The "Mount Everest" of bacteria has been discovered! It is 2cm long and visible to the naked eye!

If you just take a quick look, the slightly turbid water in the test tube seems to be no different from the water in the pond, mixed with leaves, sediments, and some thin "threads". But if you look closely, these filament-like objects are actually floating above the leaves, swaying slowly in the water...

Could it be a parasite? After all, thread-like parasites are very common in water, but when scientists took this "thin line" under a microscope, everyone was shocked - these filamentous objects turned out to be single bacterial cells visible to the naked eye!

Rendering of a giant bacterium. Image source: scitechdaily.com

Different from our usual impression of bacteria: they are tiny, most of them can only be seen under a microscope, and have a simple cell structure. The bacteria discovered by scientists this time can be called the "giants" of the bacterial world.

Part 1

Centimeter-sized bacteria visible to the naked eye

In June 2022, the well-known journal Science published a study on bacteria. Scientists discovered a new species of bacteria, Ca.Thiomargarita magnifica, which is about 0.9 cm long and can reach 2 cm in length. This size can be said to be a giant in the bacterial circle. You must know that most bacteria are still micron-sized, and 1000 microns equals 1 mm. This bacterium is 5000 times larger than most bacteria. You may not have a very intuitive impression. For example, it is like a person meeting another person as tall as Mount Everest, and the difference is instantly judged.

Comparison of giant bacteria and some other common organisms Image source: References

Maybe the picture above is not very intuitive enough. You can watch the video below to get a sense of how visible this bacteria is to the naked eye.

Giant bacteria swaying in water Video source: pictojam.com

Morphologically, the bacterium is a white filament that gradually contracts from the base to the tip to form spores. The overall appearance is smooth and there are no epiphytes. Interestingly, each filament is a separate cell with no division septum in the middle, except for the spore at the tip that is separated from the main body of the filament by closing and contraction.

The morphology of giant bacteria Image source: References

There is no doubt that this is the largest bacterium discovered so far, but this is only one aspect of T. magnifica that makes it different from others. What really subverts scientists' cognition is that the genetic material of this bacterium is enclosed by a membrane structure.

Part 2

It's bacteria, but it doesn't look like bacteria

Scientists' encounter with this giant bacterium occurred in the mangroves of the Caribbean Sea. They found some white filaments attached to the leaf sediments, but at first the scientists did not pay attention to these filaments. They just thought they were a little strange, so they did not conduct any follow-up research. This delay lasted for more than 10 years.

Finally, scientists with discerning eyes discovered the uniqueness of these white filaments and conducted in-depth research. They did not think that this was a kind of bacteria at first, because these filaments were visible to the naked eye and looked more like eukaryotes. It was not until scientists sequenced the 16S ribosomal RNA of the white filaments that they determined that this was a kind of bacteria.

(Note: 16S ribosomal RNA, referred to as 16S rRNA, is a component of the 30S subunit in the ribosome of prokaryotes and is known as the most useful and commonly used molecular marker in bacterial systematic classification studies. Unlike prokaryotes, the three types of ribosomal RNA in eukaryotes are 5.8S rRNA, 18S rRNA and 28S rRNA, and there is no 16s rRNA)

The mangrove forest where giant bacteria were found. Image source: scitechdaily.com

But as the research progressed, scientists began to wonder again. The results of 16S rRNA showed that this giant was undoubtedly a bacterium, but when they used confocal laser scanning microscopy and transmission electron microscopy to observe the membrane structure of these bacteria, they made another surprising discovery. The genetic material of these bacteria was actually covered by a membrane!

We all learned in biology textbooks that bacteria do not have nuclear membranes, only areas where genetic material is concentrated. Unlike other bacteria, T. magnifica does not let genetic material float freely in the cytoplasm, but wraps it in a membrane structure, which is similar to those of more complex cells, such as animal or plant cells. Scientists call these organelles that enclose genetic material "pepins."

Pepins in giant bacteria Image source: References

The membrane-enclosed genetic material in T. magnifica subverts people's cognition and blurs the boundary between eukaryotes and prokaryotes!

Part 3

Why can giant bacteria grow so big?

In academia, it is believed that bacteria lack active intracellular transport systems and rely mainly on chemical diffusion, which limits the size of cells. In short, bacteria are so small because as their size increases, the physiological or metabolic demands of bacterial cells grow beyond the range that the cells can maintain.

So why can giant bacteria grow so big? This has to do with the complexity of giant bacteria. In addition to the two unique characteristics mentioned above, the complexity of this giant bacterium is also far greater than that of other bacteria. The results of genome sequencing show that the genome of T. magnifica is very large, about three times that of our common bacteria, with 10 million base pairs and about 11,788 genes.

Giant bacteria have large and complex genomes Image source: References

Moreover, because pepins are widely distributed in giant bacteria, each pepin has a copy of the genome, so this giant bacterium is actually a polyploid (an individual with three or more sets of chromosomes in its cells, and for this giant bacterium, it has more than three sets of genome copies), with multiple sets of genetic material. Scientists have detected active protein biosynthesis activity in the pepins region. Although the bacteria are large in size, as long as there are a large number of pepins, they can still meet the needs of metabolic activities.

In addition, the cells of T.magnifica bacteria have a large vacuole that accounts for about 73% of the total volume. The vacuole is continuously distributed along the entire filament, which restricts the cytoplasm to the periphery of the cell. This makes it easier for giant bacteria to transport substances, and they can maintain life activities even at a huge size.

The central vacuole of a giant bacterium Image source: References

It has to be said that for more than a century, cognitive biases about the size of bacteria have caused people to turn a blind eye to giant bacteria. There is no doubt that these giant, complex bacteria may exist in obvious places, waiting for people to discover, interpret, and explore the mysteries behind biological complexity.

References:

[1]Volland, JM, Gonzalez-Rizzo, S., Gros, O., Tyml, T., Ivanova, N., Schulz, F., ... & Date, SV (2022). A centimeter-long bacterium with DNA contained in metabolically active, membrane-bound organelles. Science, 376(6600), 1453-1458.

Produced by: Science Popularization China

Author: EVEE (School of Life Sciences, Peking University)

Producer: China Science Expo

The article only represents the author's views and does not represent the position of China Science Expo

This article was first published in China Science Expo (kepubolan)

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