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Metagenomic Characterization of Chesapeake Bay Virioplankton ^,

Shellie R. Bench,^1, Thomas E. Hanson,¹ Kurt E. Williamson,^1, Dhritiman Ghosh,² Mark Radosovich,² Kui Wang,³ and K. Eric Wommack^1*

College of Marine and Earth Studies, University of Delaware, Newark, Delaware 19711,¹ Department of Biosystems Engineering and Soil Science, University of Tennessee, Knoxville, Tennessee 37996,² Center of Marine Biotechnology, University of Maryland Biotechnology Institute, Baltimore, Maryland 21202³

Received 26 April 2007/ Accepted 26 September 2007

Viruses are ubiquitous and abundant throughout the biosphere. In marine systems, virus-mediated processes can have significant impacts on microbial diversity and on global biogeocehmical cycling. However, viral genetic diversity remains poorly characterized. To address this shortcoming, a metagenomic library was constructed from Chesapeake Bay virioplankton. The resulting sequences constitute the largest collection of long-read double-stranded DNA (dsDNA) viral metagenome data reported to date. BLAST homology comparisons showed that Chesapeake Bay virioplankton contained a high proportion of unknown (homologous only to environmental sequences) and novel (no significant homolog) sequences. This analysis suggests that dsDNA viruses are likely one of the largest reservoirs of unknown genetic diversity in the biosphere. The taxonomic origin of BLAST homologs to viral library sequences agreed well with reported abundances of cooccurring bacterial subphyla within the estuary and indicated that cyanophages were abundant. However, the low proportion of Siphophage homologs contradicts a previous assertion that this family comprises most bacteriophage diversity. Identification and analyses of cyanobacterial homologs of the psbA gene illustrated the value of metagenomic studies of virioplankton. The phylogeny of inferred PsbA protein sequences suggested that Chesapeake Bay cyanophage strains are endemic in that environment. The ratio of psbA homologous sequences to total cyanophage sequences in the metagenome indicated that the psbA gene may be nearly universal in Chesapeake Bay cyanophage genomes. Furthermore, the low frequency of psbD homologs in the library supports the prediction that Chesapeake Bay cyanophage populations are dominated by Podoviridae.

Published ahead of print on 5 October 2007.

Supplemental material for this article may be found at http://aem.asm.org/.

Present address: Ocean Sciences Department, University of California Santa Cruz, Santa Cruz, CA 95064.

Present address: J. Craig Venter Institute, Rockville, MD 20850.

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