[fsckboy]

maybe they are referring to this?*

https://en.wikipedia.org/wiki/Cyanobacteria#Metabolism

"In general, photosynthesis in cyanobacteria uses water as an electron donor and produces oxygen as a byproduct, though some may also use hydrogen sulfide[77] a process which occurs among other photosynthetic bacteria such as the purple sulfur bacteria."

* I found it by searching for "oxygen"

[citrate05]

If you use hydrogen sulfide as an electron donor, you end up liberating sulfur, not oxygen.

Contrary to grandparent, in oxygenic photosynthesis, the oxygen does come from splitting water. The resulting hydride used to "charge" an electron carrier: https://en.wikipedia.org/wiki/Photodissociation#Photolysis_i...

The CO2 gets fixed into carbohydrates. Some water is generated in that process, but no molecular oxygen.