A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism.
Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsky's 1892 article describing a non-bacterial pathogen infecting tobacco plants and the discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898, more than 9,000 virus species have been described in detail of the millions of types of viruses in the environment.
Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entities.
The study of viruses is known as virology, a sub-speciality of microbiology.
Viruses infect all cellular life and, although viruses occur universally, each cellular species has its own specific range that often infects only that species.
Some viruses, called satellites, can replicate only within cells that have already been infected by another virus.
Viruses are the most abundant biological entity in aquatic environments. There are about ten million of them in a teaspoon of seawater. Most of these viruses are bacteriophages infecting heterotrophic bacteria and cyanophages infecting cyanobacteria and they are essential to the regulation of saltwater and freshwater ecosystems.
Bacteriophages, often just called phages, are viruses that parasite bacteria. Marine phages parasite marine bacteria such as cyanobacteria.
By 2015, about 40 viruses affecting marine protists had been isolated and examined, most of the viruses of microalgae.
Marine algae can be infected by viruses in the family Phycodnaviridae.
The taxonomy of this family was initially based on the host range: chloroviruses infect chlorella-like green algae from freshwaters; whereas, members of the other five genera infect marine microalgae and some species of brown macroalgae.
The phycodnaviruses contain six genera: Coccolithovirus, Chlorovirus, Phaeovirus, Prasinovirus, Prymnesiovirus and Raphidovirus.
The genera can be distinguished from one another by, for example, differences in the life cycle and gene content.
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