in marine ecology, and are also relevant within the aquaculture industry. HaRNAV and viruses from Bacillarnavirus are known to have roles in regulating dynamics and composition of their hosts’ blooms. An unclassified sequence, Baishivirus, has been suggested to be the possible pathogen of glass post-larvae disease, which is prevalent in shrimp aquaculture. Viruses detected in a cultured prawn species that had been affected by growth retardation disease have also been placed in Marnaviridae.

The name "marnaviridae" is based on its genome type (RNA virus - rnaviridae), together with the prefix "ma" being derived from the Latin word mare (sea).

The family was proposed following the discovery of an RNA virus (HaRNAV) that infects H. akashiwo off of the coast of British Columbia, which was the first report of a single-stranded RNA virus capable of causing cell lysis in phytoplankton.

HaRNAV was isolated from water collected in the Strait of Georgia in British Columbia, Canada, from a concentrated virus assemblage using the host H. akashiwo (NEPCC 522). It must not be confused with two other unrelated viruses that infect this host, H. akashiwo virus 01 (HaV-1, isolate: HaV53) in the genus Raphidovirus, and Heterosigma akashiwo nuclear inclusion virus (HaNIV).

Marnaviridae existed for multiple years with Marnavirus as the only genus and HaRNAV as the only species. After the usage of metagenomic analysis on the amino acid sequences of the capsid proteins and RdRp domains on viruses under the order Picornavirales, Marnaviridae was discovered to have a larger variety of viruses classified under it. Previously unassigned Labyrnavirus and Bacillarnavirus were also classified as genera under Marnaviridae.

Further metagenomic studies have yielded 653 Marnavirus-like sequences, prompting the proposal for a taxonomic reorganization to incorporate the new data into existing frameworks.

The following genera are recognised: