Massive Virus Found in Sewage Plunges Biologists into Debate

Biologists sifting through effluent have accidentally discovered a giant virus that may finally plop down an answer in a decade-old evolutionary debate—are viruses a fourth domain of life?

Tanya Woyke, Frederik Schulz, and their team from the Joint Genome Institute were looking for ammonia-processing bacteria in a sewage treatment plant in Klosterneuburg, Austria, when they stumbled across the jumbo virus, which they named Klosneuvirus. After analyzing its genome, they discovered that it contains the genetic information required to make proteins with 19 of the 20 amino acids used by other life forms. The virus’s size wasn’t its only unusual trait—Klosneuvirus also might not need a host cell’s machinery to thrive.

An artist's conception of what Klosneuvirus might look like

The possibility of self-sufficiency might suggest that viruses hail from a fourth domain of life (the other three being bacteria, archaea—another type of single-cell microorganism—and eukaryotes—multicellular organisms like humans). The fourth-domain theory was first floated back in 2003 when evolutionary biologist Jean-Michel Claverie and his team discovered another giant virus, Mimivirus. Claverie believes there’s a good chance that viruses were once full-fledged, single-celled organisms capable of living on their own. They evolved into modern viruses, he says, by flushing out unnecessary bits of genetic code as they became more entrenched in the parasitic lifestyle.

Woyke and Schulz, however, say that the Klosneuviruses’ mishmash genome suggests that viruses started out simply and opportunistically swiped genetic code from their host organisms. Here’s Sam Wong, reporting for New Scientist:

“This is the ultimate evidence that the virus acquired these genes from different eukaryotic cells,” [Schulz] says.

The genes mostly seem to come from single-celled eukaryotic organisms, such as protists, amoebas and ciliates, which are abundant and diverse. It’s likely that these are klosneuvirus’s hosts, but without studying the virus in culture, it’s not yet certain.

Woyke and her team haven’t yet figured out how to grow the viruses in the lab, which would help them perform a variety of tests, including ones to determine the actual function of genes that appear to code for protein-making machinery. The results of those experiments might help settle the debate. They might, at least, until the next giant virus is found.