Some truths about the Universe seem immutable. The sky is up there. Gravity crushes us. Nothing can travel faster than light. Multicellular life needs oxygen to live. Well, maybe this last thing isn't.
Scientists have just discovered that a jellyfish-like parasite doesn't have a mitochondrial genome - it's the first multicellular organism known to have this absence. This means that he is not breathing. He can live his life without oxygen.
This discovery is not only changing our understanding of how life can work here on Earth, it could also have implications for the search for extraterrestrial life.
Life and oxygen: a long love story
Life has begun to develop the ability to metabolize oxygen (i.e. breathe) more than 1,45 billion years ago. A large archaea engulfed a smaller bacterium, and eventually the two remained together.
That symbiotic relationship led the two organisms to evolve together, and eventually those bacteria that settled inside became organelles called mitochondria.
Every cell in our body except red blood cells has a large number of mitochondria, and these are essential for the breathing process. They break down oxygen to produce a molecule called adenosine triphosphate, which multicellular organisms use to power cellular processes.
We know that there are adaptations that allow some organisms to thrive in low oxygen or hypoxic conditions. Some single-celled organisms have developed mitochondria-related organelles for anaerobic metabolism; but the possibility of exclusively anaerobic multicellular organisms has been the subject of scientific debate.
At least as long as a team of researchers led by Dayana Yahalomi Tel Aviv University in Israel decided to take another look at a common salmon parasite called Henneguya salminicola.
An "Animal" that does not breathe
It is a cnidarian, belonging to the same phylum as corals, jellyfish and anemones.
Although the cysts they create in salmon meat are unsightly, the parasites are not harmful and will live with the salmon for its entire life cycle.
Hidden in its host, the little cnidarian can survive in fairly hypoxic conditions. But knowing how it does this is difficult without looking at the creature's DNA. And this is exactly what the researchers did.
The team used deep sequencing and fluorescence microscopy to conduct an in-depth study of the Henneguya salminicola and they found that he lost the mitochondrial genome. In other words, he lost his aerobic respiration capacity and almost all the nuclear genes involved in the transcription and replication of mitochondria.
These results show an incontrovertible reality: a multicellular organism has been discovered that does not need oxygen to survive.
How does he live without oxygen?
Here, this is still a mystery. It may "suck" adenosine triphosphate directly from its host, but this has not yet been determined.
But the mutation of this organism is quite consistent with a general trend in these creatures, that of genetic simplification.
For example, Henneguya salminicola it has lost much of the original jellyfish genome, but retains (strangely) a complex structure reminiscent of jellyfish stinging cells. It does not use it to sting, however, but to cling to its host: an evolutionary adaptation from the needs of the living jellyfish to those of the parasite. You can also see them in the image above: they are the things that look like eyes.
The discovery could help fisheries adapt their strategies to deal with the parasite. Although it is harmless to humans (there are quite other parasites) no one wants to buy a salmon full of weird little jellyfish.
But it is also a discovery that helps us understand how life works.
"Our discovery confirms that adaptation to an anaerobic environment is not unique to single cell eukaryotes, but has also evolved into a multicellular parasitic animal", the researchers wrote in their paper.
"H. salminicola offers an opportunity to understand the evolutionary transition from an anaerobic to aerobic metabolism ”.