Chlorovirus, also known as Chlorella virus, is a genus of giant double-stranded DNA viruses, in the family Phycodnaviridae. This genus is found globally in freshwater environments where freshwater microscopic algae serve as natural hosts. There are 19 species in this genus.
Chlorovirus was discovered in 1981 by Russel H. Meints, James L. Van Etten, Daniel Kuczmarski, Kit Lee, and Barbara Ang while attempting to culture Chlorella-like algae. During the attempted process viral particles were discovered in the cells 2 to 6 hours after being initially isolated, followed by lysis after 12 to 20 hours. This virus was initially called HVCV (Hydra viridis Chlorella virus) since it was first found to infect Chlorella-like algae.
Though relatively new to virologists and thus not extensively studied, one species, Chlorovirus ATCV-1, commonly found in lakes, has been recently found to infect humans. New studies focusing on effects of infection in mouse model are currently emerging as well.
Taxonomy
Chlorovirus is a genus of giant double-stranded DNA (dsDNA) viruses in the family Phycodnaviridae, and Baltimore group 1: dsDNA viruses. The genus contains the following species:
Acanthocystis turfacea chlorella virus 1
Hydra viridis Chlorella virus 1
Paramecium bursaria Chlorella virus 1
Paramecium bursaria Chlorella virus A1
Paramecium bursaria Chlorella virus AL1A
Paramecium bursaria Chlorella virus AL2A
Paramecium bursaria Chlorella virus BJ2C
Paramecium bursaria Chlorella virus CA4A
Paramecium bursaria Chlorella virus CA4B
Paramecium bursaria Chlorella virus IL3A
Paramecium bursaria Chlorella virus NC1A
Paramecium bursaria Chlorella virus NE8A
Paramecium bursaria Chlorella virus NY2A
Paramecium bursaria Chlorella virus NYs1
Paramecium bursaria Chlorella virus SC1A
Paramecium bursaria Chlorella virus XY6E
Paramecium bursaria Chlorella virus XZ3A
Paramecium bursaria Chlorella virus XZ4A
Paramecium bursaria Chlorella virus XZ4C
Ecology
Chloroviruses are widespread in freshwater environments in all parts of the globe and have been isolated from freshwater sources in Europe, Asia, Australia, as well as North and South America. Natural hosts of chloroviruses include various types of unicellular eukaryotic Chlorella-like algae, with individual virus species typically infecting only within a distinct strain. These algal hosts are known to establish endosymbiotic relationships with larger protists, such as Paramecium bursaria (a member of the ciliates), Acanthocystis turfacea (a centroheliozoan) and Hydra viridis (member of the hydrozoa). While an individual protist can harbour up to several hundred algal cells at any given time, free-floating algae are highly susceptible to chloroviruses, indicating that such endosymbiosis serves to provide resistance from infection.
Chlorovirus titers are variable by season and location, but typically fluctuate between 1 and 100 PFU/mL, although high abundances of up to 100,000 PFU/mL may occur in some environments. Due to the rich genetic diversity and high specialization of individual species with respect to infectious range, variations in their ecology are not unusual, resulting in unique spatio-temporal patterns, which ultimately depend on lifestyle and nature of the host. As such, previous survey data highlighted two prominent seasonal abundance peaks for both Chlorella variabilis NC64A and Chlorella variabilis Syngen viruses — one in late fall, and the other in late spring to mid-summer — which is likely attributed to the fact that they share a host species. Conversely, Chlorella heliozoae SAG viruses peaked at different times of the year and generally exhibited more variability in titers, as compared to the NC64A and Syngen viruses. Additionally, studies revealed that chloroviruses demonstrate some resilience in response to decreased temperatures observed during the winter season, characterized by presence of infectious particles under ice layers in a stormwater management pond in Ontario, Canada. Further, DeLong et al. (2016) suggest that predation by small crustaceans can play an indirect role in titer fluctuations, as degradation of protist cells passing through the digestive tract results in liberation of large numbers of unicellular algae that become susceptible to viral infection due to disruption of endosymbiosis. Overall, seasonal abundance of chloroviruses depends not only on the host species, but also on the abundance of other microorganisms, general nutrient status and ecological conditions.