How does this giant bacterium survive?

If you go to the Caribbean island of Guadeloupe and take a walk through the mangroves, you will be able to observe small whitish filaments on the decomposed mangrove leaves. surprise yourselves They are seeing bacteria with the naked eye.

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

These records have been wiped out by another bacterium, Thiomargarita magnifica, which forms the filaments we referred to at the beginning. It measures an average of one centimeter in length, and can reach up to two centimeters. Since typical bacteria are a few microns (thousandths of a millimeter), we are talking 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 confirmed, 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 sheer size of this bacterium that’s surprising, as noted in comments posted by Elizabeth Pennisi and Petra Ann Levin. Its characteristics provide a new dimension about the diversity of prokaryotes, living beings that lack a nucleus and other organelles such as mitochondria.

Why does a bacterium have to be small?

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

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

Another problem is energy production. Eukaryotes have our 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 and volume, until the production of ATP from a certain volume becomes insufficient.

Comparative scheme of Thiomargarita magnifica.

However, T. magnifica is able to overcome these two limitations. As it does? In a very clever way.

About 75% of the volume of this bacterium is occupied by a large vacuole, so that the cytoplasm is restricted to a narrow strip of about 3.3 microns thick between the vacuole and the outer membrane, a dimension that 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 pepins (something like fruit seeds). 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. Around them and other vesicles, ATP synthase is located, so that its abundance no longer depends on the internal 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. It is 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 have up to 37,000 copies of the genome. In a one centimeter bacterium we would be talking about 400,000 copies of the genome. How this huge number of copies is regulated will certainly be the subject of future research.

A bacteria clean as a paten

Do you want more surprises? The number of genes in T. magnifica (11,788) is three times the average in bacteria and is similar to that in the 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, perhaps due to the production of powerful antibiotics. It is not necessary to insist on the applied interest that this can have.

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

Since mutations occur in the process of generating copies of the genome, the daughter cells do not have the same genetic composition as their parent, something that can be considered halfway between typical bacterial reproduction and sexual reproduction.

Evolutionary and philosophical questions

The questions raised by this discovery are profound. Why did this bacterium evolve towards gigantism and increase in genome size? How is the vesicle system formed? What functions do organelles devoid of DNA? What evolutionary implications does asymmetric genome segregation have on reproduction? T? magnifica has reached the real limit of bacterial size?

I would add an almost philosophical question: how does the delocalized character of the genetic material and the production of proteins reconcile with the concept 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, something that would allow us to answer these and other questions. But what is clear is that T. magnifica poses a challenge to the usual concepts we had about prokaryotic organisms.

*This article was originally published on The Conversation.

ABOUT THE AUTHOR

Ramon Muñoz-Chapuli Oriol

Professor of Animal Biology (retired), University of Malaga

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