These giant bacteria, Thiovulum imperiosus, store their DNA in membranous compartments. It’s a method of organization that’s exceedingly rare among known bacteria.
P.E. CONTARINI
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PHILADELPHIA — Deep in the mangrove swamps of the Caribbean lives a giant bacterium with a surprise trick for organizing its DNA.
Thiovulum imperiosus wraps its genetic material in membranous fingers, squishing it against the periphery of the cell, scientists reported December 6 at the Cell Bio meeting. It’s a never-before-seen approach that’s vastly different from nearly all other known bacteria.
Scientists think most bacteria store their DNA in a floating mass inside the cell’s interior, not packed neatly in specialized compartments, said Jean-Marie Volland, a marine microbiologist at the University of California, Santa Barbara. It’s a view that aligns with textbook descriptions of bacteria as “organisms that have not evolved a lot of complexity,” he says.
His team’s work is challenging that conventional wisdom.
Volland’s collaborators discovered T. imperiosus around a decade ago while snorkeling among mangrove trees off the coast of Guadeloupe. The shallow tropical waters contain biodiversity that remains largely unexplored, Volland says. Mangrove swamps are sulfur-rich, so they smell of rotten eggs, he says. But on a good day, when the water is clear and quiet, you can see fish and sponges and a rainbow of colors.
Amidst that thriving ecosystem, scientists lit upon T. imperiosus, a bacterium so large it’s visible to the unaided eye. (Think of the tiny dot atop a lowercase i.)
Using 3-D fluorescence and electron microscopy, Volland’s team spied a giant space in the center of T. imperiosus, with bulbous compartments lining the cell’s interior edge. Squirreled away inside the compartments were blobs of cytoplasm, the cellular goo that holds the bacterium’s molecular machinery and DNA.
Microscopy images suggest how the compartments formed: Imagine the bacterium as a water balloon placed inside another balloon, with a thin space between the two. In T. imperiosus, that space has expanded dramatically, squeezing the interior balloon to the sides. There, DNA and other cellular components cram together, semienclosed in membranous pouches.
It’s the second example of DNA compartmentalization Volland has reported in giant bacteria. The first, found in a distantly related organism called Thiomargarita magnifica, appears to confine its DNA in regularly spaced bubbles. Volland had thought the T. magnifica observation might be mere curiosity. But the T. imperiosus discovery suggests that DNA compartmentalization in bacteria may be more widespread than scientists once suspected, he says.
“I think there’s a lot of complexity hiding in the bacterial world that we have not yet uncovered.”
