[jofer]

It's still quite controversial whether or not they produce oxygen that way. It's been hypothesized, but I wouldn't consider it a consensus or settled. There are also microbes that can produce oxygen without light, so there are other mechanisms to explain "dark oxygen" in deep sea ecosystems.

With that said, the simple truth of it is that we know next to nothing about these ecosystems and really can't accurately estimate impacts. They're quite possibly significant, but we just don't have much info to go off of and studies like this are sorely needed.