Scientists baffled by discovery of bacteria that ought to be too big to function

Imagine visiting a mangrove in the Caribbean and discovering a human as tall as Mount Everest.

Something similar happened to marine biologist Jean-Marie Volland – only instead of encountering a human, he encountered a species of giant bacteria.

In fact, “huge” might be an understatement. The long, filamentous microbes Volland found are more than 5,000 times larger than typical microscopic bacteria, which play important roles in ecosystems around the world.

The newly discovered species is so large that it is easily visible to the naked eye. Some specimens are up to 2 centimeters long, the diameter of a US penny.

Previously, the title of the world’s largest bacterium belonged to a microbe the size of a fruit fly’s eye.

The new record holder, described Thursday in Science magazine, resembles thin, hair-like threads. Its discoverers found it so magnificent that they named it Thiomargarita magnifica.

Aside from debunking the notion that bacteria are microscopic, the find challenges conventional wisdom about the nature of life on our planet.

The bacteria were first discovered in 2009 by Olivier Gros, a biologist at the University of the French Antilles and Guiana while collecting underwater samples in Guadeloupe, a French archipelago in the Caribbean.

Mangroves next to a body of water

One of the sampling sites in the middle of the mangroves in Guadeloupe where scientists discovered a giant unicellular bacterium.

(Olivier Gros / University of the French West Indies and Guiana)

“I noticed long, white threads clinging to sunken leaves of mangrove trees,” he said. “Given their size, I would never have imagined them as single-celled bacteria.”

However, years of follow-up studies using microscopic imaging and DNA sequencing convinced Gros and his co-workers that the filaments were a member of the Thiomargarita genus of bacteria.

T. magnifica are sulfur eaters. Rotting organic matter trapped in mangrove roots releases large quantities of foul-smelling sulfur-containing compounds that are toxic to most animals. But these bacteria gobble up sulfides to create energy in a process known as chemosynthesis.

“Little tricks on how to make a living when environmental conditions may not be entirely appropriate” allow sulfur eaters to exceed the limits of their size, said Victoria Orphan, a microbial ecologist at Caltech who was not involved in the study.

Why these bacteria grew to their gargantuan size is still up for debate, but scientists have some ideas. The centimeter long filaments of T. magnifica can help it reach higher waters and use chemical gradients to bridge a gap from where there is more oxygen to where sulfide is more abundant.

Another benefit is avoidance of predators – nobody can eat the biggest fish in a pond.

Gros said the bacteria are not abundant in the mangroves. However, the wild-caught specimens the researchers have worked with so far have been surprisingly strong and resilient.

“For the first time ever, I got to play with bacteria with tweezers,” said Volland, a marine biologist at Lawrence Berkeley National Laboratory and lead author of the article in Science. The next step, he said, is to cultivate them in a lab.

A filament of a bacterial cell of Thiomargarita magnifica.

A Thiomargarita magnifica Bacterial cell seen under the microscope.

(Jean-Marie Volland/Lawrence Berkeley National Laboratory via Associated Press)

Obtaining a bacterial cell a thousand times larger than normal is a logistical nightmare.

The microscopic size facilitates diffusion, the way bacteria transport molecules within their single-celled bodies and exchange nutrients and waste with their environment. Diffusion is a slow process and a bacterium the size T. magnifica would take several hours to get things moving and his biochemistry to shut down.

T. magnifica circumvents this problem by placing a large vacuole — essentially a scuba tank of nitrate to help it breathe — right in the center of its cylindrical body. This vacuole pushes the cell’s essential proteins, sugars, and metabolites against its membrane, facilitating diffusion.

A bacterial cell’s operating instructions are stored in its genetic material, the DNA. In common bacteria, this DNA floats freely within cell boundaries.

T. magnificahowever, has developed novel machinery that packages their DNA into numerous copies, each stored in a tiny membrane-bound compartment. Scientists named this new type of bacterial organelle pepin, after the many small seeds, or pips, found in fruits like watermelons. Thousands of pepins scattered throughout the length T. magnifica act as secure storage units for its genetic material and provide localized control centers for cellular activities.

“If you want to make proteins in response to something happening at the top or bottom of the cell, having another copy of the genome nearby might work better,” said Jonathan Eisen, a microbiologist at UC Davis, who was not involved in the study.

The packaging of a cell’s DNA in a membrane-enclosed nucleus, such as the seed of an avocado, was previously thought to be strictly limited to cells of more complex, multicellular organisms.

“That makes T. magnifica an intriguing example of a bacterium that has evolved to greater complexity,” said Volland.

Despite its colossal size, scientists believe this is unlikely T. magnifica are the giants of the bacterial world. Instead, his discovery highlights the narrow spectrum of microbes studied in the laboratory and the endless diversity of microbial life unseen in the wild.

“I regularly remind people that bacteria aren’t primitive — they evolved just like us,” Eisen said. “If we keep looking, we’ll find more exceptions.” Scientists baffled by discovery of bacteria that ought to be too big to function

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