Giant marine virus found

With a genome of more than 700 kilobases, a newly discovered virus marks the first giant virus known to infect a marine organism, and the second largest virus ever recorded.

Image: Wikimedia commons

The discovery, published this week in Proceedings of the National Academy of Sciences (PNAS), adds to the growing list of giant DNA viruses and suggests that these viruses, which appear to obtain much of their large genomes from their hosts and other microorganisms, may be more commonplace than scientists once believed."It's really interesting, and a completely different way of seeing viruses," said microbiologist linkurl:Didier Raoult;http://www.antimicrobe.org/authors/didier_raoult.asp of the University of the Mediterranean in Marseille, France, who was not involved in the research. "It's a completely new field that is emerging."These viruses "are probably playing a big role in the genetic diversity of organisms in the ocean as well," said microbiologist linkurl:James Van Etten;http://www.unl.edu/virologycenter/faculty/vanetten.shtml of the University of Nebraska, who also did not participate in the study. Not only can viruses take up genetic material from their hosts and other organisms, but they can donate genes, as well, he said."If you're teaching a beginning virology course now, it'd be pretty hard to ignore... that there are these very large DNA viruses in nature," said Van Etten, who edited the PNAS paper. Over the past decade or so, scientists have slowly begun identifying viruses that defied the conventional idea that they were tiny infectious agents with highly streamlined genomes. In 2004, researchers discovered and sequenced the 1.2 million-base pair genome of the largest known virus to date, the mimivirus (although still dwarfed by sequenced multicellular organisms, whose genomes usually exceed 100 million base pairs). This virus, and most of the other recently discovered giant viruses, has been found in amoebae, which are sometimes referred to as "melting pots" because of all the microorganisms they ingest. Inside the amoebae, these viruses and bacteria may exchange their DNA and grow their genomes. The new giant virus, dubbed CroV, is the first to be isolated from a marine organism -- a microzooplankton called Cafeteria roenbergensis. They are major consumers of heterotrophic bacteria and phytoplankton, and thus critical to maintaining the delicate balance of marine food webs. Once thought not to exist in marine environments, scientists now realize that there are some 50 million viruses in every milliliter of seawater. Every day, marine viruses kill about 20 percent of the ocean's microorganisms, which produce about half the oxygen on the planet. "These [viruses] are major players in the global ecosystem," said study author and marine virologist linkurl:Curtis Suttle;http://www.eos.ubc.ca/about/faculty/C.Suttle.html of the University of British Columbia.Like amoebae, C. roenbergensis harbor many microorganisms simultaneously, making them "a good place to exchange genes," Raoult said. "When you live in a phagocytic protist, such as this one, you meet a number of microorganisms, and then you can exchange genes and get a bigger genome."Indeed, of the 500 protein-coding genes Suttle and his colleagues found when they sequenced the virus's genome, about half were similar to those in eukaryotes, bacteria, archaea, and other giant viruses. Those with known function included genes that code for translation factors, DNA repair enzymes, ubiquitin pathway components, and tRNAs."We're finding suites of genes that you would really never expect to find in viral life, but would expect to find in cellular organisms," Suttle said."It is exciting to verify that [these large viruses] are out there," Van Etten said. There are likely many more, he added; "it's just a matter of people looking."Editor's Note: A linkurl:2005 paper in Science;http://www.sciencemag.org/cgi/content/short/309/5737/1090 also found a large DNA virus in marine microalga. The Cafeteria roenbergensis virus discovered by Suttle and colleagues is in fact the largest marine virus found to date, with a genome nearly twice the size of any other sequenced marine virus. There is no official definition of "giant virus," and conservative researchers would only include the Mimivirus and Mimi-like viruses (such as CroV), according to coauthor linkurl:Matthias Fischer.;http://www.ocgy.ubc.ca/%7Esuttle/?p=matthias As a result, some would indeed call this the first "giant" virus found in a marine organism.
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[6th August 2008]