Rogue New Form of Life

https://nautil.us/a-rogue-new-life-form-1232095/

Scientists recently discovered a microbe with one of the tiniest genomes on Earth. More surprising, the creature is almost entirely dependent on its host: Its genes don’t support any of the functions of metabolism, one of the key processes of life. As such, it challenges fundamental notions of what it means to be a living organism.
The discovery was “pure serendipity,” says Takuro Nakayama, an evolutionary microbiologist at the University of Tsukuba in Japan. Takayama wanted to study the many microbes that live within a single-celled marine dinoflagellate, Citharistes regius, a kind of plankton. But when he and his colleagues sequenced the genes of this microbial community, they kept turning up tiny, odd chunks of DNA.
Nakayama and his colleagues proposed the name Sukunaarchaeum mirabile for the newly-discovered microbe: Sukunaarchaeum after the Japanese dwarf deity Sukuna-biko-na, and mirabile for marvelous. At only 238,000 base pairs, the number of genes in the DNA of Sukunaarchaeum is smaller than that of any other known archaea. The scientists described their finding in a bioRxiv preprint earlier this year.
With its stripped down genome, Sukunaarchaeum appears to be completely dependent on its host, C. regius, for essential energy and nutrients. “It likely cannot produce its own cellular building blocks,” notes Nakayama. “No previously discovered microbe has shown such an extreme degree of metabolic dependence.”

Sukunaarchaeum seems to almost inhabit a new category of life, suspended somewhere between archaea and virus. It is like viruses—which aren’t typically considered to be “alive”—in that it has a tiny genome and is totally dependent on its host for metabolism. But unlike a virus, Sukunaarchaeum has its own ribosomes, cellular structures that synthesize proteins, and it can replicate itself without the help of a host.