Clinging to sunken debris in shallow, marine mangrove forests in the French Caribbean, tiny threadlike organisms — perfectly visible to the naked eye — have earned the title of the largest bacterium ever known.
They are about an inch long and about the size and shape of a human eyelash, knocking out the competition with 5,000 times the size of garden bacteria and 50 times the size of bacteria previously thought to be gigantic. In human terms, this is akin to encountering a person as tall as Mount Everest.
Pierre Yves Pascal
Olivier Gros, a biologist at the University of the Antilles, discovered the prokaryotes in 2009 and found them swaying gently in the sulfur-rich waters among the mangroves in the Guadeloupe archipelago. The bacteria clung to the leaves, branches, oyster shells and bottles that sank into the tropical swamp, Gros said in a news conference.
He and colleagues first thought they might be complex eukaryotic organisms or maybe a series of linked organisms. But years of genetic and molecular research have shown that each strand is in fact one towering bacterial cell, genetically related to other sulfur-oxidizing bacteria. “Of course, this was quite a surprise,” Jean-Marie Volland, a microbiologist at the Joint Genome Institute in Berkeley, Calif., said in the briefing.
This week, Gros and colleagues published an article in Science detailing everything they learned about the new, huge bacteria, which they named Candidatus (Ca.) Thiomargarita magnifica†
Their findings expand our understanding of microbial diversity in ways microbiologists never thought possible. Scientists previously hypothesized that the size of bacteria would be limited by several factors, including a lack of intracellular transport systems, reliance on inefficient chemical diffusion, and a surface-to-volume ratio needed to meet energy needs. Yet the volume of a single approx. T. magnifica cell is at least two orders of magnitude higher than the predicted maximum a bacterium can theoretically reach, Volland said.
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filaments of approx. Thiomargarita magnifica†
Jean-Marie Volland
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filaments of approx. Thiomargarita magnifica†
Jean-Marie Volland
Volland, Gros and colleagues are still learning how — and exactly why —approx. T. magnifica manages its massive size. But so far it is clear that approx. T. magnifica oxidizes hydrogen sulfide from its sulfur-rich environment and reduces nitrate. About 75 percent of the cell volume is a bag of stored nitrate. The bag presses against the cell envelope, limiting the depth nutrients and other molecules need to diffuse.
While bacteria tend to have free-floating DNA, approx. T. magnifica appears to have bundled more than half a million copies of its genome into numerous membrane-bound compartments that the researchers called pepins, after tiny seeds in fruits. The distribution of pepins on the outer edges of the bacterium would allow for localized protein production, eliminating the need to transport proteins over long distances.
The next step in studying these giant bacteria is for scientists to figure out how to grow them in labs. For now, the researchers have been collecting new specimens from the mangrove forests each time they run out. But this was tricky because they seem to have a mysterious life cycle or seasonality. Gros has been able to find one for the past two months. “I don’t know where they are,” he said.
