The Forgotten Domain

Research Blogging / by Dave Munger /

New research shows the importance of Archaea, one of three domains into which all living things are classified, for understanding all of biology.

Credit: Flickr user jurvetson

All of life can be divided into three domains: Eukarya (including plants, animals, fungi, and other organisms) Bacteria, and Archaea. But take a look at a typical US high-school biology text (in this case, my daughter’s), and you’d hardly notice it. Her book devotes a single page (out of 848) to Archaea. Sure, the concept of Archaea as a domain on the level of bacteria and eukaryotes is relatively recent, but the science behind the distinction is quite solid.

Archaea had previously been lumped in with bacteria as kingdom Monera—the so-called prokaryotes, which lack a cellular nucleus—in the five-kingdom classification system many of us (including myself) learned in high school. But as the biologist who blogs as “Lab Rat” pointed out last June, this doesn’t make much sense: “‘prokaryotes,’” she tells us, “is not much more than a scientific word for ‘blobs I don’t care about.’”

Perhaps Archaea has been relegated to its seemingly second-class status because it is a relatively recent discovery—the separate domain was first proposed in 1977. Perhaps they just seem “more unusual” than organisms like bacteria that most of us are more familiar with—after all, they’re often found in extreme environments like undersea thermal vents, high-salinity ponds, and toxic mine tailings.

But lumping Archaea in with Bacteria simply doesn’t make biological sense. While, like bacteria, archaeans don’t have a cell nucleus, they share other traits with eukaryotes like you and me. An archaean converts DNA into proteins very differently from, say, Streptococcus—in fact, protein synthesis in Archaea is more similar to the human process than to bacteria. Archaeans have cell membranes that are completely different from both bacteria and eukaryotes.

Microbiologist and computer scientist Iddo Friedberg blogged last week that archaeans don’t just live in extreme environments. They may constitute as much as 20 percent of the free-drifting microbes in the earth’s oceans. A study published this month in Environmental Microbiology describes a “giant” archaean that grows to 3 centimeters long and is nearly visible without magnification (it’s just 0.1 mm thick). The species lives in mangrove swamps in the West Indies, with each individual wearing a coat of hundreds of much-smaller bacteria, living in a mutualistic relationship—neither organism could survive without the other.

But many of the most fascinating archaeans do manage to survive in extreme environments. Microbiologist Moselio Schaechter described one such habitat in August. The abandoned Richmond Mine in California has the most acidic waters in the world, courtesy of underground microbes that consume sulfides and excrete sulfuric acid. In water acidic enough to dissolve metal, archaeans form biofilms that researchers call “blanket strips” because they look like pink fiberglass insulation. The individual archaeans are some of the smallest organisms ever found. Even their genomes are about 10 percent smaller than in other, similar organisms. Like other archaeans, their genomes share some similarities with both eukaryotes and bacteria. The researchers who first identified them, led by Jill Banfield, have found other enigmatic features such as mysterious tubular structures whose function is not yet known. The research was published in PNAS.

Since archaeans have unique cell membranes, they are typically not susceptible to viruses that infect eukaryotes and bacteria (most viruses replicate by budding off a cell’s membrane). But this doesn’t mean there are no viruses that affect the Archaea. As Columbia University virologist Vincent Racaniello writes, Archaea have their own viruses. Some of these even survive in the highly acidic environment of the Richmond Mine, or as a team led by Maija K. Pietilä found, in high-salinity ponds used to manufacture gourmet sea-salt. This virus may have a unique structure not found in viruses affecting bacteria and eukaryotes—it has to be able to survive in the same extreme environment as the organism it infects. The research was published in Journal of Virology.

Although based on its protein-synthesis apparatus Archaea is in many ways most similar to the Eukarya domain, archeans can also share and exchange DNA with the denizens of the Bacteria domain. So it seems that Archaea, straddling between the more familiar eukaryotes and higher-profile bacteria, can probably tell us something substantial about the origins of both. For much more on Archaea, try a search on ResearchBlogging.org.

Dave Munger is editor of ResearchBlogging.org, where you can find thousands of blog posts on this and myriad other topics. Each week, he writes about recent posts on peer-reviewed research from across the blogosphere. See previous Research Blogging columns »

 

Originally published September 29, 2010

Tags biodiversity biology evolution

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