Chlorovirus Diversity
Chloroviruses have been found in many places on Earth in in-land waters, and metagenomic sequence evidence suggest they may inhabit marine systems as well. Chloroviruses are known to infect certain eukaryotic green algae that reside as endosymbionts of aquatic eukaryotes, including the protists Paramecium bursaria (green paramecium), Acanthocystis turfacea (heliozoa), and the cnidarian Hydra viridissima (green hydra). These algae are referred to as zoochlorellae and some can be cultured independent of their holobiont host. Chloroviruses are 270-410 kilobase pair double-stranded DNA viruses with genome and particle sizes that place them in the grouping of "giant viruses": Phylum Nucleocytoviricota, Class Megaviricetes, Order Algavirales, Family Phycodnaviridae, Genus Chlorovirus. Chloroviruses have a lytic lifestyle that can be assessed with the plaque assay method using selected zoochlorellae. The resulting plaque morphology can be used as an initial screening method for diversity where the plaque can appear as a i) clear circle with a regular boundary, ii) cloudy circle with a regular boundary, iii) cleared area with an irregular boundary, or iv) cloudy area with an irregular boundary. These areas can be graded from large to small. In most cases the plaque morphology is reproducible with multiple rounds of isolation. The zoochlorellae hosts used in virus isolation represent subgenera boundaries and the associated viruses are referred to as the Alphachlorovirus (host: Chlorella variabilis), Betachlorovirus (host: Micractium conductrix) and Gammachlorovirus (host: Chlorella heliozoae). Along with geographic sourcing and the synthetic host ranges, the plaque morphology determinations give a large matrix of potential diversity of the genus. The BioProject "Chlorovirus Diversity" gathers sequencing data and associated metadata for a growing collection of viral isolates from natural environments. The intention of the BioProject is to be a centralized reservoir for chlorovirus information, including genomic features.