For a long time it has been thought that all eukaryotes – organisms in which the DNA is enclosed in a membrane including almost all life we can see – had to contain mitochondria. Known as the “power house” of the cell, these little subunits within the cell provide the organisms with energy and were thus thought to be essential. But it now seems that actually they might not be that essential, as researchers describe the first eukaryote known that lacks mitochondria.
Other eukaryotes had been previous contenders for the title, such as the microbe Giardia intestinalis from the gut, but further investigations had found that they simply contained highly reduced mitochondria difficult to observe. But after checking another candidate from the genus Monocercomonoides, isolated from a sample retrieved from the gut of a chinchilla owned by one of the lab members, the researchers of this latest study found no trace of the organelles at all after a genetic analysis looking for mitochondrial genes.
“In low-oxygen environments, eukaryotes often possess a reduced form of the mitochondrion, but it was believed that some of the mitochondrial functions are so essential that these organelles are indispensable for their life,” explains Anna Karnkowska, co-author of the study describing the new finding published in Current Biology. “We have characterized a eukaryotic microbe which indeed possesses no mitochondrion at all.”
Because the gut is such a low oxygen environment, many microbes that call it home already have reduced mitochondria. Coupled with the fact that they are living in an environment surrounded by nutrients, the researchers think that the Monocercomonoides merely has no need for the organelles, and so have lost them. They think instead that the microbes simply absorb the nutrients directly from their surroundings, and then break them down with enzymes in order to get energy.
Yet they also face another problem, because mitochondria do another task as well as provide the cell with energy: They also give the cell vital clusters of iron and sulfur that are needed by various proteins. To solve this, it seems like the Monocercomonoides microbe has “borrowed” genes from bacteria to give them a “cytosolic sulfur mobilization system,” which fulfils the mechanism usually found in the mitochondria.
While all the evidence seems to stack up, others suggest that the researchers will need to double check. Either way, it’s looking more and more likely that textbooks may have to be rewritten, as the researchers suspect that there are probably other microbes that also lack the organelles. “This amazing organism is a striking example of a cell which refuses to adhere to the standard cell biology textbook, and we believe there may be many more similar examples in the so far hidden diversity in the world of microbial eukaryotes, the protists,” says Karnkowska.