It seems that it would still be surprising that such a parasitical organism could exist in such an environment. Presumably, if an organism is breathing in a ne

It seems that it would still be surprising that such a parasitical organism could exist in such an environment. Presumably, if an organism is breathing in a near-zero oxygen environment, they are probably just getting the oxygen they need. If there's enough oxygen to support a parasite stealing from the first, there was enough oxygen to support the parasite getting its own oxygen. It seems to me much more likely that we'll find these parasites where there is plenty of oxygen: These organisms didn't evolve as a way of dealing with having no oxygen. They evolved because they adapted so that they did need to bother with mitochondria, because there was a plentiful supply of ATP they could steal. Since they didn't need to burn oxygen, they could do without. And the environment they are in has so much oxygen that the host organism doesn't suffer.