Bacterial cells are usually microscopic, where most bacterial cells are around two micrometres (microns), with the largest known species reaching up to 750 µm. The newly discovered giant bacteria has an average cell length greater than 9,000 µm (9 mm) and is visible to the naked eye.
“It’s 5,000 times bigger than most bacteria,” says lead author Jean-Marie Volland from the US Department of Energy Joint Genome Institute, the Lawrence Berkeley National Laboratory (US) and the Laboratory for Research in Complex Systems (US). “To put it into context, it would be like a human encountering another human as tall as Mount Everest.”
Guadeloupe is a French island group in the southern Caribbean Sea, famous for its 8000 hectares of mangrove swamp, predominantly situated within the Grand Cul-de-sac marin, which fringes Grand-Terre and Basse-Terre islands.
In 2009, marine biology Professor Olivier Gros, from the Université des Antilles (Guadeloupe), was sifting through mangrove sediments when he came across the bacteria.
“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,” says Gros.
“I thought they were eukaryotes,” adds co-author Associate Professor Silvina Gonzalez-Rizzo, of the Université des Antilles. “I didn’t think they were bacteria because they were so big with seemingly a lot of filaments.”
Both morphological and molecular approaches were used over several years to identify and characterised these unusual specimens. Combining fluorescence, x-ray, and electron microscopy with genome sequencing, it has now been confirmed that this is indeed a single-cell sulfur-oxidising prokaryote, belonging to the genus Thiomargarita.
“We realised they were unique because it looked like a single cell,” says Gonzalez-Rizzo. The fact that they were a ‘macro’ microbe was fascinating!”
The main distinction between eukaryotes and prokaryotes is that eukaryotic cells store their genetic information in a membrane-bound nucleus, while for most prokaryotic cells the DNA floats freely within the cells’ cytoplasm.
The giant bacteria keeps its DNA more organised than other prokaryotes: its DNA clusters are kept in membrane-bound compartments dubbed “pepins” after small seeds in fruit. It also has three times more genes than most other bacteria.
“The big surprise of the project was to realise that these genome copies that are spread throughout the whole cell are actually contained within a structure that has a membrane,” says Volland. “And this is very unexpected for a bacterium.”
The species has been suitably named Thiomargarita magnifica.
“’Magnifica’ because magnus in Latin means big and I think it’s gorgeous like the French word magnifique,” explains Gonzalez-Rizzo. “This kind of discovery opens new questions about bacterial morphotypes that have never been studied before.”
The paper about giant bacteria has been published in Science.