How exactly do single-celled organisms interact with their environment, and what evidence of these activities do they leave in the geologic record? Geobiologists approach such questions both from the standpoint of experimental biochemistry, and from observational geochemistry. What specific molecules allow bacteria to breathe arsenate, or respire methane? What biomarker or isotopic signals do they leave behind that might identify them? What exactly do these molecular fossils tell us about how ancient organisms lived?