Smallest Archaeal Genome Ever Found in Ocean Microbe

Scientists just announced the smallest archaeal genome ever found, and it belongs to a microbe that barely counts as a cell and barely counts as a virus. According to EurekAlert, the organism is called Candidatus Sukunaarchaeum mirabile, and it was described in the journal Current Biology.

The headline number is wild. Its genome is just over two hundred thirty-eight thousand base pairs long and codes for only one hundred eighty-nine proteins, according to Tech Times. That is fewer proteins than many large viruses carry around.

What exactly did researchers find?

Sukunaarchaeum is an archaeon, one of the three big domains of life alongside bacteria and eukaryotes. Its DNA was pulled from a single cell of a marine plankton called Citharistes regius, collected from Pacific water near Shimoda, Japan. The team sequenced it twice with two different methods and got the same gap-free circular chromosome both times.

The previous record holder, Nanoarchaeum equitans, had a genome of about four hundred ninety thousand base pairs. This new one is roughly half that size, which makes the smallest archaeal genome on record dramatically smaller than anything archaeologists of the microbe world had seen before.

A cell that outsourced everything

Here is the twist. The microbe has almost no metabolism. It cannot make its own energy, lipids or amino acids, so it has to steal what it needs from its host. Most of its genes are devoted to one job: copying its own genetic material and turning it into proteins.

Science magazine put it bluntly, noting that the microbe is not a virus, but like viruses it seemingly has one purpose, which is to make more of itself. Reporting from Science says it is a parasite that provides nothing to the single-celled creature it calls home.

Why the ribosome line matters

Biologists have long split living things into cells that make their own ribosomes and viruses that do not. Sukunaarchaeum keeps its own ribosomes, plus the machinery to copy DNA and read it out, so it lands on the cell side of that line. Yet its dependence on a host is about as total as anything known.

Evolutionary microbiologist Takuro Nakayama, who led the work at the University of Tsukuba, said: "Metabolism is one of the key components of how we often define life. This challenges that by suggesting a cell can exist almost entirely without its own." That is the kind of sentence that makes a textbook author sweat.

How this changes the definition of life

The old checklist for life leans on metabolism, genetic material and the ability to reproduce. Viruses fail the metabolism test and need a host to reproduce, so they sit outside the club. Sukunaarchaeum passes on the genetic and replication side but flunks metabolism almost completely, which blurs the boundary in a way that textbooks will have to address.

Co-author Thorsten Allers of the University of Nottingham said the organism may be close to the minimum amount of genetic information needed for something to remain an independent cell. In other words, the smallest archaeal genome could be near the floor of what a cell can get away with, at least when a host handles the groceries.

Compare that with the famous synthetic minimal bacterium built by the J. Craig Venter Institute, which needed four hundred seventy-three genes to live freely in a lab dish. The ocean parasite gets by on far less, but only because it never has to live alone. That gap shows the minimum depends on whether you count help from a host, and it gives researchers a fresh puzzle to chase.

The mystery proteins and the missing host

Not everything is solved. Eight giant proteins of unknown function eat up nearly a quarter of the genome, and they appear to sit in the cell membrane. Researchers suspect they work like grappling hooks that latch onto a host, similar to proteins seen in other parasitic archaea.

Nobody has actually seen the organism under a microscope yet. It is known only from its genetic sequence, and the team cannot confirm whether the plankton it was found in is the true host. Nakayama's next goal is to grow it in the lab and take its first picture.

Why it might be everywhere

Searches of ocean sequence databases turned up relatives of Sukunaarchaeum in samples from around the world, including near the coast of Brazil. Experts quoted by Tech Times think standard software may have been tossing out such tiny genomes as junk, so a whole hidden family could have gone unnoticed.

For anyone who likes weird biology, this is a reminder that the ocean still hides major branches of life. The smallest archaeal genome ever recorded may only be the first member of a big, strange clan. Want more science drops like this? Browse our science coverage for more.