Thiomargarita namibiensis: Meet The World's Largest Bacteria

Ever wondered how big the world's largest bacteria gets to be? Well, I did today and after 10 seconds of googling I discovered it's Thiomargarita namibiensis which on average is 0.1–0.3 mm in diameter, but sometimes can reach a diameter of  0.75 mm. To give you a perspective, the average diameter of spherical bacteria is 0.0005 - 0.002 mm ( 0.5-2.0 µm) while for rod-shaped bacteria the average length is 0.0001 - 0.001 mm (1-10 µm) and the average diameter 0.00025 - 0.001 mm (0.25-1 .0 µm).

Stained microphotograph of Thiomargarita namibiensis bacteria (public domain)

And to give you an even better perspective, if T. namibiensis was a blue whale, then the average bacterial cell would be slightly smaller than a new-born mouse. T. namibiensis is actually big enough to be visible with the naked eye, almost as big as the periods of this post!

The species was first discovered by a group of German, Spanish, and American researchers sampling sediment off the coast of Namibia and was officially described in 1999.

Image illustrating morphology of Thiomargarita namibiensis. (credit)

The bacterium's name means "Sulfur Pearl of Namibia", from the greek word θείο meaning "sulphur", the Latin word "margarita" for pearl and.. namibiensis for Namibia. All this, a direct reference to the location where the bacterium was first found and the fact that it uses sulfates for energy:

The bacterium is chemolithotrophic and is capable of using nitrate as the terminal electron acceptor in the electron transport chain. The organism will oxidize hydrogen sulfide (H2S) into elemental sulfur (S). This is deposited as granules in its periplasm and is highly refractile and opalescent, making the organism look like a pearl. [source]

But why does it get so big? Well, about 98 % of its body is consisted of a liquid container, called vacuole, that stores nitrates. As mentioned before, these bacteria generate energy from reactions between sulphide and nitrates, but the latter reach the sediment they inhabit only so often:

While the sulfide is available in the surrounding sediment, produced by other bacteria from dead microalgae that sank down to the sea bottom, the nitrate comes from the above seawater. Since the bacterium is sessile, and the concentration of available nitrate fluctuates considerably over time, it stores nitrate at high concentration (up to 0.8 molar) in a large vacuole like an inflated balloon, which is responsible for about 80% of its size. When nitrate concentrations in the environment are low, the bacterium uses the contents of its vacuole for respiration. Thus, the presence of a central vacuole in its cells enables a prolonged survival in sulfidic sediments. The non-motility of Thiomargarita cells is compensated by its large cellular size. [source]

Prior to the discovery of T. namibiensis, the previous holder of the record was Epulopiscium fishelsoni, at 0.2 - 0.7 mm (200–700 μm) long, making it also visible to the naked eye. Epulopiscium means "banquet of fish" from the latin word epulum ("feast" or "banquet") and the latin piscium ("of fish"), a reference to the bacteria's symbiotic relationship with surgeonfish. As for the specific epithet fishelsoni, it is in honor of Lev Fishelson, a Polish-born Israeli ichthyologist who was part of the team who first found the bacterium while studying the intestines of a brown surgeonfish.

Size comparison between Epulopiscium fishelsoni, E. coli and a Paramaecium (credit)

These bacteria have been isolated from many different surgeonfish species from all around the world but the details behind their symbiotic relationship still remain unclear. The prevalent theory is that the bacteria somehow help the fish in breaking down algal nutrients. 

What is most interesting about T. namibiensis is probably its unique viviparous (giving birth) reproductive cycle which may also the reason behind it's gigantic size: 

Unlike most bacteria, which undergo binary fission, Epulopiscium reproduces exclusively through an unusual form of sporulation in which anywhere from one to twelve daughter cells are grown inside of the parent cell, until the cell eventually lyses (and dies). Although sporulation is widespread among other bacteria (such as Bacillus subtilis) in the phylum Firmicutes, spore formation is usually brought about by overcrowding, the accumulation of toxins in the environment or starvation, rather than a standard form of reproduction. Also, endospores are dormant, while new Epulopiscium cells are active. [source]

And speaking about the world's largest bacteria, I think I have to give a shout out to Syringammina fragilissima. It might not be a bacterium but it is an one-cell organism like bacteria. And not just any single one-celled organism. It's a true behemoth, with the largest ever recorded specimen being 20 cm (8 in) across! Here's a photo of the specimen:

credit 

Although it looks like a sponge, it's actually a Xenophyophore, a group of giant multinucleate unicellular organisms found on all major oceans, at depths ranging from 500 to 10,600 meter. Actually, I have made a whole post about S. fragilissima, just click the link to check it out and learn more about it!

So, that's enough talk about gigicantic uni-cell organisms for today. Don't forget to click the various links if you want to learn all there is to know about the organisms mentioned in this post! 

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