Tiny filamentous organisms clinging to sunken debris in shallow mangrove forests in the French Caribbean and perfectly visible to the naked eye have earned the title of largest bacteria ever known.
About one centimeter long, they’re roughly the size and shape of a human eyelash, and they outperform the competition at 5,000 times the size of garden bacteria and 50 times the size of bacteria previously thought to be giants. In human terms, this is like meeting a person as tall as Mount Everest.
Olivier Gros, a biologist at the University of the Antilles, discovered the prokaryotes in 2009 and noticed them swaying gently in the sulfur-rich waters among the mangroves of the Guadeloupe archipelago. The bacteria clung to the leaves, branches, oyster shells and bottles submerged in the tropical swamp, Gros said in a news conference.
He and his colleagues first thought they might be complex eukaryotic organisms, or perhaps a chain of interconnected organisms. But years of genetic and molecular research have shown that each string is actually a towering bacterial cell genetically related to other sulfur-oxidizing bacteria. “Of course, that was quite a surprise,” said Jean-Marie Volland, a microbiologist at the Joint Genome Institute in Berkeley, California, in the briefing.
This week Gros and colleagues published an article in Science setting out everything they learned about the new, giant bacteria they’ve baptized Candidatus (ca.) Thiomargarita magnifica.
Their results 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 demands. However, the volume of a single Approx. T. magnifica cell is at least two orders of magnitude larger than the predicted maximum that a bacterium can theoretically reach, Volland said.
Volland, Gros, and colleagues are still learning how—and exactly why—Approx. T. magnifica manages its enormous size. But so far that is clear Approx. T. magnifica oxidizes hydrogen sulfide from its sulfur-rich environment and reduces nitrate. About 75 percent of its cell volume is a sack of stored nitrate. The sac presses against the cell envelope, limiting the depth at which nutrients and other molecules must diffuse.
While bacteria tend to have free-floating DNA, Approx. T. magnifica seems to have more than half a million copies of its genome clustered in numerous membrane-bound compartments, which the researchers named pepins after small seeds in fruits. The distribution of pepins on the outer edges of the bacteria could allow for localized protein production, eliminating the need to transport proteins over long distances.
The next step in studying these gigantic bacteria is for scientists to figure out how to grow them in laboratories. For now, the researchers have collected new specimens from the mangrove forests each time they run out. However, this was difficult as they seem to have some mysterious life cycle or seasonality. Gros hasn’t been able to find any in the past two months. “I don’t know where they are,” he said.
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