There are three known domains of life: Bacteria; Archaea, another type of single-celled organism; and Eukaryotes.
Now scientists believe they may have discovered a fourth domain, a distinct, previously unknown branch of life.
Fox News has reported a study by the French National Research Agency at Aix-Marseille University that has uncovered two large viruses named “Pandoraviruses,” a reference to Pandora, the mythical Greek figure who opened a box and released evil into the world.
Our knowledge of Earth’s microbial biodiversity is still incomplete, says virologist Jean-Michel Claverie, a coauthor of the study, which has been published in this week’s issue of the journal Science.
Pandoraviruses may expand our knowledge of life on Earth because they represent a fourth domain of microbial organisms.
One of the viruses, Pandoravirus salinus, was unearthed from sediments collected off the coast of Chile. The other, Pandoravirus dulcis, was discovered in a freshwater pond near Melbourne, Australia.
The average virus is about one one-hundredth the size of the average bacterium. Most viruses are too small to be seen directly with an optical microscope. Both of the new viruses are so large that they can be seen using a traditional light microscope.
Their genome of P. salinus is 1.91 million DNA bases long, while that of P. dulcis is 2.47 million DNA bases. The size of largest of the previous viral genome, Megavirus chilensis, is 1.18 million bases. A typical flu virus can have as few as 10 genes, and M. chilensis, the largest virus previously discovered, has only about 1,000 genes. Pandoraviruses have more than 2,000 new genes coding for proteins and enzymes, whose purpose is unknown.
A genomeconsists of all of the DNA in an organism. The human genome is 3 billion bases long, arranged on 23 pairs of chromosomes. A geneis a piece of genetic material inside the chromosome. Genes are arranged in a line along the DNA molecule. Each gene contains several thousand bases. The sequence of bases in the gene determines the properties of the gene. There are between 30,000 and 40,000 genes in our bodies. Proteins make up a large part of each cell. There are an estimated 100,000 different proteins in the human body.
There are three tests for determining whether an organism is a virus. One is that viruses spawn hundreds of new copies in one cycle, rather than splitting in two, like typical bacterium or cells. The second is that Pandoraviruses lack the genes needed for energy production. Third, they cannot produce proteins without infecting single-celled organisms known as amoebas, which serve as one of many viral hosts and are the ones that Pandoraviruses seem to prefer. Viruses require living cells for multiplication.
Pandoraviruses satisfy all three tests for a virus.
Claverie and his team theorize that the ancient ancestors of Pandoraviruses were once free-living cells that gradually lost most of their genes as they became parasitic.
There are three known domains. A bacterium is a single-celled organism. It is a type of prokaryote that does not have a distinct nucleus. Archaea are a major group of prokaryotes. Archaea are also single-celled organisms, but they can thrive in extreme environmental conditions. A eukaryote is an organism made up of one of more cells with distinct nuclei. Animals and plants fall within this domain.
Pandoraviruses may constitute a fourth domain. Its biochemical and regulatory functions may have significant biotechnical and biomedical applications.