Using quantitative PCR, the abundances of six phytoplankton viruses DNA polymerase (polB) gene fragments were estimated in water samples collected from Lake Ontario, Canada over 26 months. Four of the polB fragments were most related to marine prasinoviruses, while the other two were most closely related to cultivated chloroviruses. Two Prasinovirus-related genes reached peak abundances of 41000 copies ml À1 and were considered 'high abundance', whereas the other two Prasinovirus-related genes peaked at abundances o1000 copies ml À1 and were considered 'low abundance'. Of the genes related to chloroviruses, one peaked at ca 1600 copies ml À1 , whereas the other reached only ca 300 copies ml À1 . Despite these differences in peak abundance, the abundances of all genes monitored were lowest during the late fall, winter and early spring; during these months the high abundance genes persisted at 100-1000 copies ml À1 while the low abundance Prasinovirus-and Chlorovirus-related genes persisted at fewer than ca 100 copies ml À1 . Clone libraries of psbA genes from Lake Ontario revealed numerous Chlorella-like algae and two prasinophytes demonstrating the presence of candidate hosts for all types of viruses monitored. Our results corroborate recent metagenomic analyses that suggest that aquatic virus communities are composed of only a few abundant populations and many low abundance populations. Thus, we speculate that an ecologically important characteristic of phycodnavirus communities is seed-bank populations with members that can become numerically dominant when their host abundances reach appropriate levels.

Extant and newly designed primers for the polymerase chain reaction (PCR) were used to amplify phycodnavirus DNA polymerase ( polB) gene fragments from numerous samples collected at different times of the year from 3 freshwater environments in Ontario, Canada. Overall, a total of 143 cloned PCR fragments were sequenced and 106 putative phycodnavirus polB gene fragments were identified. Although most of these 106 gene fragments were very closely related (i.e. > 97% identical) to polB sequences from chloroviruses, or environmental sequences related to prasinoviruses, 16 represented 2 new types of phycodnavirus polB genes. More specifically, polB fragments that formed a new clade of chloroviruses were amplified from Lake Ontario using newly designed Chlorovirus-specific PCR primers, and a polB sequence most closely related to genes from the prymnesioviruses PgV-03T and CbV-PW1 was amplified from a pond sample from Mississauga, Ontario, using the degenerate algal virus-specific PCR primers AVS1 and AVS2. Thus, the results of the present study provide evidence for a new type of Chlorovirus, and the first observation of polB sequences from freshwater phycodnaviruses that are presumed to infect algae other than chlorophytes.