Scientists have discovered the world’s largest known bacterium, found in the form of white threads the size of human eyelashes, in a Guadeloupe swamp.
At about 1 cm long, the strange organism Thiomargarita magnifica, is about 50 times larger than any other known giant bacteria and the first to be visible to the naked eye. The thin white threads were discovered on the surfaces of decaying mangrove leaves in shallow tropical sea swamps.
The discovery was surprising because, according to models of cell metabolism, bacteria simply shouldn’t get that big. Previously, scientists had suggested an upper possible size limit of around 100 times smaller than the new species.
“To put it in context, it would be like meeting a human with a human the size of Mount Everest,” said Jean-Marie Volland, a scientist at Lawrence Berkeley National Laboratory who co-authored the study.
The organism was discovered by Olivier Gros, a professor of marine biology at the Université des Antilles in Guadeloupe, while looking for symbiotic bacteria in the mangrove ecosystem.
“When I saw them, I thought, weird,” Gros said. The lab first performed microscopic analysis to determine that the strands were single cells. Closer inspection also revealed a strange internal structure. In most bacteria, the DNA swims freely around the cell. Thiomargarita magnifica appears to keep its DNA more organized in membrane-bound compartments throughout the cell. “And that’s very unexpected for a bacterium,” Volland said.
The bacterium was also found to have three times as many genes as most bacteria and hundreds of thousands of genome copies scattered throughout each cell, making it unusually complex.
Scientists are not yet sure how the bacteria evolved to be so large. One possibility is that it adapted to avoid predators. “If you get a hundred or a thousand times bigger than your predator, you can’t be consumed by your predator,” Volland said.
However, growing larger would have meant losing some of the traditional advantages of bacteria, including the unique ability to move and colonize new niches. “By escaping the microscopic world, these bacteria have definitely changed the way they interact with their environment,” Volland said.
The bacteria hadn’t been found elsewhere — and had disappeared from the original spot when the researchers returned recently, perhaps because they’re seasonal organisms. But in the article, published in the journal Science, the authors conclude that the discovery “suggests that large and more complex bacteria may be hiding in plain sight.”