Who is the "giant" among microorganisms?

Recently, French scientists isolated a new type of bacteria called Thiomargarita floridula (Giant Thiomargarita floridula) from the mangrove mudflats of the Caribbean Sea. Its size has reached an astonishing 2 centimeters and is easily visible to the naked eye. It is really astonishing that the bacteria have grown to the same size as worms.

Thiomarginatus giganteus (white bright line) compared to a coin

(Image source: Reference 5)

So why do we define Thiomargarita as a bacterium? Why does it grow to such a huge size? Are there any other giants like this among other microorganisms? To answer these questions, we have to start with the classification of organisms and the connections and differences between various microorganisms.

1. The most mainstream biological classification method - the three-domain theory

The classification of organisms is a very important scientific issue for humans. Only by correct classification can we more effectively recognize and understand different organisms, so as to better utilize or protect them. At present, in the biological community, the more mainstream cognition is to divide organisms into three major categories: bacterial domain, archaea domain and eukaryotic domain. This classification method is based on ribosomal RNA sequences, rather than simply relying on the anatomical characteristics of each species, and is relatively scientific.

Schematic diagram of the three domain theory

(Image source: Reference 2)

It is worth mentioning that viruses do not belong to any of the above categories. This is because viruses do not have the ability to survive independently from cells, and only show life-like characteristics after infecting cells. Therefore, in the above classification method, viruses often exist as a non-biological supplement.

In the three-domain theory, there are certain similarities between the organisms in the bacterial domain and the archaeal domain. Both are single-cell organisms, and the cell types are prokaryotes. However, due to fundamental differences in cell membrane lipids and ribosomal RNA, bacteria and archaea are divided into two domains. In fact, their ancestors have evolved independently from each other since about 2.7 billion years ago, and can be said to be two completely different organisms.

It should be noted that although the word "archaea" is included in the name of archaea, from an evolutionary perspective, archaea are not "ancient bacteria" nor are they the ancestors of modern bacteria. In fact, archaea are evolutionarily closer to eukaryotes and are even widely considered to be the ancestors of eukaryotes.

Diagram of eukaryotic and prokaryotic cells

(Image source: Wikipedia)

Although there are also single-celled organisms with very simple structures among eukaryotic organisms, their cells are essentially eukaryotic cells, which are fundamentally different from prokaryotic cells in structure. Eukaryotic cells contain nuclei, hence the name eukaryotic. Eukaryotic cells also contain other organelles, such as mitochondria, chloroplasts, Golgi bodies, etc. Compared with eukaryotic cells, prokaryotic cells generally do not have an intracellular membrane, a formed nucleus wrapped by a nuclear membrane, no chromosomes in the cell, and the DNA chain is not coiled and exists in a free form in the cytoplasm. There are also no membrane-bearing organelles (such as mitochondria or chloroplasts) in the cytoplasm.

In short, the structure of eukaryotic cells is much more complex than that of prokaryotic cells. All kinds of animals and plants we see in daily life (including humans ourselves, of course) are composed of eukaryotic cells and belong to eukaryotic organisms.

Colorful eukaryotic organisms

(Image source: Wikipedia)

2. Microbial Giant Selection: Qualification is very important

After talking about the classification of organisms, let's take a look at how microorganisms should be classified. Microorganisms are a general term for tiny organisms that are difficult to see directly with the naked eye. There is actually no generally accepted statement about their classification, nor is there a sufficiently rigorous biological basis.

At present, the biological community tends to divide microorganisms into the following six categories: algae, protozoa, fungi (fungi), bacteria, archaea, and viruses. The first three of these six microorganisms belong to the eukaryotic domain, bacteria and archaea each form a domain, and viruses belong to non-living things. However, for ordinary people like us, everyone has experienced the power of viruses, and in our daily cognition, viruses have long been regarded as representatives of microorganisms.

It must be pointed out that among the three eukaryotic organisms, algae, protozoa, and fungi, although many members are tiny organisms that cannot be seen without a microscope, there are also many "giants". For example, mushrooms, the representative of fungi, can easily grow to an astonishing size. For example, in 2018, Chinese scientists discovered a mushroom that was known as the largest fungus on earth, and its length reached 12 meters.

However, existing literature has reported that scientists from the US Forest Service discovered a strain of Armillaria ostoyae in Oregon in 1998. DNA testing at different locations showed that individuals of this fungus covered an area of ​​890 hectares and may have grown in the forest for about 2,400 years.

Some people may say that mushrooms are multicellular fungi, so it is not uncommon for them to grow very large. This is true, but many single-cell algae and single-cell protozoa can also grow to a size that is completely embarrassing to call themselves microorganisms. For example, the kelp mentioned in the high school biology textbook is a single-cell algae. Its nucleus is located at the base, surrounded by false roots, and the "cap" and "handle" and other structures extend upward. The size of the kelp is actually 2 to 5 centimeters, which is not related to microorganisms. Some more exaggerated single-cell algae and single-cell protozoa can reach tens of centimeters or even meters in length.

Several giant single-celled organisms, from left to right: Cyclocystis sphaerocephala, Pteridophyta sphaerocephala, and Umbellifera

(Image source: Wikipedia)

Of course, such huge single-celled species usually do not have just one nucleus inside their cells, but multiple nuclei forming a so-called "syncytium". The reason why they are called single-celled species is that there is no cell wall or cell membrane to separate different cells inside the structure that makes up their body.

In this case, the real microbial giants can only be selected from bacteria, archaea and viruses.

3. Microbial giants that subvert cognition

Let's look at bacteria first. Before the discovery of this super giant bacterium, the largest bacterium in the world (in volume) was held by Namibian Thiomarginatus. In 1999, this bacterium was discovered in marine sediments on the Namibian continental shelf. Its average diameter is 0.1 to 0.3 mm, and the largest individual is even close to 1 mm, which is large enough to be observed with the naked eye. The E. coli that we are very familiar with is only a few microns in length, and the volume difference between the two is thousands of times.

The giant thiophagous bacteria discovered this time have directly refreshed the previous record by thousands of times. Their length can reach an exaggerated 2 cm. Not only can they be seen with the naked eye, but they can also be easily picked up with tweezers. Their shape is similar to human eyelashes. If the height of an adult is used to compare the size of ordinary bacteria, the height of the giant thiophagous bacteria is Mount Everest, which is more than 5,000 times that of ordinary bacteria.

Size comparison of Thiomarginatus giganteus (A/B) and Escherichia coli (Image source: Reference 1)

Compared with ordinary bacteria, the DNA of Thiomarginatus giganteus is larger, and the amount of genetic information is about three times that of E. coli. As a prokaryote, it does not have a nucleus structure like eukaryotes, but inside its cells, it has multiple small organelles that carry DNA. Around these DNA "cores", proteins are synthesized in large quantities, thus growing to a huge size. Its discoverers initially did not think that this was a bacterium at all, but after DNA testing, it was confirmed that this behemoth was indeed an independent individual.

Most archaea are extremophiles that live in extreme environments, and most people don’t know much about them. The largest archaea currently measure about a dozen micrometers, and the largest rod-shaped archaea can be nearly 100 micrometers (0.1 mm) long. However, perhaps due to the constraints of the living environment, humans have not yet discovered any “huge” archaea.

Archaea are frequent visitors to deep-sea hydrothermal vents (Image source: Wikipedia)

Viruses have a simple structure and lack the ability to survive independently outside cells, so they are excluded from the three-domain theory. Simple structure, of course, means small size. It is for this reason that viruses were discovered hundreds of years later than bacteria. Only after the invention of the electronic telescope did humans truly observe viruses. Take the new coronavirus as an example. Its size is only 120 nanometers, which is a few hundredths of the volume of E. coli. However, in recent years, many giant viruses have been discovered one after another, which has greatly overturned humans' previous understanding of viruses.

In 2003, French scientists discovered Mimivirus, which is about 400 nanometers in size. Its discovery also marks the beginning of giant viruses entering the human vision. At present, the largest virus discovered by humans is the Pittovirus reported in 2014. This virus is a double-stranded DNA virus found in Siberian permafrost ice core samples, with a maximum length of nearly 2 microns and a diameter of 0.5 microns. It looks very much like the wide-mouthed pottery jars used in ancient Rome, hence the name. After thawing, the Pittovirus has the ability to infect amoeba and kill it. Therefore, in addition to the topic of size, people have also raised unprecedented concerns about the possibility of unknown pathogens being released after the thawing of permafrost due to global warming.

Size comparison of the wide-mouthed jar virus and the new coronavirus (right)

(Image source: Wikipedia)

In a sense, microorganisms are the real masters of the Earth, and they appear in every corner of the Earth. From the fiery deep-sea hydrothermal vents to the permafrost that has been frozen for tens of millions of years, microorganisms have lived and multiplied tenaciously, and ultimately played a key role in the Earth's energy and material cycles. The Earth shapes microorganisms, and microorganisms in turn shape the Earth. Their magical diversity in evolution has allowed humans to continuously break through the concept of "biology". Perhaps in the future, there will be even larger microorganisms to refresh the list of microbial giants.

References

A centimeter-long bacterium with DNA contained in metabolically active, membrane-bound organelles

https://www.science.org/doi/10.1126/science.abb3634

Chapter 30: The inner body of "Ancestor" 2.7 billion years ago

https://gendai.ismedia.jp/articles/-/73944

Want to observe cells without a microscope? These "giant" single-celled organisms can satisfy you

https://tech.sina.com.cn/roll/2020-01-06/doc-iihnzhha0600724.shtml

Detailed structure and analysis of the world's largest low-temperature electron microscope

https://www.nips.ac.jp/release/2017/11/post_352.html

Largest known bacteria in the world are visible to the naked eye

https://www.newscientist.com/article/2325909-largest-known-bacteria-in-the-world-are-visible-to-the-naked-eye/

Produced by: Science Popularization China

Produced by: Guo Fei (Yantai University)

Producer: China Science Expo