Breathing and mitochondria
Macroscopically speaking, breathing represents the process of oxygen entering the body. However, in the micro-cell world, breathing usually represents the process of energy production, including anaerobic breathing and aerobic breathing. The former usually appears in yeast and some anaerobic bacteria. In the case of muscle cell hypoxia, the human body can also produce lactic acid through anaerobic respiration. But the efficiency of energy release in this process is not too high.
Aerobic respiration is indispensable for multicellular organisms. Compared with anaerobic respiration, it is more efficient and releases more energy. Cells usually use glucose as energy, and oxygen is more like an intermediary to assist aerobic respiration. The last step of aerobic respiration requires oxygen to receive electrons and release ATP produced by metabolism. Without oxygen, excess electrons will gradually accumulate in cells, leading to the stagnation of electron transfer chain and the abnormality of ATP. This is fatal to multicellular life.
Mitochondria are organelles that complete this precise procedure and ensure the normal operation of complex cell life. Mitochondria still retain most of the DNA related to aerobic respiration, which exists independently of the nuclear genome. The scientific community speculates that this unique cell is also a kind of bacteria, and after being swallowed by eukaryotes, the two have reached a state of mutual benefit. Bacteria gradually evolved into mitochondria as seen today, which are responsible for helping to convert food and oxygen into energy that cells can use.
Mitochondria are indeed indispensable partners for multicellular life that depends on aerobic respiration. However, living in an anoxic environment, their demand for mitochondria does not seem so great. It is speculated that the mitochondria in these organisms will gradually transform into mitochondrial-related organelles (MRO), gradually lose mitochondrial genes, and finally completely eliminate the mitochondria in the body.
Looking for creatures without mitochondria
A study published in Current Biology first discovered eukaryotes without mitochondria. At that time, a cooperative team from the Czech Republic and Canada isolated a microorganism from animals, but the results of gene sequencing showed that this microorganism did not have a gene encoding mitochondrial protein.
This species, called Monocercomonoides, lived in a low-oxygen environment for a long time, and their ancestors on the evolutionary tree themselves contained mitochondria, which indicated that they lost mitochondria during evolution. Instead, it obtained another cytoplasmic sulfur mobilization system from bacteria to replace mitochondria.
Before this research, many scientists were looking for eukaryotic organisms without mitochondria, including plants, animals, fungi and protozoa. At that time, Karnkowska, the author of the study, thought that there were many protozoa without mitochondria, just like Trichomonas unicornis. In his view, the vitality of eukaryotes is much stronger than we thought.
Karnkowska's idea is correct, because the research recently published in the Proceedings of the National Academy of Sciences even surpassed his prediction at that time, and found an animal without mitochondria for the first time. Of course, this animal is not the huge individual we usually see, but a parasite that lives on fish. It looks strange, similar to tadpoles, and has alien-like "eyes" on its head.
When the researchers examined the organisms of various species under the parasitic Myxosporidia, they unexpectedly found that a species called Henneguya salminicola had no mitochondrial genome. Because their corresponding mitochondrial genes were not found in gene sequencing, this means that they have no mitochondria at all.
In terms of species relationship, Myxomycetes are distant relatives of jellyfish, both of which belong to Echinotheca (also known as coelenterata). Myxomycete salmon is still a multicellular organism, although it has lost nerve cells and muscle cells in the long evolution process. For example, two stinging cells look like alien eyes, which can help slime mold salmon to firmly adsorb when they are close to the host.
Evolutionary decision
As for why myxomycetes finally became mitochondria-free, the study speculated that this should be related to their living environment. Its own biological behavior is actually the same as that of parasites. It usually roams freely in the water, and its main life cycle is completed in two hosts respectively. One is fish, especially salmon; The other is annelid worm.
In the parasitic environment, oxygen has become a luxury. So in the long process of evolution, it gradually gave up the mitochondria that needed oxygen to provide energy. This is actually the inevitable result of evolution. When mitochondria are useless, the body not only needs to consume extra energy to make it and maintain its operation, but also gets no benefit, which is extremely uneconomical for living things, so gradually getting rid of this burden is the best choice.
From the perspective of evolution, life tends to become more complex and diverse. However, the slime mold of salmon also shows us that not all life thinks so. In the eyes of these animals, complexity is more like a burden, and there is no need for complexity and diversity in their parasitic tasks. Nerves and muscles are redundant, and mitochondria become unnecessary. In this way, slime molds from salmon gradually become like single-celled organisms. In this regard, the newspaper reporter Dorothée Huchon was also very surprised. He said, "Myxomycetes have lost almost everything, and now they don't even need to breathe."
In fact, in this new study, the author also detected other myxomycetes. As a close relative of Myxomycete Salmon, this Myxomycete has mitochondrial genes. At present, it is not clear why myxomycetes are so unique.
How does Myxomycetes salmon survive without mitochondria? Huchon speculates that they may have an undiscovered system and ability to obtain ATP directly from the host to meet their energy needs. But how they skillfully completed this process, there are more unknowns waiting for scientists to discover. As Karnkowska said, the vitality of eukaryotes is much stronger than we thought.