Three billion years ago, things on Earth were different. For one thing, there wasn't all this oxygen all over the place — the first cyanobacteria had to figure out a way to live on volcanic carbon dioxide, water and sunlight. These old-timey organisms lived anaerobically, without oxygen. Strangely enough, what we're breathing today is the type of atmosphere they made possible for us, because the byproduct of their food production — oxygen — eventually overtook Earth's atmosphere. Anaerobic organisms were forced into oxygenless nooks and crannies of the planet and kept simple and single-celled.
It's an Animal, But Without a Mitochondria
However, when there is a rule, there is usually an exception, and a group of scientists have discovered a small, parasitic cnidarian — a relative of a jellyfish — that apparently doesn't use oxygen to respire. They published their findings in the Feb. 24, 2020 issue of the Proceedings of the National Academy of Sciences.
This animal, Henneguya salminicola, is a tiny alien-headed parasite with a long tail that feeds on the muscle tissue of salmon and other fish. It's a eukaryote — a member of a broad group of organisms that includes most of the living things you can see with your naked eye: animals, plants, fungi, etc. The cells of eukaryotes contain all sorts of fancy organelles that their more primitive counterparts, the prokaryotes, don't have. One of those organelles is the mitochondria, a structure that has a tiny genome all its own, separate from the rest of the organism, and which eukaryotic cells use to produce energy, with the help of oxygen.
But within the larger groups we call "eukaryotes," there are a few single-celled, non-animal species that are anaerobic. They don't have mitochondria, but have something scientists call "mitochondrion-related organelles." Henneguya salminicola is the first animal to have this feature.
It's all very strange, but how did they get this way?
Evolution of an Anomaly
"The ancestors of Henneguya almost certainly had mitochondria," says study co-author Stephen Atkinson, a research professor in the Department of Microbiology at Oregon State University, in an email interview. "All its closest relatives have mitochondria, so the evolution to an anaerobic lifestyle and loss of functional mitochondria appears to be a recent adaptation of that species alone — at least that we know of so far!"
In the cells of typical animals, the mitochondria use oxygen in a multistep process to create chemical energy. The research team found that the parasite has just had to adapt to an environment with very little available oxygen. Without the need for mitochondria, it lost the genetic instructions for at least several parts of the processes that use oxygen — for instance, Henneguya salminicola has lost its mitochondrial genome, which other animal cells need because they contain the instructions for using oxygen. By losing the genome, the parasite saves energy by not having to copy genes for things it no longer needs.
But how can it survive without oxygen in the first place?
"We presume that it must instead absorb molecules related to energy production from the host cells, which have already done part of the processing," says Atkinson. "Stealing something from the host is fundamental to parasitism!"
What Does it Mean to Be an Animal?
Like many important discoveries, this finding was totally unexpected — the researchers were hoping to compare the genomes of two small parasites, and each time they tried to run that of Henneguya salminicola, something was obviously very weird. Looking into it further, they found its cells contained a little empty bag where a mitochondria might once have been.
"This discovery has expanded our understanding of what it means to be an 'animal' by showing that even complex life can evolve in a way to succeed in environments without oxygen," says Atkinson. "Knowing that anaerobic animals can exist alerts us to the fact that we must be on the lookout for this in other species — and maybe look in anaerobic environments for animals where we never would have looked before. Specifically for myxozoan parasite research, it means we will look for unusual or missing mitochondria in other species from now on, to try and discover the connections between hosts, tissue and environment that lead to loss of mitochondrial function to take advantage of anaerobic metabolism."
The discovery of anaerobic mechanisms in these parasites could also open a new avenue for treatment, as specific drugs have been used to target other anaerobic parasites.