Some truths about the Universe and our experience in it seem to be unchanging. The sky is high. Gravity is bad. Nothing can travel faster than light. Most cellular life needs oxygen to survive. Except we might need to rethink that last one.
In 2020, scientists discovered a jellyfish-like parasite that lacks a mitochondrial genome – the first multicellular organism ever found to be absent. That means it doesn’t breathe; in fact, it lives its life independent of oxygen.
This discovery not only changes our understanding of how life might work here on Earth – it may also have implications in the search for extraterrestrial life.
Life began to develop the ability to convert oxygen – that is, to breathe – sometime over 1.45 billion years ago. The big archaeon swallowed the small bacteria, and somehow the new bacterial home was mutually beneficial, and the two stayed together.
That symbiotic relationship caused the two organisms to evolve together, and eventually the bacteria housed inside became organelles called mitochondria. Every cell in your body except red blood cells has a large number of mitochondria, and these are essential for the process of respiration.
They break down oxygen to produce a molecule called adenosine triphosphate, which many cellular organisms use to power cellular processes.
We know 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 only multicellular organisms has been the subject of scientific debate.
That was, until a team of researchers led by Dayana Yahalomi of Israel’s Tel Aviv University decided to look again at a common salmon parasite called Henneguya salminicola.
(Stephen Douglas Atkinson)
It is a cnidarian fish, belonging to the same phylum as corals, jellyfish, and anemones. Although the tumors it forms on the flesh of the fish are unsightly, the parasite is harmless, and will live with the salmon for its entire life cycle.
While hidden inside its host, the small cnidarian can tolerate completely hypoxic conditions. But exactly how it does it is difficult to know without looking at the organism’s DNA – so that’s what the researchers did.
They used detailed sequencing and fluorescence microscopy to make a close examination of the H. salminicola, and found that it had lost the mitochondrial genome. In addition, it also lost the capacity for aerobic respiration, and almost all the nuclear genes involved in the transcription and replication of mitochondria.
Like single-celled organisms, it had organelles related to mitochondria, but these are also unusual – they have folds in the inner membrane that are not normally seen.
Similar taxonomic and microscopic techniques in closely related cnidarian fish parasites, Myxobolus squamalisserved as a control, and clearly expressed the mitochondrial genome.
These results showed that here, at last, was a multicellular organism that did not need oxygen to survive.
Time H. salminicola still mysterious, the loss is entirely consistent with the general trend of these creatures – one of simplifying nature. Over many, many years, they essentially diverged from the free-living ancestor of jellyfish to the simpler parasites we see today.
(Stephen Douglas Atkinson)
They have lost a large part of the original jellyfish genome, but it has remained – oddly enough – a complex structure that resembles the stinging cells of the jelly. They do not use these to bite, but to cling to their hosts: an evolutionary change from the needs of free-living jellyfish to parasitic ones. You can see them in the picture above – they are eye-like things.
The discovery could help fishermen adjust their strategies to deal with parasites; even though it is harmless to humans, no one wants to buy salmon that is full of weird little jellyfish.
But it is also an amazing invention for helping us understand how life works.
“Our findings confirm that adaptation to anaerobic environments is not unique to single-celled eukaryotes, but has also evolved in multicellular, parasitic animals,” the researchers explained in their paper, published in February 2020.
“So, H. salminicola provides an opportunity to understand the evolutionary transition from aerobic to unique anaerobic metabolism.”
The study was published in PNAS.
An earlier version of this article was published in February 2020.
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