Bigger than a soil worm or a fruit fly. Scientists have found a giant bacterium in the mud of a mangrove forest on the Caribbean island of Guadeloupe. One cell of this thin, extremely long bacterium is on average the size of an eyelash hair, almost an inch long. That is five thousand times larger than the normal size of a bacterium. The longest copies of this Thiomargarita magnificaas the bacterium is provisionally called by the researchers, even measured two centimeters. An article with a detailed description of this spectacular discovery appeared on Thursday in science

The French microbiologist Olivier Gros of the Université des Antilles found the bacterium for the first time in 2009. He saw them as white threads on a fallen mangrove leaf that had sunk to the bottom. Even he, as an expert on microorganisms, didn’t realize they were bacteria. “At first I thought it must be a eukaryotic organism,” Gros says. Eukaryotes are organisms with a cell nucleus containing their DNA, and with cell organelles such as mitochondria and chloroplasts.

Gros took them to the lab and studied them under the light microscope and then also with the electron microscope. “I was looking for a nucleus or mitochondria that are characteristic of eukaryotic cells. But I found nothing at all! I did discover sulfur granules in the cytoplasm. In that respect, it resembled the filaments that sulfur-oxidizing bacteria sometimes form, with the difference that these filaments seemed to consist of only one cell.”

The research only took off years later when a postdoc performed a genetic analysis in the lab. “That made it clear at once and without a doubt that this was a bacterium that belonged to the genus” Thiomargarita† That’s how we came up with the name Thiomargarita magnifica

Size up

“This is a great story,” responds Gijs Kuenen, emeritus professor of microbiology at TU Delft. The largest bacteria is now a size larger than Thiomargarita namibiensis, the giant bacterium discovered thirty years ago off the coast of Namibia.” This is a round bacterium that grows to a maximum size of 0.75 centimeters. Gerard Muijzer, professor of microbial systems ecology at the University of Amsterdam, speaks of an “incredible discovery” for his field. “I also still tell my students in lectures that bacteria are small and simple creatures, but these kinds of finds contradict that more and more.”

The discovery is so spectacular because Thiomargarita magnifica seems to scoff at all the restrictions that normally apply to bacteria. As single-celled organisms that depend on chemical diffusion of both food and waste, they are forced to remain microscopic. Bacteria also lack a nucleus and intracellular transport, their DNA floats freely in the cell and can only give its instructions locally.

Two copies of the newly discovered giant bacteria† To propagate, they tie off cells at the ends.
Photo Olivier Gros

The giant bacterium has managed to overcome those limitations, the French and American researchers described during a press conference earlier this week. The cell has a large central vacuole (fluid bladder) that presses the remaining cell contents against the outside of the cell. As a result, the diffusion of molecules in and out of the cell is no longer a problem. But there’s more: in the cell, Gros’ team discovered not only large sulfur grains, but also small round structures that they call “pepins” (French for “seeds”). The pepins appear to contain copies of the bacterial DNA as well as ribosomes, cell parts that are needed to make proteins. They are located throughout the cell and contain a total of 40,000 copies of the cell’s genetic material. For example, T. magnifica succeeds in making the necessary proteins and enzymes distributed throughout the cell, and does not run into problems because of its length.

Toxic Compounds

The giant bacterium is a so-called sulfur bacterium: it gets its energy from oxidizing sulphides in the soil. These sulfur compounds are left behind after the digestion of leaf remnants on the bottom of the permanently submerged mangrove forest. These are toxic compounds for most other organisms, but sulfur bacteria can actually grow well on them. “The fact that these sulfur bacteria live in this extreme environment with a really crazy gradient of sulfur and nutrient-rich on the one hand and oxygen-rich on the other is beneficial, because it means they have little competition from other organisms,” says Kuenen. Its height brings T. magnifica Kuenen thinks that this has a special extra advantage: “While the bacterium is in the sulphide with its toes at one end, it can snack on oxygen at the other end.”

You’d expect a single-cell of this size to be very fragile, but that’s not the case, Gros says. “You can just manipulate them with tweezers without damaging them. They are extremely tough.”

The bacterium attaches itself to solid surfaces in the black mud of the mangrove forest, says Gros. “We found them not only on fallen leaves of the red mangrove, but also on glass bottles and plastic litter.”

On Guadeloupe, the researchers found them in various places in the mangrove forest, but not everywhere. Apparently the bacterium makes quite specific demands on its environment, concludes Gros. “The current should not be too strong and you need a black layer of oxygen-depleted mud. But even in places with ideal conditions you will not always find them. And for the past two months I’ve been looking in places where we’ve seen them before, but haven’t found them again. I don’t know where they went. Maybe there is some seasonal influence, I don’t know. I’m pretty sure we’ll find them elsewhere too.”

Coincidentally, Muijzer has just completed his own research into sulfur bacteria in a mangrove forest on Bonaire. “We did not find this giant bacteria there,” he says. “But now that this is known, we are going to take a closer look at the molecular signals in our samples.”

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