Largest bacterium ever found is surprisingly complex

Black and white image of the filaments of Ca. Thiomargarita magnifica

The filamentous Thiomargarita magnifica cells have more complex internal organization than do typical bacteria.Credit: Olivier Gros/Lawrence Berkeley National Laboratory

Lurking on rotting leaves sunken in the mangroves of Guadeloupe in the Caribbean live some extraordinary thread-like creatures. These filament-like organisms, up to a centimetre in length, are the biggest single-cell bacteria yet to be found. Named Thiomargarita magnifica, they live by oxidizing sulfur, and are 50 times bigger than any other known bacteria.

Biologist Olivier Gros found the bacteria in 2009 while exploring the mangroves of Guadeloupe, where he works at the University of the Antilles in the French West Indies. “At the beginning, I thought it was something like a fungi or something — not bacteria, but a eukaryote, maybe,” Gros says. Unlike bacteria and archaea, which are simple microorganisms, eukaryotes — which include animals and plants — have complex cells containing a nucleus and organelles such as mitochondria.

When he got back to his laboratory at Pointe-à-Pitre in Guadeloupe, Gros examined his discovery under a microscope. It was then that he realized he wasn’t looking at a eukaryote — and that he’d found something special. In 2018, marine biologist Jean-Marie Volland at Lawrence Berkeley National Laboratory in California looked at the bacteria more closely using a range of methods, including transmission electron microscopy and an imaging technique called fluorescence in situ hybridization. In this way, he helped to confirm that it was a single cell. The authors reported their results in a preprint in February, and have now published them in Science1.

There are other whoppers in the Thiomargarita bacteria family, but the next-largest is only around 750 micrometres in length. Other filament-like bacteria are also found in the mangroves, but these all consist of tens or hundreds of cells. “What is very unique about the T. magnifica is that the entire filament, which is among the longest filaments in the mangrove, is just one cell,” says Volland.

Central to the bacterium is its vacuole — an inert, fluid-filled membrane. Around the edge of this are membrane-bound structures, which the authors call pepins and describe as being similar to the organelles found mostly in eukaryotic cells.

The filaments of Ca. Thiomargarita magnifica are pictured next to a coin

Thiomargarita magnifica filaments next to a US 10-cent coin.Credit: Tomas Tyml/Lawrence Berkeley National Laboratory

Thiomargarita magnifica is remarkable for more than its size. In other bacteria, genetic material floats freely inside the cell, usually in the form of just one circular chromosome. In T. magnifica, the team saw that the genetic information was stored in hundreds of thousands of pepins. Each of these contains DNA and ribosomes, molecular machines that translate instructions from DNA to make proteins. The pepins collectively host up to 700,000 copies of the genome.

Many questions remain. Among these are whether the specific habitat of the mangrove, which has high levels of sulfur-containing molecules and sulfur-eating microbes, is crucial to the existence of this bacterium. And the pepins themselves needs a closer look to determine whether they all contain the same mix of genetic material, ribosomes and proteins. “We have not sequenced individual pepins — we have sequenced the entire cell, which contains hundreds of thousands of pepins,” says Volland. In particular, the researchers don’t know whether each pepin contains just one copy of the genome, or more than one.

Now that T. magnifica has been discovered, Gros expects other teams to go off in search of even larger bacteria — which might be hidden in plain sight, he says. Petra Levin at Washington University in St Louis, Missouri, says that the discovery challenges conventional wisdom that bacteria have lower size limits than eukaryotic cells. “There’s probably an upper limit on cell size at some point, but I don’t think it will be peculiar to bacteria or archaea or eukaryotes.”

“We really should not underestimate evolution, because we can’t guess where it’s going to go,” says Levin. “I would not have guessed this thing exists, but now that I see it, I can see the logic in the evolution to this point.”

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