Thiomargarita magnifica is, so far, the largest known bacterium. It is visible to the naked eye, as it is the size of an eyelash. At 5,000 times larger than most bacteria and 50 times larger than any other known giant bacteria, Thiomargarita magnifica is unique. “To put it in context, it would be like a human encountering another as high as Mount Everest ,” said marine biologist Jean-Marie Volland, lead author of the study.
The centimeter-long T. magnifica bacterium was discovered on one of the Guadeloupe islands in 2009 while marine biology professor Olivier Gros was looking for bacteria that use sulfur to generate energy. However, when Gros poured his sample of swamp water into a petri dish, what he saw were fine, noodle-like threads visible to the naked eye floating on leaves and soil. “When I saw them, I thought: ‘weird’,” he said. “At first, I thought it was something curious, some white filaments that must be attached to something in the sediment, like a leaf.”
Years later, several scientists have examined the strange organism under the microscope. They have studied it using fluorescence, X-rays, electron microscopy and have sequenced its genome. Researchers have confirmed that it is a giant single-celled bacterium .
The results of the research have recently been published in Science . The researchers reveal in the publication several curious mechanisms that could explain why this bacterium has reached a size that exceeds the limits of what is theoretically possible.
Bacteria belong to the group of prokaryotic organisms, that is, they do not have internal membranes that isolate their genetic material. T. magnifica , however, does have internal membranes in which its DNA and ribosomes are stored. “Because it secretes its genetic material into membrane-bound organelles, T. magnifica challenges our concept of a bacterial cell ,” the study authors write.
By having internal membranes, the gigantic bacterium can distribute the mitochondria to the outside, which is where ATP (adenosine triphosphate) is formed, the energy of the cell. As we have mentioned, other bacteria do not have internal membranes, so the only place to place the ATP-generating machines (ATP synthase) is the cell envelope that encapsulates the entire organism. Since it is difficult to transport this energy very far, most bacterial cells are limited in size.
Most bacteria are also smaller because they have to be able to double in volume before dividing into gods to reproduce. T. magnifica , by contrast, reproduces by shedding a small portion of itself to create a daughter cell . Thus, it overcomes this other limitation.
Researchers have discovered that the genome of T. magnifica has a much higher number of genes than other bacteria: 11,788 genes, compared to 3,935 genes of an average prokaryote.
Referencia: Volland, J.M. et al. 2022. A centimeter-long bacterium with DNA contained in metabolically active, membrane-bound organelles. Science. DOI: 10.1126/science.abb3634
