Tiny microbe challenges the definition of cellular life

Summary

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.It turns out that these DNA chunks belong to some unusual archaea—a branch on the tree of life populated by single-celled microbes that can often survive in extreme environments. (Archaea are similar to bacteria, but distinct in their structure, genetics, and metabolism.)How did Sukunaarchaeum end up with such a strikingly tiny genome? ADVERTISEMENT Nautilus Members enjoy an ad-free experience. Log in or Join now . 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.So how did Sukunaarchaeum end up with such a strikingly tiny genome? Over the course of evolution, genetic instructions for life often become increasingly complex. But evolution can also go in the other direction, leading to greater simplicity in the genome. This so-called genomic reduction, where organisms end up with fewer genes than their ancestors, is typically observed in the domains of bacteria and archaea. What struck Nakayama and his colleagu...