Biologists have discovered the largest bacterium ever found, with a single cell measuring a mammoth 2 cm (0.8 in) long. Visible to the naked eye, this new species has some bizarre characteristics that make it like a missing link in the evolution of complex cells like those in humans.
Most species of bacteria measure between one and five micrometers long, but the biggest previously known was Thiomargarita namibiensis, which tops out at 750 micrometers or 0.75 mm. But this newly identified species blows everything else out of the water – its average length is a whopping 9,000 micrometers (0.9 cm/0.4 in), with the largest recorded specimen reaching 2 cm. This single cell is longer than your everyday housefly.
This gigantic size completely upends the accepted scientific understanding of how big bacteria could possibly get. It was long believed that the size of bacteria was limited by the distance that the molecules they exchange with their environment could travel. If nutrients can’t make the journey from their membrane to their interior, and if toxins can’t do the reverse trip, the organism wouldn’t be viable.
So how does the giant new bug pull this off? It actually contains a water-filled sac that makes up 73 percent of its volume, which pushes its cellular contents up against the outer membrane, so vital molecules don’t have to travel across its entire width.
This is the same mechanism that allows the previous record-holder, T. namibiensis, to grow larger than most bacteria. Noting these similarities and others found in a genetic analysis, the team proposed that the new species belongs to the same genus, suggesting the name Thiomargarita magnifica.
The organism was actually first discovered about a decade ago, as bundles of white filaments growing on decaying mangrove leaves in a Caribbean swamp. But at the time, its discoverer, marine biologist Olivier Gros, didn’t realize they were bacteria, let alone single cells. That identification only came more recently, with detailed analysis.
Its physical size isn’t the only giant thing about this bacterium. Genetic analysis revealed that its genome is also massive, containing 11 million bases and around 11,000 genes, which is almost three times larger than most bacterial genomes. On closer inspection, the team found that the genome is quite repetitive, with more than half a million copies of certain sequences.
But the bizarre bacterium only gets stranger. This DNA was packaged inside a membrane sac, something that’s normally characteristic of more complex lifeforms – bacteria normally have their DNA floating free through the cell. This blurs the line between the two main classifications of lifeforms, prokaryotes and eukaryotes.
The research has yet to be published in a journal, but is available on the preprint server bioRxiv.
Source: Science