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 its atmosphere. Up here, this chemical changed the composition of pre-existing gases so drastically that it caused a global ice age. And the name of this powerful, poisonous, world-changing gas? Oxyg
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.