According to relevant definitions, microorganisms should be small and can only be seen under a microscope. However, a bacterium recently discovered by scientists in mangroves in the Caribbean has overturned people’s previous understanding of microorganisms. The linear single cell of the microorganism can be seen by the naked eye, which can reach 2 cm, which is equivalent to a peanut, 5000 times larger than other microorganisms. More importantly, this giant bacterium has a huge genome. Unlike other bacteria that float freely in cells, it is wrapped in a layer of membranous tissue, which is a more complex feature of cell structure, just like cells in the human body.

Recently, once the relevant information of this giant bacterium was released, some researchers were shocked and began to analyze and evaluate its characteristics. This is a giant microorganism that has never been found before, which is 10 times higher than the upper limit of bacterial volume recognized by human beings.

The discovery is incredible and eye opening. This oversized bacterium is even larger than fruit flies and nematodes. The latter is a common research target in the laboratory. In the laboratory research, in order to carry out the bacterial infection test, they will infect smaller bacteria with fruit flies and nematodes and observe the changes after infection.

In addition to overturning the volume and complex structure of microorganisms, this giant bacterium may also be an important link missing in the evolution of complex cells. For a long time, researchers have divided life forms into two categories: prokaryotes, including bacteria and single celled microorganisms called Archaea; Eukaryotes include everything from yeast to most multicellular life forms, including humans. The DNA of prokaryotes is in a free floating state, while the DNA of eukaryotes is wrapped in the nucleus. Eukaryotes also divide various cell functions into vesicles called organelles, and can move molecules from one vesicle to another, which prokaryotes cannot achieve.

However, the newly discovered giant bacteria blurred the boundary between prokaryotes and eukaryotes. About 10 years ago, Olivier Gros, a marine biologist at the University of the French Antilles, accidentally found a strange creature in a local swamp, which grew in filaments on the surface of rotten mangrove leaves. It was not until five years later that he and his colleagues realized that these organisms were actually a special kind of bacteria. Recently, when researchers conducted in-depth analysis of the bacteria and tried to fully understand its biological characteristics, they realized that the research results refreshed people’s previous understanding of the concept of bacteria.

Some microorganisms, such as slime molds and blue-green algae, form clearly visible stems or filaments composed of piles of cells, but the research team used various microscopy and staining methods to verify that mangrove filaments are actually the structure of each cell. In addition, the cell includes two membrane capsules, one of which contains the DNA of all cells. This membrane capsule is an organelle, This is a major new scientific development, which means that the two branches of life are not as different as previously thought. Maybe it’s time to reconsider our definitions of eukaryotes and prokaryotes.

Another capsule is filled with water, which may be one of the reasons why the bacteria grow unusually large. Previously, microbiologists thought that bacteria must be very small, in part because they need to eat and breathe, and detoxify through the diffusion of molecules inside the cell, and the moving distance of these molecules is very limited. In 1999, researchers found a huge sulfur eating microorganism on the coast of Namibia, about the size of a poppy seed. The microbial body is large because its cell material is squeezed on the outer cell wall by a huge cystic structure filled with water and nitrate. The basic molecules of this microorganism can still diffuse, because only the cells at the edge are living. Later, scientists also found other large sulfur eating microorganisms, but their filament structure is composed of multiple cells.

Like the microorganisms found in Namibia, the newly discovered mangrove bacteria also have a huge saclike structure - about water, accounting for 73% of the total volume. This similarity and genetic analysis led the research team to classify it into the same genus as most other giant microorganisms and suggested naming it Thiomargarita magnafica.

The largest Thiomargarita magnafica micro organism found now is up to 2 cm long. If the microorganism is not trampled, swallowed, blown away by the wind or swept away by the waves, its size will become larger.

The analysis results show that the membrane capsule filled with DNA of this microorganism is also squeezed at the edge. When researchers sequenced the DNA, they found that its genome is very large, with 11 million bases, including about 11000 significantly different genes. Generally, the bacterial genome has an average of about 4 million bases and 3900 genes.

By labeling DNA with fluorescent substances, the microbial genome is so large because the same piece of DNA has more than 500000 copies. The protein manufacturing plant called ribosome also exists in the capsule filled with DNA, which may make the gene code more efficient to encode protein. Bacteria are often considered to be a small, simple and underevolved life form, also known as “protein capsule”, but by studying this largest microbial structure, we will have a new understanding of microorganisms. (Ye Qingcheng)