Some truths about the universe and our experience of it seem immutable. The sky is above. Gravity sucks. Nothing can travel faster than light. Multicellular life needs oxygen to live. Except we might have 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 lack one. That means he’s not breathing; in fact, it lives its life completely independent of oxygen.
The discovery doesn’t just change our understanding of how life might work here on Earth—it may also have implications for the search for extraterrestrial life.
Life began to develop the ability to metabolize oxygen—that is, to breathe—sometime 1.45 billion years ago. The larger archaeon engulfed the smaller bacterium and the bacterium’s new home was mutually beneficial and the two stayed together.
This symbiotic relationship led to the two organisms co-evolving, and eventually these bacteria settled inside became organelles called mitochondria. Every cell in your body except red blood cells has a large number of mitochondria, which are necessary for the process of respiration.
They break down oxygen to form 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 unicellular organisms have evolved mitochondria-related organelles for anaerobic metabolism; but the possibility of exclusively anaerobic multicellular organisms has been the subject of some scientific debate.
That was until a team of researchers led by Dayana Yahalomi from Tel Aviv University in Israel decided to take another look at a common salmon parasite called Henneguya salminicola.
(Stephen Douglas Atkinson)
It is a cnidarian, belonging to the same phylum as corals, jellyfish and anemones. Although the cysts it creates in the flesh of the fish are unsightly, the parasites are not harmful and will live with the salmon throughout its life cycle.
A small cnidarian, hidden inside its host, can survive quite hypoxic conditions. But how exactly it does it is hard to know without looking at the creature’s DNA — so the scientists did.
They used deep sequencing and fluorescence microscopy for the detailed study H. salminicolaand discovered that he had lost his mitochondrial genome. In addition, it also lost the ability for aerobic respiration and almost all nuclear genes involved in mitochondrial transcription and replication.
Like single-celled organisms, it has evolved organelles related to mitochondria, but these too are unusual—they have folds in the inner membrane that are not usually seen.
The same sequencing and microscopic methods in a closely related cnidarian fish parasite, Myxobolus squamaliswas used as a control and clearly showed a mitochondrial genome.
These results showed that there was finally a multicellular organism that did not need oxygen to survive.
While H. salminicola is still something of a mystery, the loss is quite consistent with the overall trend in these creatures – that of genetic simplification. Over many, many years, they basically morphed from a free-living ancestor of the jellyfish into the much simpler parasite we see today.
(Stephen Douglas Atkinson)
They have lost most of the original jellyfish genome, but have retained – strangely – a complex structure resembling the stinging cells of a jellyfish. They do not use them to sting, but to cling to their hosts: an evolutionary adaptation from the needs of free-living jellyfish to the needs of parasites. You can see them in the picture above – they are the things that look like eyes.
The discovery could help fishermen adapt their strategies for dealing with the parasite; although it is harmless to humans, no one wants to buy salmon riddled with tiny, strange jellyfish.
But it’s also one heck of a discovery that helps us understand how life works.
“Our discovery confirms that adaptation to an anaerobic environment is not unique to unicellular eukaryotes, but has also evolved in a multicellular parasitic animal,” the researchers explained in their paper published in February 2020.
“Therefore, H. salminicola provides an opportunity to understand the evolutionary transition from aerobic to exclusively anaerobic metabolism.”
The research was published in PNAS.
An earlier version of this article was published in February 2020.