Giant virus found in marine predatory plankton

Researchers have identified a marine giant virus that infects Cafeteria roenbergensis, a widespread planktonic predator that occupies a key position in marine food webs, according to a study.

So-called giant viruses have puzzled evolutionary biologists since the discovery of Acanthamoeba polyphaga mimivirus, which infects freshwater amoebae and has a genome of 1.2 million base pairs that is larger than the genomes of some cellular organisms.

Curtis A. Suttle and colleagues analyzed an unknown virus infecting C. roenbergensis that had been isolated in Texas coastal waters in the early 1990s.

According to the authors, the pathogen's genome contains approximately 730,000 base pairs, which would make it the largest known marine virus. The virus, named CroV, possesses numerous genes that are typically used by living cells to repair DNA damage and to synthesize proteins and sugars.

CroV also has genes that encode some of the proteins that viruses need to replicate but must obtain from a host organism.

Because viruses cannot replicate independently, they are classified as "non-living," but giant viruses like CroV that possess functioning components of the replication machinery challenge this classification.

CroV, the authors report, may also represent a major group of largely unknown but ecologically important marine giant viruses.