The Nitrogen Fix

The family is allied with an alien, an octopus-like being, who can survive in the new atmosphere. Humans must live in shelters with oxygen-generating plants, or use suitable breathing equipment. Some of Earth's original life forms have mutated to survive in the changed atmosphere. Since almost no metals can exist in the corrosive atmosphere, any technology is based on ceramics or glass.

Some humans are suspicious of the aliens, and even blame them for the change to the atmosphere, since they seem to be adapted for it. The family have an almost fatal encounter with a group of such people, who are holding another alien hostage. However, the two aliens are able to pool memories biochemically, so that they become the same personality in two bodies. Their combined knowledge and skills help the humans to escape.

At the end the aliens reveal that they are basically tourists or scientists, and they travel from one system to another over thousands of years. Atmospheres "mature" when the nitrogen absorbs all the oxygen, the cause being the inevitable evolution of bacteria containing an enzyme that catalyzes this reaction. The aliens do not understand how the bacteria could have become sufficiently widespread, since the enzyme is a compound of a metal that not only does not form many compounds, but also is found only in rare deposits on the Earth.

Then they mention that the metal is gold, and the humans explain how widely humans have distributed it in the form of jewelry.