Oceans reveal secrets of viruses

Earth teems with bacteria-eating viruses. There are perhaps 10 times as many of these viruses as of all other living things put together. Yet researchers don’t know how many virus species there are, how they are distributed around the globe, or how diverse their genes are. Now, a team of scientists has begun to answer those questions, completing the first survey of virus DNA in oceans around the world.

Scientists knew that each milliliter of ocean water holds about 50 million virus particles and that those organisms kill 20 percent of the bacteria in the ocean every day (SN: 7/12/2003, p. 26: All the World’s a Phage). In the process, viruses move gene sequences from one bacterium to another, speeding evolution and turning some bacteria virulent.

Curtis Suttle of the University of British Columbia in Vancouver began collecting samples of ocean water from around the world in 1998. He saved them, waiting for genomic sequencing to become inexpensive enough to reveal the genes of all the viruses in the water at once.

That time finally came. Suttle, Forest Rohwer of San Diego State University, and several colleagues analyzed 1,300 samples of ocean water from the Arctic, British Columbia’s coast, the Gulf of Mexico, and the Sargasso Sea. They report in the November PLoS Biology that 91 percent of the DNA sequences in the water were previously unknown.

The DNA sequences suggest that each area of the ocean contains its own characteristic community of viruses. The researchers were surprised to find that many viruses found in the Sargasso Sea have a single strand of DNA rather than the typical two strands. Such viruses had never previously been found in the ocean.