THE MARTIAN SURFACE SHOWS AMPLE SIGNS OF AQUEOUS ALTERATION SUGGESTING THE PRESENCE OF LIQUID WATER IN ITS PAST SOME OF WHICH WAS LIKELY DRIVEN BY HYDROTHERMAL ACTIVITY. AS VOLCANOES HAVE BEEN ACTIVE FOR MUCH OF MARS S HISTORY THESE HYDROTHERMAL SITES MAY HAVE BEEN AMONG THE LONGEST-LIVED HABITABLE ENVIRONMENTS ON ANCIENT MARS. CERRO NEGRO VOLCANO IN NICARAGUA HAS BASALTS WITH SIMILAR COMPOSITION TO MARTIAN REGOLITH AND ALSO SHOWS COMPARABLE MINERAL ALTERATIONS TO THOSE SEEN AT ANCIENT MARTIAN SITES. A PRELIMINARY LOOK AT THE MICROBIOLOGY OF ITS ACTIVE FUMAROLES SUGGESTS A DIVERSE ECOLOGY SURVIVING IN EXTREME CONDITIONS BY ADOPTING AN ENDOLITHIC LIFESTYLE. UNDERSTANDING MICROBE-MINERAL INTERACTIONS AND NUTRIENT CYCLING IN FUMAROLE-HOSTED ENDOLITHS INFORMS METABOLIC SYSTEM MODELS THAT COULD BE APPLIED TO SIMILAR SYSTEMS FOUND ON MARS. THIS PROPOSAL FOCUSES ON THE FOLLOWING QUESTIONS USING THE CERRO NEGRO FUMAROLIC SYSTEM AS AN ANALOG FOR POTENTIAL MARTIAN HABITATS: I. WHAT METABOLIC STRATEGIES ARE PRESENT AND USED BY MICROBIAL COMMUNITIES IN CERRO NEGRO FUMAROLES? II. WHAT CONSTRAINTS DO THE GEOCHEMISTRY AND MINERALOGY OF THE SYSTEM IMPOSE ON THESE COMMUNITIES? III. HOW DO CHEMICAL AND THERMAL GRADIENTS AFFECT SPATIAL RELATIONSHIPS IN THE MICROBIAL COMMUNITY? IV. WHAT TYPES OF BIOSIGNATURES ARE DETECTABLE FROM ENDOLITHIC BEHAVIOR OR MICROBIAL BIOMINERALIZATION? TO ADDRESS THESE QUESTIONS NEXT-GENERATION SEQUENCING TECHNIQUES ALONG WITH CO-REGISTERED GEOCHEMICAL AND MICROSCOPIC DATA WILL BE USED TO DETERMINE THE FUNCTIONAL AND SPATIAL RELATIONSHIPS BETWEEN MICROBIAL SPECIES METABOLISMS AND ENVIRONMENTAL PARAMETERS. THIS WILL ALLOW CONSTRUCTION OF A MODEL OF COMMUNITY METABOLISM NUTRIENT FLOW WITHIN THE SYSTEM AND CORRELATIONS BETWEEN MICROBIAL DIVERSITY AND GEOCHEMICAL PARAMETERS. THESE DATA WILL DRIVE HYPOTHESIS GENERATION AND TESTING REGARDING MICROSCALE COMMUNITY SPATIAL ORGANIZATION WHICH WILL BE CONFIRMED WITH FLUORESCENT IN SITU HYBRIDIZATION. MODELING COMMUNITY METABOLISM AND ORGANIZATION ARE IMPORTANT TO UNDERSTAND WHICH MICROBES ENGAGE IN ENDOLITHIC BEHAVIOR HOW THEY INFLUENCE THEIR ENVIRONMENT AND UNDER WHAT CONDITIONS THESE COMMUNITIES FORM DETECTABLE BIOSIGNATURES. TOGETHER THIS WILL AID MISSION PLANNERS AND SCIENTISTS OF MARTIAN EXPERIMENTS IN UNDERSTANDING DATA FROM CURRENT INSTRUMENTATION AS WELL AS AIDING IN INSTRUMENT DESIGN AND SELECTION FOR FUTURE LANDERS.