The P-T extinction (a.k.a. the Great Dying) tends to be considered the worst - for example, Wikipedia states:
It is the Earth's most severe known extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct. It is the only known mass extinction of insects. Some 57% of all families and 83% of all genera became extinct. Because so much biodiversity was lost, the recovery of life on Earth took significantly longer than after any other extinction event, possibly up to 10 million years.
Presumably the "96% of all marine species" (which references two books, neither of which I have) is in terms of multicellular marine species or some similar subcategory that's easier to observe in the fossil record (I could be wrong).
My question is this: Would the Great Oxygenation Event not have killed a higher proportion of organisms (taking into account all organisms)?
I know that we don't have any records that can tell us how many died in the GOE and hence this question is necessarily speculative, but surely nearly all organisms pre-GOE would have been obligate anaerobes (and therefore found oxygen to be toxic)?
What I'm unsure about is how single-celled organisms fared in more recent (Phanerozoic) extinctions, as most discussion seems to be in terms of opisthokonts (i.e. animals + fungi) and archaeplastidans (i.e. plants + algae).