Small Wonder: Microbial hitchhiker has few genes

In gravel and rocks taken from a volcanically heated ocean bed north of Iceland, German scientists have identified an unusual microbe that appears to have the fewest genes of any living organism so far studied. The spherical microbe is just 400 nanometers in diameter and lives on the surface of a slightly larger organism, the investigators report in the May 2 Nature.

“There are only several hundred genes in it. This is the smallest genome known so far,” study coauthor Karl O. Stetter of the University of Regensburg in Germany told Science News.

“This newly identified entity may represent a whole new group within Archaea, the most recently discovered and still most mysterious of life’s three domains,” say Yan Boucher and W. Ford Doolittle of Dalhousie University in Halifax, Nova Scotia, in a commentary accompanying the report.

Over the past decade, most biologists have accepted the once-controversial notion that single-celled archaea represent a branch of life distinct from eukaryotes, which include plants and animals, and bacteria (SN: 8/24/96, p. 116: https://www.sciencenews.org/sn_arch/8_24_96/fob1.htm). Viruses have a questionable claim to life since they can’t reproduce outside a host cell.

Stetter and his colleagues came across their novel archaea while working with other microbes isolated from ocean-floor material picked up on a dive several years ago. When they looked through a microscope at a type of archaeum known generally as Ignicoccus, they noticed some smaller, spherical objects attached to it.

At first, the scientists thought they were observing a virus on the archaeum. Stetter’s team was surprised, however, when a common research technique called polymerase chain reaction (PCR) failed to identify the microbe’s DNA. Using other methods, the researchers ultimately sequenced a piece of DNA that biologists use to classify organisms. While it bore some similarity to those of other archaea, the sequence was different enough that the scientists concluded they had identified a

new category within the archaea.

The complicated route to the microbe’s discovery means that PCR-based methods may still be inadequate to reveal life’s true diversity, contend Boucher and Doolittle.

Because Ignicoccus is Latin for “fireball,” Stetter named its microbial partner Nanoarchaeum equitans to indicate a dwarf archaeum riding a fiery sphere. The researchers haven’t observed any structures connecting the two microbes, but N. equitans doesn’t seem able to survive without its larger relative. Ignicoccus likely secretes a chemical upon which N. equitans has become dependent, notes Andreas Teske, a microbiologist at the Woods Hole (Mass.) Oceanographic Institution. “It’s an amazing story,” he says.

The San Diego-based biotech firm Diversa, which Stetter cofounded, is working with the German researchers to sequence the entire genome of N. equitans, and the effort is nearly complete. While declining to specify the number of genes in N. equitans, Stetter says it has many fewer than the 470 or so belonging to the intracellular bacterium Mycoplasma genitalium, the previous record holder for smallest genome.

N. equitans may have such a minimalist genome because it represents a primitive life-form reminiscent of early life on the planet or because it’s a highly evolved microbe that has streamlined its genome to the bare essentials.

Teske is curious to know how an organism with so few genes can survive the heat and pressure of the ocean environment where N. equitans was found.

The microbe’s discovery may also offer insight into the smallest number of genes needed to sustain a cell (SN: 6/12/99, p. 377). N. equitans “is very close to the theoretical minimum,” says Stetter. “It could harbor some important clues about the origin of life or early life on Earth.”