This microscope photo, provided by Lawrence Berkeley National Laboratory in June 2022, shows thin strands of bacterial Thiomargarita magnifica cells to a US penny coin. Thomas Timl / Associated Press
The largest known bacterium – a noodle-shaped organism found in shallow mangrove swamps in the Caribbean and large enough to be seen with the naked eye – redefines what is possible for bacteria, the most ancient form of life on Earth.
Scientists said Thursday that the bacterium, called Thiomargarita magnifica, is remarkable not only because of its size – colossal to a single-celled organism up to eight-tenths of an inch (2 cm) long, but also because its internal architecture is different from other bacteria.
DNA, the body’s scheme, is not free-floating inside the cell, as in most bacteria, but is contained in a number of small sacs attached to a membrane. Membrane-bound structures in cells are called organelles.
“It’s thousands of times bigger than ordinary bacteria. Detecting this bacterium is like encountering a human being as tall as Everest, “said marine biologist Jean-Marie Woland of the US Department of Energy’s Joint Genome Institute and the California Complex Systems Research Laboratory, co-leader of the study. published in the journal Science.
The bacterium was found in several places in Guadeloupe, a French archipelago in the Caribbean. It was first observed in the sulfur-rich seawater of the Guadeloupe swamp by the microbiologist of the Université des Antilles and co-leader of the study, Olivier Gro.
“In 2009, I found long white threads attached to a sunken leaf of a mangrove tree. I found such threads intriguing. I took them back to the lab to analyze them, “Gross said. “It’s a big surprise to me that I have such a huge bacterium living in the mangrove forests of Guadeloupe.
The normal bacterial species is 1-5 micrometers long. This species is on average 10,000 micrometers long (four tenths of an inch / 1 cm), with some Thiomargarita magnifica twice as large.
“It’s an order of magnitude larger than what we thought was the maximum possible size for a bacterium,” Woland said. “They are about the same size and shape as the lashes.”
The largest bacterium known to date had a maximum length of about 750 micrometers.
Bacteria are unicellular organisms that live almost everywhere on the planet, vital to its ecosystems and most living things. Bacteria are thought to have been the first organisms to inhabit the Earth and remain quite simple in structure billions of years later. Human bodies are teeming with bacteria, only a relatively small number of which cause disease.
Thiomargarita magnifica is not the largest single-celled organism known. This honor is the aquatic alga Caulerpa taxifolia, reaching 6-12 inches (15-30 cm) in length.
The Caribbean mangrove swamps are full of organic matter, and the microbes in the sediment break down this matter and produce high concentrations of sulfur. The sulfur-rich environment offers an energy source for bacteria such as Thiomargarita magnifica.
Researchers called its DNA-carrying organelles “pepins” in French for small seeds in fruits.
“Apart from two exceptions, there were no other bacteria known to hold their DNA in a membrane-bound organelle. This is actually a characteristic of more complex cells that have a membrane-bound nucleus, such as human cells or animal and plant cells, “Woland said.
Mapping of its genome has shown that Thiomargarita magnifica has lost some of the genes needed for cell division and has more than the usual number of copies of genes responsible for cell elongation.
“This may partly explain why the cell grows into such an elongated thread. The genome is also very large and contains three times the average number of genes that are normally found in bacteria. “Half of these genes have no idea what they’re for,” Woland said.
This bacterium illustrates how life on Earth still has surprises awaiting discovery, he said.
“Life is fascinating, very diverse and very complex,” Woland added. “It’s important to stay curious and have an open mind.”
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