Roughly 3 billion years ago, a single-celled photosynthetic bacterium began burping a new chemical that was poisonous to nearly every species on Earth. Over the following hundreds of millions of years, more microorganisms began producing this toxic gas, first saturating Earth's oceans and eventually...
Nice article, but that headline is so misleading, I'm not inclined to upvote it.
tl;dr: Microorganisms used to live without oxygen. Some organisms started sprewing oxygen. Oxygen killed off most of these oxygen-free microorganisms. Life evolved and adapted. There's this lake in which the bottom layer is oxygen-less and oxygen-free microorganisms exist there. Scientists are doing a bunch of studies of this ecosystem.
I wouldn't call those organisms "alien." They were first in this planet!
Bacteria are always single-celled, and quite frankly, I don't know why the ones in question are such a big deal, there are plenty of prokaryotes that thrive under anaerobic conditions.
Having seen enough exceptions in biology, I wouldn’t be surprised if someone found a multicellular bacterial species that violates everything we know about bacteria. Biology is completely wild, and it’s really hard to come up with a rule or a category that always works and nobody has any problems with it.