seawater viral libraries were novel and bore no significant similarity to any known genes in the databases. The approach appeared reasonably comprehensive for double-stranded DNA phages; it recovered most major families, including those with bacterial or algal hosts. Similar applications in marine sediments yielded parallel results, but with some interesting differences. Among sediment viral assemblages, even greater novelty was detected: more than 75% of the viral sequences recovered resembled nothing in the databases. The double-stranded viral DNA sequences identified in sediments suggested an important role for temperate phages, for example, viruses that can integrate into their host’s genome. A large comparative analysis of seawater viral assemblages collected from diverse locales recently indicated that marine viral species have a global distribution (Angly et al. 2006).

Applications for analyzing RNA-based virus assemblages from seawater have also been developed, and have revealed new groups of RNA-viruses that infect marine planktonic protists and animals. In total, the early viral metagenomic analyses provide a solid starting place for exploring and interpreting the genomic diversity in naturally occurring viruses. They also provide a foundation for the next steps in microbial-community genomics, the integrated analyses of viruses and their cellular hosts collected from the same metagenomics samples and analyzed simultaneously.

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