BioWorld International Correspondent

LONDON - A new living organism, so minute that its genome must be close to the minimum size required for life, has been discovered living in rock and gravel on the seabed near Iceland, where the water is close to boiling point.

Analysis of its genetic material to date suggests that it is the only representative of a new phylum of the Archaea - one of the three great branches of living things, along with bacteria and eukaryotes. Karl Stetter, professor of microbiology at the University of Regensburg in Regensburg, Germany, who led the team that made the discovery, told BioWorld International that he believes that there may be "further continents of microbial life yet to be discovered."

"Everyone had assumed that if you used probes based on a certain small subunit ribosomal RNA, then you would detect all life forms," he said. "But this organism is so different that these probes did not work. We have embarked on a search using new probes that will detect this very different genetic material, and our unpublished observations so far suggest that our hunt has been successful and that we have found other relatives of this new species."

Stetter clearly takes pleasure in naming the numerous new thermophilic species he has discovered over the past 22 years. He and his colleagues have named the latest novelty Nanoarchaeum equitans. The name stands for "riding the fire sphere," because N. equitans is found on the surface of another species of Archaea called Ignicoccus, or "fireball." The group called the new phylum the Nanoarchaeota, which stands for "the dwarf archaea."

Stetter and his colleagues, from the university and the Max Planck Institute for Medical Research in Heidelberg, Germany, report their observations in a letter in the May 2, 2002, Nature titled, "A new phylum of Archaea represented by a nanosized hyperthermophilic symbiont." They postulate that because the conditions in which the organism is found - high temperatures and absence of oxygen - resemble the conditions on Earth 3.8 billion years ago, the Nanoarchaeota may be the living representatives of highly primitive forms of microbial life.

Stetter runs the Regensburg Archaea Center, which has eight glass-lined steel and titanium fermenters capable of producing kilograms at a time of archaea and hyperthermophilic bacteria - a term he coined some time ago to describe those which grow fastest at temperatures of 80 degrees Celsius and above. He is also co-founder of Diversa Corp., of San Diego, which concentrates on investigating potential applications of hyperthermophilic organisms.

Apart from heat-stable enzymes, Stetter predicts that the novel life forms he has begun to identify may provide fertile hunting grounds for compounds that may be pharmacologically active, either as antibiotics or as candidate drugs to treat various diseases.

As part of their systematic search for novel hyperthermophiles, Stetter and his team examined rocks and gravel taken from a depth of 120 meters at the Kolbeinsey ridge, north of Iceland, where the temperature of the sea water is close to boiling point. By removing oxygen from the culture and adding hydrogen, sulfur and carbon dioxide, while maintaining a temperature of 90 C, they were able to encourage the growth of a recently described archaeon called Ignicoccus.

Unlike previously isolated organisms of this genus, the cells were covered by extremely small spherical particles. To the researchers' surprise, the small particles could be stained with a stain specific for DNA, even though the genetic material within them could not be amplified with the usual PCR primers. Attempts to grow the particles independently failed.

Examination with electron microscopy showed those minute cells were about 400 nanometers in diameter and enclosed by a surface layer. The team has so far been unable to determine how the N. equitans are attached to the Ignicoccus cells, and found that they could dislodge them with sound waves.

Stetter told BioWorld International, "At the time we submitted the Nature paper, we thought that the genome of N. equitans was around 0.5 megabases, which is extremely small. Now, however, Diversa and Celera Genomics, of Rockville, Md., already have sequenced the genome and it turns out it is even smaller than that." The sequencing project only took about one day, Stetter said, but he added that annotating the genes within the sequence will take much longer.

It will be "particularly exciting," Stetter predicted, to study such a tiny genome. "It should have clues about what the minimal requirements for life are. We also want to know why these tiny little guys sit on the big cells - why they ride the fireball - and how come the big cells do not seem to be influenced in any ways by the small ones."

Commenting on the paper in a "News and Views" article in the same issue of Nature, titled "Something new under the sea," Yan Boucher and W. Ford Doolittle of Dalhousie University in Halifax, Nova Scotia, said that although Stetter and his colleagues speculate that N. equitans represents a very early branch of the Archaea, it could equally be "a highly derived descendent of some already known group, off on an evolutionary tangent of its own."