Biogeography of serpentinite-hosted microbial communities Microbial habitats hosted in ultramafic rocks constitute substantial, globally-distributed portions of the subsurface biosphere, occurring both on the continents and beneath the seafloor. The aqueous alteration of ultramafics, in a process known as serpentinization, creates energy rich, high pH conditions, with low concentrations of inorganic carbon which place fundamental constraints upon microbial metabolism and physiology. Despite their importance, very few studies have attempted to directly access and quantify microbial activities and distributions in the serpentinite-hosted ecosystems. We have initiated microbiological studies of alkaline springs and their associated carbonate deposits at three separate continental sites of serpentinization in Newfoundland, Italy, and California, and we are comparing these results to previous analyses of the Lost City hydrothermal field, near the Mid-Atlantic Ridge. We anticipate that the data from these samples will yield insight into the biogeography of serpentinite-hosted microbial communities and provide targets for further investigation of important biogeochemical processes.