Oxygen
From Tardis Wiki, the free Doctor Who reference
Oxygen was one of the primary elements required for the function of many carbon-based life forms.
It was most vital on space stations or spacecraft to allow said life forms to breathe. (DW: The Moonbase)
By the 51st century, Human space vessels could house oxygen factories in the form of synthetic forests. These used light from space to produce oxygen. (DW: Flesh and Stone)
