Producer: Popular Science China
Producer: Ye Peiyuan (Shanghai Ocean University, Yunhai Science Popularization Team)
Producer: Computer Network Information Center, Chinese Academy of Sciences
In November 2020, the 10,000-meter-class submersible "Strivers" manufactured in China successfully sat at a depth of 10,909 meters in the Mariana Trench; in October this year, the "Strivers" officially put into regular scientific research applications in the Mariana Trench.
With the continuous development of deep submersible technology, people have gradually discovered that the vast deep sea is not a dead silence, and countless creatures are thriving in this dark place.
For example, the Mariana lionfish, which lives about 8,000 meters below the surface of the sea, is the "deepest deep-sea fish" that has been discovered. In contrast, the depth of human diving is generally within 10-20 meters, and the maximum depth is only 300 meters.
You know, at 8,000 meters underwater, the hydrostatic pressure is about 800 atmospheres, which is almost equivalent to an adult bull standing on your fingernails. Without a deep submersible, it would have been impossible for humans to reach such a deep sea.
So how do deep-sea fish come under such enormous pressure? Is it because they have a good mindset?
Anti-stress, starting from the "break away" of the maw
When you swim, you may have the experience that when you dive into the bottom of the pool, you will feel a feeling of pressure on your eardrums, or even a slight pain. This is because the water pressure on the outside of the eardrum is significantly greater than the air pressure on the inside, causing the eardrum to be subjected to an inward pressure.
From this we can conclude that as the water depth increases, the water pressure will be much greater than the atmospheric pressure, causing the surrounding water to begin to squeeze the inflated object inward.
Image credit: Pexels
Most bony fish are an inflated object in some sense because they have an inflated swim bladder in their body. For bony fish living in shallow waters, the swim bladder is a very important structure that helps fish adjust buoyancy to float or dive.
But for deep-sea fish, the inflated swim bladder is like a fragile balloon, and the huge external water pressure will squeeze and ravage the balloon without reservation until it explodes into pieces.
As a result, many deep-sea fish have evolved to "abandon" the "dangerous" structure of the swim bladder and instead rely on certain lipids to provide buoyancy.
Compared with fish in shallow seas, deep-sea fish have less bone and muscle content, while there are relatively more lipids and gelatinous substances. In addition, the proportion of cartilage in the bones of deep-sea fish is also much higher than that of shallow fish.
For deep-sea fish, this is all a necessary "compromise" to adapt to life in the deep sea. The so-called "too rigid is easy to fold", compared to bones and muscles, lipids and gums can better help fish cope with great stress.
Another benefit of this body structure is that a lower proportion of bones and muscles reduces the energy expenditure of deep-sea fish, while a higher proportion of lipids can also store more energy, which is crucial for fish in the deep sea with poor nutrients and thin oxygen.
A few years ago, the "water drop fish" that was rated as the world's ugliest creature, the soft cryptocephalic dufar, is a good example.
The drip fish that is caught ashore is often a pool of pink objects lying on its stomach, living like a slime with a big nose.
However, in the deep sea, the shape of the dripfish is no different from that of ordinary fish, but in the process of being caught ashore, due to the rapid reduction of pressure, their body structure is destroyed, which has become what we see. And where they live, it is the gelatin of this body that helps them survive.
Drop fish (Image source: Wikipedia)
Previous studies have found mutations in the genome of the Mariana lionfish that regulate bone development and ossification of bone tissue.
This mutation causes the calcification process of the Mariana lionfish skeleton to terminate prematurely, resulting in cartilage in most of its bone composition. The resistance of cartilage to high pressure is much stronger than that of hard bone tissue.
However, this is not the full skill of deep-sea fish.
Molecular mechanisms of special phenotypes of lionfish in the Mariana deep sea (Image source: Reference 1)
Powerful anti-stress ability to penetrate deep into cell membranes
You know, still water pressure is not a macroscopic object, it is not like a hand that squeezes the deep-sea fish, it will only affect the deep-sea fish from the macroscopic body structure. Hydrostatic pressure is pervasive, and both macroscopic and microscopic structures are attacked by it.
When we focus our attention on the microscopic world, we will find that in a high-pressure environment, the fluidity of cell membranes will decrease. Simply put, the cell membrane of a cell becomes "hard" in the deep ocean, which is not a good thing.
Cell membranes are an important gateway to control the entry and exit of substances into and out of cells, and hardening of the membrane can make it more difficult for substances to enter and exit the cell. Extracellular nutrients cannot enter the cell, and the waste products generated within the cell are difficult to transport out of the cell, and the organism will not survive.
It's like a delivery man trying to deliver food through a crowded intersection: originally he just had to squeeze through the cracks, but as a result, a mysterious force pushed everyone together, making everyone crowded, and the delivery man tried his best to squeeze through, then he would feel so stressed.
Scientists have found that deep-sea fish have more unsaturated fatty acids on their membranes than shallow fish, which allows their cell membranes to maintain a high level of fluidity in high-pressure environments, improving the efficiency of material transportation.
For example, vegetable oil has a higher content of unsaturated fatty acids than animal oil, so vegetable oil is generally liquid at room temperature, while animal oil is mostly solid. It's hard to get a coin to penetrate a piece of butter, but it's easy to get it to fall from the surface of a bottle of peanut oil to the bottom of the bottle.
A high proportion of unsaturated fatty acids can make deep-sea fish still have a "soft" cell membrane even in a high-pressure environment, but if a deep-sea fish is caught ashore, its cell structure will also be destroyed, because when it is in a low-pressure environment, the fluidity of the cell membrane is too strong, and the cell membrane is too "soft", resulting in cells that are easily damaged.
Analysis of 9 bony fish gene families found significant amplification of gene families associated with fatty acid metabolism in MHS (Image source: Reference 1)
Lipids aren't the only substances affected by high pressure, and proteins can't escape this ubiquitous pressure. Normally, proteins affected by high pressure undergo structural changes and loss of function, and the normal functioning of proteins is essential for the survival of organisms.
Fortunately, deep-sea fish also have corresponding coping strategies for this. Amino acids at certain protein-specific sites in deep-sea fish are replaced by other amino acids, improving their resistance to stress. For example, α actin in deep-sea fish has been replaced by amino acids at multiple sites, including the binding sites of calcium ions and ATP. Amino acid replacement at these two sites ensures that actin still works properly in high-pressure environments.
In addition, the number and variety of chemical bonds in some proteins vary. This change leads to a change in the tertiary structure of the protein, thereby strengthening the rigidity of the protein structure and improving its adaptability to high-pressure environments. Just like when you build a building block, you put two extra tapes on the outside of the blocks, which is definitely much more stable than not putting on the tape.
It has also been found that the content of trimethylamine oxide (TMAO) in deep-sea fish is much higher than that in shallow fish. Trimethylamine oxide is a very important protein stabilizer that helps denatured proteins regain their original structure and thus their normal function.
The large amount of trimethylamine oxide in deep-sea fish can help the proteins in their cells maintain their original structure and function, thereby ensuring the activity of the cells.
Interestingly, as fish die, oxidized trimethylamine gradually breaks down into trimethylamine, which is an important source of fishy smell in the sea. That is to say, the more deep-sea fish, the heavier the fishy smell after death, and friends inland always feel that the fishy smell of fish is not difficult to understand.
These changes in gene coding and regulatory sequences may help MHS increase intracellular TMAO levels to enhance protein stability (Image source: Reference 1)
Life is full of surprises and challenges. If you also want to give up for a moment, you can think of deep-sea fish - they face so much pressure but do not give up, but from the protein level to change themselves, adapt to the environment, become the master of the environment, what reason do we have to give up, hurry up and change ourselves, overcome the pressure!
Resources:
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5、Winnikoff J R , Wilson T M , Thuesen E V , et al. Enzymes feel the squeeze: Biochemical adaptation to pressure in the deep sea[J]. Biochemist, 2017, 39(6):26-29.
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