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Electron microscopy image of a Pandoravirus particle.

An image of a Pandoravirus particle, created using an electron microscope.

Image courtesy Chantal Abergel and Jean-Michel Claverie

Christine Dell'Amore

National Geographic

Published July 18, 2013

Scientists have found the biggest viruses known, and these pandoraviruses have opened up entirely new questions in science—even suggesting a fourth domain of life, a new study says.

Each about one micron—a thousandth of a millimeter—in length, the newfound genus Pandoravirus dwarfs other viruses, which range in size from about 50 nanometers up to 100 nanometers. A genus is a taxonomic ranking between species and family.

In addition to being huge, pandoraviruses have supersize DNA: 2,500 genes as compared with 10 genes in many viruses. (Get a genetics overview.)

Microbiology was similarly upturned about ten years ago when scientists found the genus Mimivirus—the first large virus of its kind at about 0.7 micron.

Following the discovery of Mimivirus and an even larger behemoth called Megavirus chilensis, "we have been thinking deeply into the limits of viruses, and this is why we're open more than other labs to finding exotic things—we push the envelope of what we would consider possible," said study co-author Jean-Michel Claverie, a microbiologist at Aix-Marseille Université in France, who is part of a research team with microbiologist Chantal Abergel.

So the pair and their colleagues began hunting for more giant viruses in water sediments, where other big viruses have been found due to the abundance of amoeba prey.

Sure enough, they found two: Pandoravirus salinus, from the mouth of Chile's Tunquen River, and Pandoravirus dulcis from a freshwater pond near Melbourne, Australia—both of which parasitize amoebas. (Also see "Virus-Infecting Virus Fuels Definition of Life Debate.")

"Finding such a new type of virus that is so different happens once every 50 years—it's a major discovery," said the team, whose study appears today in the journal Science.

Why haven't scientists found pandoraviruses before?

There are several reasons, but a simple one is that many scientists still assume viruses are small.

"When people look into cells and when they see things that don't have the right dimension or don't have regular assets or geometries, they don't think of viruses—they think its some kind of bacteria," Claverie and Abergel said.

When the scientists then try to cultivate these supposed bacteria in the laboratory and fail, it doesn't surprise them, since up to 60 percent of bacteria in the oceans can't be grown in the lab.

The study authors also note that Pandoravirus may had already been found 13 years ago—but scientists just didn't know what it was. (See more pictures of viruses.)

When the team screened scientific literature on parasites that eat a type of amoeba called Acanthamoeba, they found mention of Pandoravirus-like particles.

How are pandoraviruses different than other viruses?

Simply put, they have little in common with other viruses—"something that came as a surprise to us," the team said.

For one, the virus reproduces in a curious fashion. Most viruses start a new cell by building an empty "box" and filling it up with DNA over time. But curiously, pandoraviruses do both of these processes at the same time in a process the team calls "knitting." (Read blog post: "An Infinity of Viruses.")

Perhaps most striking, 93 percent of pandoraviruses' 2,500 genes cannot be traced back to any known lineage in nature. In other words, they are completely alien to us.

Such foreign genes, the team suggests, is evidence for the "controversial existence of a fourth domain of life," in addition to bacteria, archaea, and eukaryota, the latter of which includes complex life like us.

The three-domain system is "probably pretty wrong—we are missing some part of the puzzle here," the team said.

What should you know about these viruses?

First and foremost, that they're not harmful to people, the team emphasized: Most viruses infect other microbes.

In fact, many pandoraviruses and similar marine viruses may have a beneficial—and unseen—role in nature. (See National Geographic's pictures of marine microbes.)

For instance, viruses prey on and thus regulate a lot of the ocean's phytoplankton, which produces half of our planet's oxygen and forms the base of the ocean's food chain.

Overall, the team added, the discovery of pandoraviruses "demonstrates our shallow knowledge of microbiology on Earth."

Follow Christine Dell'Amore on Twitter and Google .

22 comments
VIctor Medvil
VIctor Medvil

 You know people can make artificial virii right? No offense but I believe this to be relevant in this discussion, They keep Acquire Reproduction from host Hijack Host ribosomes thus can reproduce themselves. They Evolve as mutations in the transcription and translation happen. Respond to Environment the Ebola virus even has immune-suppression features that actively attack the cell.

Ted Pavlic
Ted Pavlic

When you say "0.5 in diameter", you should say "0.5 micro in diameter." Otherwise you make the pandoravirus sound like it's a half inch wide...

john coffey
john coffey

Perhaps this could be a way to fight off amoebic infections in humans? A very horrible infection of the brain that starts through the sinus? 

Don Bodat
Don Bodat

please allow me a tiny correction. A micron does not measure length. The correct expression is micrometer (µm) which is

  • 10 -6 meter
  • One thousandth of a millimeter
  • ;-)

    Clem Cirelli
    Clem Cirelli

    But give them some more grant money and I'll bet they'll find something just before it runs out.

    Josue Edwards
    Josue Edwards

    is it degenerative or is it like chicken pox, once you've had it you cant get it 

    Matt Ewald
    Matt Ewald

    Or maybe these huge viruses are a result of human interference, rather than nature.

    Matt Ewald
    Matt Ewald

    Or maybe these huge viruses are a result of human interference, rather than nature.

    Matt Ewald
    Matt Ewald

    Or maybe these huge viruses are a result of human interference, rather than nature.

    Codifex Maximus
    Codifex Maximus

    How can a virus bud having no cellular machinery to accomplish the task?  The virus can't knit itself for the very same reason.  It is NOT a cell therefore it is not life but a delivery system and a payload of genetic material.

    Stian Weiseth
    Stian Weiseth

    I saw a ted-talk where a certain percentage of samples taken from human noses where of unknown origin. Could air-born mega-viruses  account for some of this?

    Meister Omega
    Meister Omega

    They're probably the longest surviving form of life on the planet. Early RNA, and viruses were most likely the first form of life which evolved on this planet. There's probably a good chance these may be the "missing link" of sorts between viruses, and bacteria.

    Tom Doerksen
    Tom Doerksen

    @Meister Omega Virus are definitely not "the first form of life" that evolved. If you study the evolution of Viruses, it becomes apparent that they had to evolve in tandem with bacteria. Viruses most certainly didn't evolve into bacteria. It seems like a reasonable idea at first but the fact of the matter is, viruses can't survive without other organisms to prey on. They therefore couldn't evolve with out a pre-existing organism: bacteria.

    Codifex Maximus
    Codifex Maximus

    @Tom Doerksen @Meister Omega I tend to think of viruses as cellular mechanism to exchange genetic information.  Later, it turned into a kind of genetic warfare.


    Probably, viruses are responsible for rapid genetic mutation in times of extreme survival pressure.

    JC Gibson
    JC Gibson

    @Tom Doerksen @Meister Omega  

    Good point. But Biology 101 also hints that all life could have started out from RNA the spliceosome thing. And a bunch of RNA clump together just look more and more like virus now. And we thought the free RNAs would be ingested by any organism today any time they present themselves. But now, they don't get ingested and break down, they proliferate evidently.

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