 A basic reason to pay attention to microbes can be inferred from claims that humans live in a fundamentally microbial world (e.g., Staley et al. 1997). Statements of this sort refer to the pervasiveness of microbes throughout the planet, and the fundamental contributions they make to every ecosystem. Two aspects of microbial processes in the world stand out to make this view possible: biogeochemical cycles and microbial symbiosis. Both matter a great deal to human existence, from the maintenance of the planet as we know it to the health of our own bodies.