The cycle of life is a "circle" - life itself provides the requirements for more life. We did not even have free oxygen before cyanobacteria, and animals then evolved to use the oxygen and return carbon dioxide for plants to use.
The ocean is getting more acidic because of climate change, and this is killing off the plankton, among other things. I give you one guess at what is at the base of the food chain.
If the ocean gets too acidic and too warm, it will kill all the life in it, and we will get high levels of
hydrogen sulfide.
Seems pretty deadly to me.