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The following is a transcript from Scientific American.

If you were searching for a life form unlike anything known to science, where would you look? Scientists at Charles University, in Prague, looked in the poop of a pet chinchilla. "They were isolating a lot of various strains from various environments—including their own pets." Anna Karnkowska, an evolutionary biologist who collaborated on the work, who is now at the University of British Columbia. "Scientists like to isolate things from everywhere possible."

They found a relative of the parasite Giardia, called Monocercomonoides. It’s eukaryotic, meaning it has organelles and a nucleus enveloping its DNA. Just like our cells, or the ones in plants, or fungi. But unlike bacteria, which don’t have those things.

And upon closer examination, the new critter was different from every other eukaryote known to science. Because it’s missing a key organelle—it seems to have no mitochondria.

You probably learned in biology class that mitochondria, let’s all say it together, are the powerhouses of the cell. They charge up energy-rich molecules, when oxygen's around. But they do other stuff too: like manufacture certain essential proteins. This newly discovered microbe, and others that live in low-oxygen environments, use different, oxygen-free pathways to make energy. But they usually still have mitochondria for that protein assembly job. The new bug apparently pulls off the protein-synthesis by using a system that it picked up from bacteria, in what’s known as a lateral gene transfer. The findings are in the journal Current Biology.

Karnkowska says the microbe's ancestors probably had mitochondria at some point—then lost them once they’d acquired the bacterial system. And she says other microbes might share a similar story. "We know now that probably there are more of these weird things." And as this study suggests, they could be found in some weird places.


Do the newly discovered microbes have mitochondria?

Judging from the whole transcript, it seems to me that the answer is NO. But the sentence "they usually still have mitochondria for that protein assembly job" in the fourth paragraph looks rather confusing for me. What should "they" refer to?

If "they" refers to "this newly discovered microbe" and "others that live in low-oxygen environments", then it contradicts to the overall tone that the newly discovered microbe does not have mitochondria.

If "they" refers only to "others that live in low-oxygen environments", it does not make sense to me: when one describes some properties/facts about A and B and then uses the pronoun "they", how could it refer to B?

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You need to parse this carefully. "They" in the fourth paragraph refers to microbes that live in low oxygen environments. Although they don't use mitochondria for generating energy these low oxygen microbes use mitochondria for other purposes. In that respect the newly found microbe is different. It too lives in a low oxygen environment and uses other methods for generating energy, however, in this specific case it does not have mitochondria for the other purposes it serves.

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