 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.