For the new study, a team led by evolutionary biologist Anna Karnkowska, a postdoc, and her adviser, Vladimir Hampl, of Charles University in Prague, checked another candidate, a species in the genus Monocercomonoides. The single-celled organism came from the guts of a chinchilla that belonged to one of the lab members. The team decided to test it because it belonged to a group of microbes that scientists posited had lost their mitochondria.

When the researchers sequenced Monocercomonoides’s genome, they found no signs of mitochondrial genes (the organelles carry their own DNA). Digging deeper, they determined that it lacks all of the key proteins that enable mitochondria to function. “The definition of eukaryotic cells is that they have mitochondria,” says Karnkowska, who is now at the University of British Columbia, Vancouver, in Canada. “We overturn this definition.”

Monocercomonoides may not need mitochondria because of where it lives—in the intestines of chinchilla hosts, which it doesn’t appear to harm. Nutrients are abundant there, but oxygen, which mitochondria require to produce energy, is scarce. Instead of relying on mitochondria, the organism likely uses enzymes in its cytoplasm to break down food and furnish energy, the authors suggest. But energy production is not the only problem that Monocercomonoidessolved. Mitochondria provide another cellular service: synthesizing clusters of iron and sulfur that are essential helpers for a variety of proteins. It turns out that Monocercomonoides has come up with a workaround by borrowing some bacterial genes that perform the same function, the scientists reveal online today in Current Biology.

Source: First eukaryotes found without a normal cellular power supply