Other Life, Other Chemistry

I am working, very slowly, on a hard SF novel about the first extrasolar colony. The colonists find a world which has a general chemistry very similar to Earth's. However, the details have some surprising differences. 

They find the remains of a sapient culture, apparently gone for centuries. The colonists' chief engineer warns them:

"By the way, don't eat their stored food. A detailed chemical analysis of several types shows they apparently considered cadmium an essential trace element."

This is several chapters after the botanists worry about not finding enough cobalt in the soil:

"Cobalt is an essential trace element for terrestrial animals. Among other things, it is at the core of every B-12 molecule. The vitamin's chemical name, cobalamin, in fact comes from the cobalt in it. Plants take up the metal incidentally, animals eat the plants and make B-12, and the acid in human stomachs releases this for absorption."