The Bahamian Islands, characterized by San Salvador (24 05' N, 74 30' W), contain numerous hypersaline lakes and ponds (45 to 160 ppt). These lakes are subject to intense irradiance (> 2300 μE m^-2 s^-1), high temperatures (> 30 C) and chronic nutrient (e.g. nitrogen) depletion. On San Salvador, luxuriant microbial mats blanket the shallow sediments in many of the lakes and contribute significantly to production and material flux within the lakes. The ability of the mats to fix atmospheric N₂ is important for ensuring N-availability and sustaining productivity. On a yearly basis, water availability is a key determinant for mat productivity and growth. When lake-levels recede during the dry season (December to April), extensive areas of the mats are desiccated. During desiccation, the mats incur further osmotic, temperature and irradiance stresses. Due to the severe water-stress conditions, the mats typify microbial growth at .the edge of life. To minimize the effects of decreased water availability incurred through osmotic and desiccation stress, many prokaryotic organisms produce secondary metabolites such as trehalose and glycine betaine and/or exopolymeric substances (EPS). Because water stress is an overarching characteristic of the lakes, a large portion of the carbon and nitrogen budgets may be coupled to the production and degradation of metabolites produced in response to water stress. For the San Salvador mats, as well as other non-hypersaline mat communities world wide, EPS represent a significant structural component, protective feature, and pool of reduced carbon. However, little work has been done to assess regulation of EPS production, content, and degradation in relationship to the structure, composition, stress endurance, and nutrient flux of mat communities. This project will establish a microbial observatory for the hypersaline lakes and ponds of San Salvador Island. In addition to representing hypersaline biomes, the lakes of San Salvador are pristine systems free of anthropogenic impacts. As such, they enable us to exam how large scale climatic oscillations and other natural environmental changes impact microbial community structure and function. The overall research objective of this study will be to assess the influence water availability has on structural diversification, community composition, production, and carbon sequestration in microbial mats. The specific goals for this observatory are to: 1) Describe the structural and microbial diversity of the different mat communities in relation to water availability. 2) Assess the influence water availability has on primary production, EPS production, and EPS degradation. 3) Isolate and characterize novel anhydrophilic organisms and biogeochemically important genes. 4) Develop a conceptual model linking climate fluctuations, water budgets, primary production, and EPS production and turnover. Fieldwork will be based out of the Bahamian Field Station (BFS) on San Salvador Island, which is close, and ideally suited to support proposal activity. Every year, the BFS hosts students and researchers from a multitude of United Sates and overseas institutions. In cooperation with the BFS staff, a prime focus for this project will be the establishment of educational and research opportunities for students wishing to better understand the structure, function, and environmental controls of halotolerant, extremophilic microbial communities on San Salvador. Information and data for this project will be made available at BFS, through the project’s central web site, publications, and educational efforts.