Largest Bacterium Ever Discovered Has An Unexpectedly Complex Cell (science.org)

A newly described bacterium living in the Caribbean "is visible to the naked eye, growing up to 2 centimeters -- as long as a peanut -- and 5,000 times bigger than many other microbes," writes Elizabeth Pennisi via Science.org. "What's more, this giant has a huge genome that's not free floating inside the cell as in other bacteria, but is instead encased in a membrane, an innovation characteristic of much more complex cells, like those in the human body."

From the report:

The bacterium was unveiled in a preprint (PDF) posted online last week and it has astounded some researchers who have reviewed its features. Aside from upending ideas about how big -- and sophisticated -- microbes can become, this bacterium "could be a missing link in the evolution of complex cells," says Kazuhiro Takemoto, a computational biologist at Kyushu Institute of Technology.

Researchers have long divided life into two groups: prokaryotes, which include bacteria and single-cell microbes called archaea, and eukaryotes, which include everything from yeast to most forms of multicellular life, including humans. Prokaryotes have free-floating DNA, whereas eukaryotes package their DNA in a nucleus. Eukaryotes also compartmentalize various cell functions into vesicles called organelles and can move molecules from one compartment to another -- something prokaryotes can't. But the newly discovered microbe blurs the line between prokaryotes and eukaryotes. [...] Furthermore, that cell includes two membrane sacs, one of which contains all the cell's DNA, [researchers] report in their 18 February preprint on bioRxiv. Volland calls that sac an organelle and that's "a big new step" that implies the two branches of life are not as different as previously thought, [Verena Carvalho, a microbiologist at the University of Massachusetts, Amherst] says. "Perhaps it's time to rethink our definition of eukaryote and prokaryote!" agrees Petra Levin, a microbiologist at Washington University in St Louis. "It's a supercool story."

The other, water-filled sac may be the reason the bacterium could grow so big. [...] The DNA-filled sac, also squished along the inner edge of this bacterium, proved extraordinary as well. When researchers at the Department of Energy Joint Genome Institute sequenced the DNA inside, they found the genome was huge, with 11 million bases harboring some 11,000 clearly distinguishable genes. Typically, bacterial genomes average about 4 million bases and about 3900 genes. By labeling the DNA with fluorescent tags, [researchers] determined the bacterium's genome was so big because there are more than 500,000 copies of the same stretches of DNA. Protein production factories called ribosomes were inside the DNA-filled sac as well, likely making the translation of a gene's code into a protein more efficient.

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