If bacteria have to be small to function, how does this giant survive?

Bacteria 'Thiomargarita magnifica' photographed next to a penny.  Tomas Tyml/Lawrence Berkeley National Laboratory

Bacteria ‘Thiomargarita magnifica’ photographed next to a penny. Tomas Tyml/Lawrence Berkeley National Laboratory

If you go to the Caribbean island of Guadeloupe and walk through the mangroves, you can observe small whitish filaments on the decomposed leaves of the mangrove trees. surprise yourself They see bacteria with the naked eye.

The giant bacteria were already known: Epulopiscium fishelsoni It is a bacterium that inhabits the intestine of the surgeonfish and reaches 0.6 millimeters in length. His record lasted until the discovery of Thiomargarita namibiensisa filamentous bacterium that reaches 0.75 millimeters.

These recordings were wiped out by another bacterium, Thiomargarita magnifica, which forms the filaments of which we spoke at the beginning. It measures on average one centimeter long, and can reach up to two centimeters. Since typical bacteria measure a few microns (thousandths of a millimeter), we are talking about colossal sizes, 5,000 times larger than usual.

But not only the size of Thiomargarita magnifica surprise. Let’s see.

T. magnifica was discovered in 2009, and was thought to be a fungus. When its bacterial nature was proven, a team of American and French researchers undertook an investigation whose results have just been published in the journal Science. And it’s not just the disproportionate size of this bacterium that surprises, as mentioned in the comments posted by Elizabeth Penisi there Petra Ann Levin. Its characteristics bring a new dimension to the diversity of prokaryotes, living beings lacking a nucleus and other organelles such as mitochondria.

Why does a bacterium have to be small?

The exceptional size raises important questions right from the start. What size restrictions limit the growth of bacteria? In other words, why are bacteria so small? The first restriction concerns the transport of substances.

Eukaryotic organisms have complex cellular transport systems, but prokaryotes lack them. This causes molecules to be transported by diffusion, a very slow process that limits cell volume.

Another problem is energy production. Eukaryotes have their own mitochondria, but bacteria produce ATP, the energy transport molecule, through the enzyme ATP-synthase, located on the inner surface of the cell membrane. An increase in size decreases the ratio between surface area and volume, until the production of ATP from a certain volume becomes insufficient.

However, T. magnifica is able to overcome these two limitations. How does he do? In a very smart way.

About 75% of the volume of this bacterium is occupied by a large vacuole, so that the cytoplasm is restricted to a narrow band of about 3.3 microns thick between the vacuole and the outer membrane, a dimension which allows the diffusion of molecules.

On the other hand, cellular processes are decentralized. On the periphery of the bacterium are vesicles that store sulfur (energy source) and others that the authors called Seed (something like fruit pips). These nuggets contain DNA and ribosomes, and messenger RNA and proteins are generated on them.

What is surprising is that the nuggets are bounded by membranes, similar to the nucleus of eukaryotes. Surrounding them and other vesicles is ATP synthase, so its abundance no longer depends on the inner surface of the cell. Although simple organelles have been described in other bacteria, this is the only known case in prokaryotes of genetic material located in a membrane-bound vesicle.

The nuggets contain genomic DNA. They are actually multiple copies of the genome. This phenomenon occurs in other giant bacteria, sometimes numbering tens of thousands of copies. In T. magnifica it has been estimated that each millimeter of bacteria can contain up to 37,000 copies of the genome. In a bacterium of one centimeter, we would speak of 400,000 copies of the genome. The regulation of this large number of copies will certainly be the subject of future research.

A bacterium clean like a paten

Want more surprises? The number of genes in T. magnifica (11,788) triples the average of bacteria and is similar to that of yeast Saccharomyces cerevisiae, a eukaryote. This set includes genes related to sulfur oxidation and carbon fixation, but the very high number of genes related to secondary metabolism, the synthesis of bioactive compounds, was surprising.

This could explain why the surface of this bacterium always appears clean, without other bacteria adhering to it, possibly due to the production of strong antibiotics. It is not necessary to insist on the applied interest that this can have.

Finally, let’s talk about the reproduction of T. magnifica. Its filaments usually have constrictions at the distal end which isolate small portions interpreted as the daughter cells of the reproductive process. What is unique in this case is that the daughter cells only receive a sample of the maternal genome, about 1% of the copies.

Since mutations occur during the process of generating copies of the genome, the daughter cells do not have the same genetic makeup as their parent, which can be considered midway between typical bacterial reproduction and sexual reproduction. .

Evolutionary and philosophical questions

The questions raised by this discovery are profound. Why did this bacterium evolve to gigantism and increase the size of the genome? How is the vesicular system formed? What are the functions of organelles lacking DNA? What evolutionary implications does asymmetric genome segregation have on reproduction? doT. magnifica Has the actual bacterial size limit been reached?

I would add an almost philosophical question: how does the delocalized nature of genetic material and the production of proteins reconcile with the notion of the individual? Are we facing a new type of individual in the world of living beings?

At the moment, it has not been possible to cultivate this bacterium, which would allow us to answer these and other questions. But what is clear is that T. magnifica This is a challenge to the usual concepts we had about prokaryotic organisms.

This article was originally published on The conversation. read it original.

Ramón Muñoz-Chápuli Oriol does not receive a salary, perform consulting work, own stock, or receive funding from any company or organization that may benefit from this article, and has stated that he had no relevant connection beyond the cited academic position.

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