The Santa Elena Ophiolite comprises 250 km2 of ultramafic rocks and mafic associations along the northwestern Pacific coast of Costa Rica. Uplifted ultramafic rocks originating from the Earth’s mantle represent a reservoir of carbon and reducing power, which during the process of serpentinization, are liberated into the surface environment, potentially supporting microbial growth. The extremely variable weather conditions that occur on the Santa Elena Peninsula between the wet (May – October) and dry (November – April) seasons result in a unique hydrogeological environment. Contrary to other continental serpentinization environments (e.g., Tablelands, Canada Western Coastal Range, USA Gruppo di Voltri, Italy), in this tropical scenario, precipitation quantities and intensities are usually greater and occur during extended periods of times, which facilitate infiltration, accelerating water-rock interactions and liberating carbon and energy to the surface environment. These extreme seasonal events provide a unique opportunity to investigate the response of microbial metabolisms (at the interface of deep carbon and energy fluxes) under a dynamic subsurface hydrology setting. During an expedition in March 2013, a series of alkaline springs along the peninsula were sampled. Preliminary V4-V5 sequence data suggests that these fluids had low diversity and that indicated the presence of microorganisms involved in hydrogen, methane, and methanol metabolism (Sanchez-Murillo, et al. unpublished), including bacteria from the genera Hydrogenophaga and Methylibium and from the family Methylophilaceae, as well as archaea from the orders Methanobacteriales, Methanocellales and Methanomicrobiales. In this proposal we intend to demonstrate the links between geochemistry, hydrology, and microbial activities related to subsurface processes at Santa Elena Ophiolite.