Scientists have found the ocean's largest virus. Fortunately, this microbial monster is a menace only to a particular single-celled organism.

Based on the size of its genome, or complete DNA sequence, the microbe dubbed CroV is the second to be considered a "giant virus." The only virus with a larger genome lives in fresh water.

CroV's enormous, and surprising, genetic code further blurs the boundary between viruses and cellular life, according to the researchers who described it.

CroV is equipped with genes that allow it to repair its genome, synthesize sugars and even gain more control over the machinery that it hijacks within the host cells to replicate itself.

"They take over the cell, and they basically run the cell," said Matthias Fischer, who described CroV for his doctoral dissertation at the University of British Columbia. He added that the production of new CroV viruses within an infected cell resembles an assembly line.

Viruses are essentially genetic material wrapped in a thin protein coat, and they must use the goods of a host in order to make more of themselves. Traditionally, viruses were considered nonliving. However, these discoveries about CroV add more weight to the argument that viruses are alive, Fischer said.

Big virus, tiny host

Fischer found that CroV's genome contains approximately 730,000 base pairs, the building blocks of DNA. By comparison, the largest virus on record, Mimivirus, has a genome of about 1.2 million base pairs. Prior to confirmation in 2003 that Mimivirus was indeed a virus, the largest known virus had a genome of around 331,000 base pairs, according to Fischer.

Despite its size, CroV is a threat only to the relatively small. It infects a common, single-celled grazing creature called Cafeteria roenbergensis. In fact, the virus is approximately a twentieth the size of its host. (For a person who stood 5-foot-6, or 1.7 meters, this would translate roughly into being infected by a virus the size of a softball.)

Not surprisingly, the infection kills Cafeteria roenbergensis, according to Curtis Suttle, also a University of British Columbia researcher who worked on the study. This tiny creature may be the most abundant eukaryote, or complex celled organism, in the ocean and perhaps the world, he said. This category includes all animals, plants and many other organisms.

Mimivirus, meanwhile, lives in fresh water and infects amoebas, which, like C. roenbergensis, are single-celled creatures.

Hodgepodge of genes

C. roenbergensis's diet of bacteria and viruses may explain the strange collection of genes possessed by the giant virus that infects it. Perhaps the oddest of these include the genes that code for the entire pathway to create a key component to a bacterial outer membrane.

"Who knows why that is in there?" said Suttle. "As far as we know, it (CroV) doesn't interact with bacteria at all."

CroV may have acquired these genes by picking up DNA from the remains of a bacterium eaten by a cell the virus later infected, according to the researchers. Something similar also may have occurred with Mimivirus, which infects a bacteria-eating amoeba and also appears to contain genes of bacterial origin. This is a possible explanation for the origin of 10 to 20 percent of the giant virus genes.

Other genes within CroV are even more mysterious. The researchers could not recognize 51 percent of the genes they encountered in the new virus. This is actually a low proportion – about 90 percent of the genes within certain viruses are unknown, Fischer said.

"Every virus you pull out has a new set of genes that is unique to this virus, that has never been seen before," he said.

This makes it unlikely that many viral genes have cellular origins. It is currently hypothesized that viral genes are ancient, and have never been part of cellular organisms, he said.

The research was published Oct. 25 in the online early edition of the Proceedings of the National Academy of Sciences.

This article was reprinted with permission from LiveScience.

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