They are estimated to be the most abundant biological entities in both marine and freshwater environments (with abundances typically in the range of 10 7 to 10 9 particles/ml) and likely contain the largest pool of unknown genetic diversity on Earth ( 7, 22, 45, 63). Phage research, particularly in aquatic environments, has uncovered the extraordinary abundance of viruses as well as their impact on the ecology of our planet ( 3, 44). Viruses that infect heterotrophic bacteria, cyanobacteria, and eukaryotic phytoplankton dominate the world's oceans and play an important role in structuring the marine microbial food web. Thus, while polymorphism in a viral signature gene, such as g23, can be a powerful tool for inferring evolutionary relationships, the degree to which this approach can capture fine-scale variability within virioplankton populations is less clear. Unlike g23 polymorphism, RAPD-PCR fingerprints showed a greater temporal than geographic variation. In contrast, RAPD-PCR fingerprinting detected high genotypic similarity within PFGE bands from the same location, time, and genome size class without the requirement for DNA sequencing. The surprising occurrence of T4-like g23 within small genomic groups (23 to 64 kb) indicated that the genome size range of T4-like phages may be broader than previously believed. T4-like phages containing similar g23 proteins were found within both small- and large-genome populations, including populations from different geographic locations and times. Seasonal changes in overall virioplankton composition were apparent from pulsed-field gel electrophoresis (PFGE) analysis. This study examined two of these approaches-T4-like major capsid protein (g23) gene polymorphism and randomly amplified polymorphic DNA-PCR (RAPD-PCR) fingerprinting-to ask how well each technique could track differences in virioplankton populations over time and geographic location. However, there are few approaches that can provide data on the genotypic identity of viral populations at low cost and with the sample throughput necessary to assess dynamic changes in the virioplankton. Thus, it is likely that the genetic identity of viral populations comprising the virioplankton also change over temporal and spatial scales, reflecting shifts in viral-host interactions. Viral production estimates show that virioplankton communities turn over rapidly in aquatic ecosystems.
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