Bigger than any insect: a giant bacterium discovered in Guadeloupe

ANDonly a noble figure does Thiomargarita magnifica something very special: the organism is one centimeter long and is visible to the naked eye. This certainly seems remarkable, because the life form that the scientists, in collaboration with the French biologist Jean-Marie Volland of the University of California, Berkeley, discovered in the mangrove swamps of Guadeloupe is a unicellular organism. And it’s remarkable, but also not entirely unusual. There are several marine protozoa that are anything other than microbes in terms of microscopic life, such as bladder algae valonia ventricosaalso called an “navy bulletin” in English, whose cells can reach a diameter of up to four centimeters.

Ulf von Rauchhaupt

Editor of the “Science” section of the Sunday newspaper Frankfurter Allgemeine.

but Thiomargarita magnifica not algae or any other higher organism in the cellular-biological sense, but bacteria. The largest known type of bacteria, sulfur bacteria Thiomargarita namibiensis, is 180 micrometers long on average, isolated specimens reach an impressive 750 micrometers or 0.75 millimeters, for which you need at least a magnifying glass as a visually impaired person. As the name suggests, it is T. magnificus is related to this now deposed record holder, but grows up to 50 times its size, and thus larger than some animals, such as nematodes or fruit flies. The vast majority of bacteria, on the other hand, do not reach more than a few micrometers. This is coming up T. magnificus five thousand times. “It’s like meeting a man the size of Mount Everest,” says Jean-Marie Volland.

Nitrate soul

It was discovered T. magnificus as an unusual accumulation of long white fibers on the surface of rotting leaves in the mangrove swamps of a Caribbean island belonging to France. As Volland and co-authors now report in Science, the macroscopic microbe “breathes” nitrates and oxidizes sulfur in this process. The name Thiomargarita literally means “sulfur pearl” and comes from the sulfur granules that members of this genus carry in their cytoplasm.

3D rendering of segmented Thiomargarita magnifica cells from hard X-ray tomography.  Cells of different lengths are probably specimens of different stages of development.


3D rendering of segmented Thiomargarita magnifica cells from hard X-ray tomography. Cells of different lengths are probably specimens of different stages of development.
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Bild: Science

Giant bacilli store nitrates in a giant sac, or vacuole, that occupies most of their cells, leaving only two to three microns of space under the cell membrane for sulfur granules and other organelles. So it solves T. magnificus The problem with all giant protozoa: how fast enough to transport the molecules responsible for metabolism through the vast dimensions of the cell, when mere diffusion allows only the movement of biomolecules at a typical rate of one millimeter per hour.

Another trick of large protozoa is the distribution of genetic material with instructions for the synthesis of important biomolecules throughout the cell. One T. magnificus-The cell contains an estimated 40,000 copies of its genetic information and takes the principle of the decentralized genome to the extreme in that it packs each of these DNA sets individually into its own vesicles, the so-called pepins. Peter Anne Levin of the University of Washington at St. Louis writes in his publication Science in the accompanying commentary, the data suggest that these pepins are key sites for protein synthesis. “At the same time, the structure of pepins suggests that they function almost like autonomous organisms in the cell as a whole.”

Fascination, but also a mystery Thiomargarita magnifica however, it is its sheer, unexpected size. This raises questions similar to those posed by paleontologists in the face of huge herbivorous sauropods from the Jurassic and Cretaceous periods: Why – through which evolutionary processes – could these bacteria have become or had to become so large? A: Have they reached the upper limits of what can happen to bacterial cells in terms of their size and related metabolic problems?

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