Most plants would die because they rely on CO~2~ for photosynthesis.

Many sea animals would die. Oceans absorb CO~2~ which forms carbonic acid (H~2~CO~3~) in water. Oceans are slightly alkaline due to dissolved salts (bicarbonate and carbonate) and the carbonic acid from the absorption helps to create a stable pH. Many sea animals are highly adapted to a specific pH and would die if the ocean got either too acidic or too alkaline, so they are pretty doomed in either case.

Many humans would die because agriculture would collapse. Also breathing pure oxygen over a long period of time would be very bad because of oxygen toxicity. Yeah, pure oxygen is toxic for humans lol

Land animals, I'm not so sure, but I assume most of them would die too.