Peanut-sized mega-bacteria discovered in the Caribbean

Peanut-sized mega-bacteria discovered in the Caribbean

Thiomargarita magnifica, which can reach 2 cm in length, is a real giant in the world of bacteria.

In popular jargon, the term “bacteria” is often replaced by “microbe”, in reference to their small size which makes them invisible to the naked eye… at least, for the majority of them; this is not the case for this new species with such an impressive size that it has just set a new world record.

Until now, this title of largest bacterium ever documented went to Thiomargarita namibiensis, a veritable bacteriological colossus capable of reaching 0.75mm. But this new species simply shattered that record; individuals measure on average… 0.9 cm.

The record for an individual cell has even been set at 2 cm, a size comparable to that of a peanut! It’s also more than 20 times the previous record, and researchers expect some specimens could be even larger.

A real micro-colossus…

This veritable colossus completely redefined what researchers thought they knew about bacterial growth. Until then, specialists were convinced that the size of bacteria was limited by their ability to exchange molecules with their environment. However, the larger the organization, the more complicated these logistics become.

Indeed, to enter or leave the cytoplasm of the cell (the central pocket which contains in particular the organelles, functional equivalents of the organs on the scale of a cell), each compound must interact specifically with an element of the cellular machinery placed on the diaphragm.

It is very convenient for small bacteria; since they have a small volume, it is therefore easy for the compounds to find their way in or out. But the problem is that in this specific context, when we increase the size of an organism, its volume increases faster than its surface; the ratio between surface area and volume therefore decreases as the organism grows.

This surface to volume ratio (S/V) is a problem that goes well beyond bacteria; it is absolutely central in general biology. This parameter is directly responsible, among other things, for the size limit of many animal species; if you have never met a human the size of a diplodocus, it is above all a story of S / V ratio.

…that defies all theoretical limits

To overcome this conceptual limit, many species have developed countermeasures through natural selection; in the majority of species, we find, for example, more or less developed circulatory systems. That’s what your blood is for: it allows each molecule to be delivered directly to its destination, instead of patiently waiting for it to get there on its own by diffusion – which would leave you with all the time to choke or die from hunger.

But unlike us these organisms do not have a complex circulatory system; they are not even entitled to a rudimentary brewing system. Nutrients and other elements must find their way by simple diffusion into the fluid of the cytoplasm. Indeed, like any living being, it needs to be constantly fed, and this diffusion is a relatively slow phenomenon.

Consequently, the more the size increases, the more the surface/volume ratio increases, the more distance the molecules have to travel, and the more difficult it is for the cell to sustain itself. There is therefore a theoretical threshold beyond which the cell would simply no longer be viable. In this case, how to explain the presence of such a colossus?

To grow to this extent, this newly discovered bacterium uses a very ingenious evolutionary sleight of hand; it has another fluid-filled pocket in the middle of its cytoplasm. This bag, which represents almost ¾ of the total volume of the bacterium, allows molecules to be transported to the membrane without having to rely entirely on diffusion.

An XXL genome

This same mechanism had already been identified in Thiomargarita namibiensis, the previous defending champion. Genetic analysis incidentally revealed that the new category giant belonged to the same group; so she was baptized Thiomargarita magnificent.

This same genetic analysis revealed another fascinating aspect of Thiomargarita magnificent. It’s not just its physical dimensions that are extreme: its genome, too, is simply massive. It contains more than 11 million bases which define no less than 11,000 genes. Simply extraordinary figures, and close to three times above average bacterial genomes!

And the researchers were not at the end of their surprises. In effect, Thiomargarita magnificent had one last big surprise in store for them; they also noted the presence of ribosomes directly adjacent to the cell’s DNA.

One of the missing pieces of the great puzzle of evolution?

Very briefly, these ribosomes are cellular organelles which play the role of protein factories. This is a significant finding; according to the researchers, this arrangement is probably a way of optimizing the process of translation and transcription which makes it possible to synthesize proteins from the recipe defined by the DNA.

Concretely, it is a more optimized architecture compared to most bacteria; in fact, the fact of separating the genetic machinery in this way is an approach that one finds rather in organisms much more sophisticated. Enough to redefine a whole section of fundamental research in bacteriology, just that!

Too often, bacteria are equated with simple, small, and “low-evolved” life forms.”, explains Chris Greening, a microbiologist at Monash University. “Common “protein bags”, in short. But this bacterium shows us that this observation could not be further from reality!

This is a particularly striking element for researchers. In effect, Thiomargarita magnificent may well contain some crucial information for understand the origin of life as we know it. According to Kazuhiro Takemoto, a bioinformatician based at the Kyushu Institute of Technology, it could be “a missing link in the history of the evolution of complex cells. It is therefore a major discovery at all levels!

The text of the study is available here.

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