Carbon dioxide and oxygen is balanced in nature by plants , animals and other organisms . in day we know that plants inhale carbon dioxide and we humans and animals exhale carbon dioxide.  so the plants give out O2 and we take in. in night there is no sunlight and photosynthesis does not happen
.so the plants take in O2 .so in the morning the carbon dioxide exhaled by plants and animals. are taken in by the plants and given out by us.
   Algae present in the oceans give lots of O2 and the O2 is widely used by steel industries.