Microbial habitats in extreme environments and organic transformations at ridge-crest hydrothermal systems are receiving increased attention as the novel properties of microbes that live in these environments become increasingly important in terms of bioprospecting for natural products from which new drugs can be made. This research is theoretical in nature and designed to guide the discovery of such micro-organisms by calculating "tipping points" (i.e., temperature, pressure, and compositional conditions that trigger the most dramatic changes in mineral reactions and/or fluid evolution). The focus of the work is on known hydrothermal vent systems from the Ridge 2000 Integrated Study Sites where extensive hydrothermal vent fluid compositions and mineral data are present. Goals of the project are to use thermodynamic constraints and calculations to predict the supply of chemical energy to microbial communities that inhabit seafloor hydrothermal vents. It will also focus on adding estimates of hundreds of additional organic compounds to thermodynamic databases to allow more complete and realistic calculations of organic transformations and the abiotic synthesis of organic compounds. Calculations will examine the full compositional ranges of igneous basement rocks and sediments that exist in mid-ocean ridge settings. By focusing on locating and understanding tipping points, the proposed work departs from most previous theoretical work, and provides a new way of looking at the evolution of hydrothermal systems and identifying the best locations to sample for high temperature or other unique microbial life forms. Broader impacts of the work include student training, building infrastructure for science in terms of providing much needed thermodynamic data for organic compounds, and public outreach.