Newly Discovered Bacterium is So Massive it's Visible to the Naked Eye

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Single filament of Ca. Thiomargarita magnifica, shown on its side. Credit: Jean-Marie Volland

A team of researchers has discovered and characterized bacteria that is 5,000 times larger than most other bacteria. It is so large, in fact, that it is visible to the naked eye—no microscope needed.

“To put it into context, it would be like a human encountering another human as tall as Mount Everest,” said Jean-Marie Volland, co-author of a new study on the bacteria and a scientist at the U.S. Department of Energy Joint Genome Institute (JGI).

The bacterium was discovered by Olivier Gros, a marine biology professor at the Université des Antilles in Guadeloupe in 2009. The researcher was looking for sulfur-oxidizing symbionts in mangrove sediments not far from his lab when he first encountered the bacteria.

“When I saw them, I thought, ‘strange,’” said Gros. “In the beginning I thought it was just something curious, some white filaments that needed to be attached to something in the sediment, like a leaf.”

But that wasn’t the case at all.

Conducting microscopy studies over the next couple of years, the researchers realized the thin, noodle-like threads were actually sulfur-oxidizing prokaryotes belonging to the genus Thiomargarita. Silvina Gonzalez-Rizzo, an associate professor of molecular biology at the Université des Antilles and co-first author on the study, aptly named the bacteria Ca. Thiomargarita magnifica—because magnus in Latin means big.

At the JGI, Volland began studying Ca. T. magnifica to better understand what this sulfur-oxidizing, carbon-fixing bacterium was doing in the mangroves.

Using various microscopy techniques—such as X-ray tomography, confocal laser scanning microscopy and transmission electron microscopy— Volland visualized entire filaments up to 9.66 mm long and confirmed they were indeed giant single cells rather than multicellular filaments, as is common in other large sulfur bacteria. He also employed confocal laser scanning microscopy and transmission electron microscopy to visualize the filaments and the cell membranes in more details.

Taking an even closer look at the cell membranes, Volland noted novel, membrane-bound compartments that contain DNA clusters. He dubbed these organelles “pepins,” after the small seeds in fruits. These DNA clusters were plentiful in the single cells.

The team was also surprised to learn about the cell’s genomic complexity. According to the study, published in Science, the bacteria contain three times more genes than most bacteria and hundreds of thousands of genome copies that are spread throughout the entire cell.

“The big surprise of the project was to realize that these genome copies that are spread throughout the whole cell are actually contained within a structure that has a membrane,” said Volland. “This is very unexpected for a bacterium.”

For most bacteria, their DNA floats freely within the cytoplasm of their cells. However, as Volland and team observed, this newly discovered species keeps its DNA more organized.

“This project has been a nice opportunity to demonstrate how complexity has evolved in some of the simplest organisms,” said Shailesh Date, founder and CEO of the Laboratory for Research in Complex Systems, and one of the article’s senior authors. “One of the things we've argued is that there is need to look at and study biological complexity in much more detail than what is being done currently. So, organisms we think are very, very simple might have some surprises.”

While the research revealed many new surprises, it also paved the way for many new questions. Chief among them is Ca. T. magnifica’s role in the mangrove ecosystem—where Gros’ discovery of the bacteria in the murky waters kicked off the entire project.

Another question is whether the pepins played a role in the evolution of the bacteria’s extreme size, and whether or not pepins are present in other bacterial species. The precise formation of pepins and how molecular processes within and outside of these structures occur also remains to be studied.

 

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