Mitochondria, organelles central to cellular metabolism and energy production, are typically a hallmark of the eukaryotic cells that make up plants, animals, fungi, and many other organisms. Now scientists report the first known example of a eukaryotic organism that lacks any semblance of the organelle: a single-celled, gut-dwelling microbe in the genus Monocercomonoides.

Related to the intestinal parasite Giardia, Monocercomonoides belongs to a larger group of organisms that live in low-oxygen environments. Many of its members possess reduced forms of mitochondria, but the Monocercomonoides microbe appears to be missing this organelle altogether.

“Most of us like to think of cells as following textbook definitions, but this is a nice, surprising example of how diverse eukaryotic cells can be, and just how many weird cells are out there,” says study co-author Anna Karnkowska, an evolutionary biologist who worked on the project as a postdoc at Charles University in Prague. The findings are detailed in a study published today in Current Biology.

After early experiments failed to turn up obvious signs of mitochondria, Karnkowska and her colleagues decided to sequence the genome of the microbe, looking for DNA related to a range of known mitochondrial proteins in other organisms. They found no such sequences, nor did they find DNA that coded for telltale signals that help import proteins into the mitochondrial compartment from other parts of the cell.

“That’s what really persuaded me,” says Thomas Richards, an evolutionary biologist at the University of Exeter in England, who was not involved in the study. The transfer system is usually one of the best ways to find evidence of a mitochondrial organelle.”

Michael Gray, a biochemist at Dalhousie University in Halifax, Nova Scotia likewise finds the paper’s assertions convincing. “They’d taken a lot of trouble to try to find evidence of mitochondrial proteins using several different approaches,” he says, “so I think their conclusion is pretty robust.”

The authors propose that the Monocercomonoides microbe survived the loss of mitochondria by first acquiring, over the course of its evolution, biochemical machinery from bacteria. Specifically, the team discovered that the organism had gained an alternative system to help build iron-sulfur clusters—one of the core functions of mitochondria in eukaryotes. These complexes work together with certain metabolic enzymes to carry out essential biochemical reactions.

Because mitochondria-like organelles have previously been reported in a closely related organism the researchers suggest that the Monocercomonoides microbe shed its mitochondria some time after the two microbes branched off from a common ancestor.

The paper’s findings challenge conventional thinking about just how essential mitochondria are for eukaryotic cells, says Richards. “We’d been working under the unwritten rule that every eukaryote had mitochondria of some form, even if highly reduced,” he says, “but this paper suggests that that idea is flexible.” Karnkowska, now a visiting researcher at the University of British Columbia in Vancouver, Canada, says the findings raise the possibility of other eukaryotes without mitochondria. Senior author Vladimír Hampl, an evolutionary biologist at Charles University, is leading the team’s search for additional examples among close relatives of Monocercomonoides.