No abstract

Pulsed field 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 Յ2 d during an intense dinoflagellate bloom in coastal Pacific waters. Different DNA banding patterns from diverse locations also indicated spatial variability in composition, but all 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 Ͼ200 kb, and the overall mean VLG size was 50 Ϯ 4 kb (mean Ϯ SD, n ϭ 30). On average in surface seawater, Ͼ90% 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.The abundance, distribution, and dynamics of viruses in various aquatic environments have been extensively investigated at the assemblage level (Wommack and Colwell 2000). In contrast, the diversity within natural viral assemblages and how assemblage composition changes over space and time are less well known. Studies of host range, morphology, and genetic variability using traditional culturebased and new PCR-based techniques all suggest that viral diversity in seawater is quite high (reviewed by Wommack and Colwell 2000). However, since many potential host organisms have not yet been cultivated and the range of genetic variability among aquatic viruses is still poorly known, it has been argued that many viruses of potential ecological importance could be missed by these techniques .Pulsed field gel electrophoresis (PFGE) was introduced as another useful technique for resolving different viruses in complex assemblages based simply on their genome size (Klieve and Swain 1993). Such an approach allows an overview of the viral diversity that is faster and less expensive

Viruses span an impressive size range, with genome length varying a thousandfold and virion volume nearly a millionfold. For cellular organisms the scaling of traits with size is a pervasive influence on ecological processes, but whether size plays a central role in viral ecology is unknown. Here, we focus on viruses of aquatic unicellular organisms, which exhibit the greatest known range of virus size. We outline hypotheses within a quantitative framework, and analyse data where available, to consider how size affects the primary components of viral fitness. We argue that larger viruses have fewer offspring per infection and slower contact rates with host cells, but a larger genome tends to increase infection efficiency, broaden host range, and potentially increase attachment success and decrease decay rate. These countervailing selective pressures may explain why a breadth of sizes exist and even coexist when infecting the same host populations. Oligotrophic ecosystems may be enriched in “giant” viruses, because environments with resource‐limited phagotrophs at low concentrations may select for broader host range, better control of host metabolism, lower decay rate and a physical size that mimics bacterial prey. Finally, we describe where further research is needed to understand the ecology and evolution of viral size diversity.