World’s largest bacteria discovered in Caribbean mangroves

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There is nothing more natural than calling bacteria “microbes”, that is, life forms that we can only see with microscopes, but a newly discovered bacterial species completely breaks this rule. Measuring about 1 centimeter, clearly visible to the naked eye and with an appearance reminiscent of strands of white hair, it was found in the mangroves of Guadeloupe, in the Caribbean. And the massive size (for a bacterium) is only part of its weirdness.

The internal organization of the cells of the species—provisionally designated as Thiomargarita magnificent— also appears to be far more complex than expected for creatures of this type, which could provide valuable insights into the process that led to the emergence of more complicated living things, such as the ancestors of animals and plants.

Coordinated by French marine biologist Jean-Marie Volland, who works at the Lawrence Berkeley National Laboratory (USA), the research describing the megabacterium has just been published in the specialized journal Science.

There is no reason to fear any harmful effect of the “macrobe” on human health. The species survives through chemosynthesis, a process equivalent to photosynthesis in plants, but using chemical compounds as a base, not light (in the case of the giant bacterium, it depends mainly on the chemical element sulfur). That is, it produces its own food and does not parasitize anyone.

“We don’t know the details of the evolutionary history of Thiomargarita magnificabut we know that she lost some of the genes [grosso modo, trechos funcionais do DNA] considered essential for bacterial cell division. On the other hand, we know that it has multiple copies of some genes that are involved in cell elongation. This may explain their unusual cellular morphology,” Volland told Sheet.

The filaments that correspond to the bacteria were found stuck to the submerged leaves of the mangrove, in brackish and shallow water. Scientists have confirmed that each 1-cm-long strand actually corresponds to a single cell — the data is important because other bacteria are able to form multicellular colonies that are also visible to the naked eye. The cells described are now at least 50 times larger than any other bacteria found before.

There are two main hypotheses to explain why it is so large, according to the French researcher. “Giant bacteria may have acquired this extreme cell size to escape their predators. You can imagine that if you become hundreds of times bigger than your enemies, you no longer have to worry about the risks of being eaten.” On the other hand, the size could also be useful to capture more chemical energy from the sulfur present in the mangrove environment.

Be that as it may, getting that big also involves a number of challenges. Bacterial cells in general are much simpler than the so-called eukaryotes (a group that includes all living beings with many cells, like us, and also some complex microbes, such as malaria).

They lack, for example, sophisticated systems for transporting nutrients and other substances from one part of the cell to another. This would be one of the reasons why bacterial cells are normally much smaller than human ones: they wouldn’t be able to distribute supplies to all their parts. In addition, their DNA is normally scattered throughout the cytoplasm (the “stuffing” of the cell), without a specific nucleus dedicated to storing it, as in eukaryotes.

It is at this point that the magnificent T. really stands out. She seems to have solved this problem by creating things similar to the internal organization of the cells of the most complex living things. What is most striking is the presence of many small “power plants” separated by a membrane inside the cell, containing both DNA and ribosomes, the assembly lines of proteins. In addition, it has 40,000 copies of its genome (the set of DNA) scattered throughout its cells. With this, no cellular region would be without the raw materials necessary for its functioning.

“THE magnificent T. is not the missing link between prokaryotes and eukaryotes, but it gives us a unique opportunity to study a modern microbe that is currently evolving towards greater cellular complexity”, summarizes Volland.

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