What is the fundamental unit of life?
Any primary school student will tell us that it is the cell.
Generally, we consider cells as the basic building blocks of life just as atoms are the building blocks of matter.
But cells, we soon learn in high school, have a lot of components inside them. Called cell organelles, the most important of them in almost all life forms – whether plant or animal – are the mitochondria.
Cells breathe because of mitochondria. They are the power-centres of the cell. They take oxygen and release energy in the form of a molecule called ATP (adenosine tri phosphate).
ATP are called the biological energy coins. If an organism breathes oxygen, then at the cellular level mitochondria are at work. In other words, wherever there is an oxygen-breathing life form, there is mitochondria.
Now, an animal, an evolutionary relative of the jelly fish, a parasite living in the muscles of the salmon fish, causing what is called the tapioca disease, was being studied by Dorothee Huchon, a zoologist from the Tel Aviv University, and a team of scientists.
The animal called ‘Henneguya salminocola’ with just 10 cells surprised the scientists who were sequencing its genome.
Its mitochondrial genes were missing, but not the mitochondria themselves.
The cells of Henneguya have organelles which are very similar to the mitochondria. These mitochondria-related-organelles simply lack the genome needed to produce the enzymes which are a must for cellular respiration.
Living inside a salmon, it is already living in an oxygen-deficient environment. So how does it produce energy it needs to survive?
According to Huchon, the parasite at the cellular level takes the ATP from the host. However, the picture is still not clear.
Not the first time
Now organisms living in no-oxygen conditions are quite common. But mostly they are primitive organisms called prokaryotes – organisms which have not developed cell membranes.
But eukaryotes, organisms which have developed cell membranes always have mitochondria.
A decade ago, another multicellular organism had been discovered living in conditions which scientists consider as having no oxygen.
Three species belonging to a phylum Loricifera were discovered living in the completely dark and oxygen-less environment of the Mediterranean Ocean floor.
There are debates as to whether these animals can live without oxygen completely. But what we know for sure is that these organisms have cell organelles called hydrogenosemes.
These are organelles which can act like mitochondria producing energy without oxygen.
Usually, organisms which contain hydrogenosemes can switch their energy production to these organelles when they encounter environments where oxygen is meagre. But here in these three species of Loricifera, only hydrogenosemes have been found and mitochondria are absent.
It should be remembered that the hydrogenosemes are mitochondria adapted for anaerobic energy production (in absnece of oxygen).
In 2016, a microscopic parasite in the guts of rodents in South Africa, ‘Monocercomonoides’, became the first organism to be discovered which had evolved cell membranes but no mitochondria.
In 2019, a team of international scientists along with Uwe John of Alfred-Wegener-Institut, Germany, reported an interesting eukaryotic organism: a marine plankton Amoebophyra ceratii, that breathed oxygen. But it lacked mitochondrial DNA.
The plankton belongs to a branch of organism whose nuclear genome can be as many as 100 times (from 10 times) larger than the human genome. What has happened in this particular case is that the mitochondrial DNA necessary for aerobic respiration has been transferred to, and integrated with, the DNA material in the nucleus of the cell.
So here we have a transfer of genetic material from within a cell organelle to the cell nucleus.
Now, the current discovery of multi-cellular organism without mitochondrial genome has to be then seen in this background of an astonishing variety of adaptations in the way mitochondria and cell as a whole relate to themselves.
The discoveries show that even at the sub-cellular level of life, there is an astonishing diversity – a testimony to the vibrancy of evolution and the way life can evolve quite a variety of pathways for respiration – something we take for granted.
“There is grandeur in this view of life …”, said the sage of biology and he also suggested that there could be processes other than natural selection at work.
There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.Charles Darwin
Only what Darwin called ‘the war of nature’ one should say becomes ‘the dance of nature’ as our canvas expands and our vision deepens.
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