Oxygen

Oxygen was one of the primary elements required for the functioning of many carbon-based life forms.

It was vital on space stations and spacecraft to allow said life forms to breathe. (DW: The Moonbase)

Adric, Varsh and a number of other Alzarians used oxygen cylinders against the Marshmen, causing an change in the atmosphere too quickly for them to adapt to. They then proceeded to flood the ship with the gas, forcing the Marshmen to flee. Their Starliner has an electrolytic power supply, enabling it to create it own oxygen. (DW: Full Circle)

By the 51st century, Human space vessels could house oxygen factories in the form of synthetic forests, nicknamed "treeborgs". These used light from space to produce oxygen. (DW: Flesh and Stone)