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."