Genome size distributions indicate variability and similarities among marine viral assemblages from diverse environments

Pulsed held gel electrophoresis (PFGE) was used to determine the size distributions of virus-like DNA in seawater from diverse environments (Arctic Ocean, Ross Sea, coastal Pacific Ocean, and northern Adriatic Sea). Changes in DNA banding patterns indicated that shifts in the viral assemblage composition occurred on the order of less than or equal to2 d during an intense dinoflagellate bloom in coastal Pacific waters. Different DNA banding patterns from diverse locations also indicated spatial variability in composition, but an of the samples analyzed had similar features. Size frequency distributions for virus-like genomes (VLGs) were multimodal, with major peaks occurring around 31-36 kilobases (kb) and 58-63 kb. The smallest discrete band resolved was 26 kb, the largest was >200 kb, and the overall mean VLG size was 50 +/- 4 kb (mean +/- SD, n = 30). On average in surface seawater, >90% of the VLGs occurred in the 26-69 kb size range, and at least half were between 28 and 45 kb. This first extensive survey of viral genome sizes in seawater indicates that most marine viruses have physical properties similar to other known viruses. The distributions revealed that the vast majority of the detected VLGs had sizes typical of bacteriophages, whereas only a few percent were in the size range of known algal viruses.