SULFUR CYCLING PLAYS AN EMINENT ROLE FOR LIFE ON EARTH. CONSIDERING THE UBIQUITY OF SULFUR IN OUR SOLAR SYSTEM IT IS REASONABLE TO PRESUME THAT SULFUR ALSO PLAYS A ROLE IN "LIFE ELSEWHERE". CONSEQUENTLY BIOSIGNATURES OF SULFUR-UTILIZING ORGANISMS ARE A TARGET IN ACHIEVING THE GOAL OF THE NASA PLANETARY SCIENCE RESEARCH PROGRAM TO "ASCERTAIN THE CONTENT ORIGIN AND EVOLUTION OF THE SOLAR SYSTEM AND THE POTENTIAL FOR LIFE ELSEWHERE". ON EARTH SULFUR CYCLING OPERATES IN A CURIOUS FASHION: WHILE SULFUR OXIDATION OPERATES OVER MULTIPLE INTERMEDIATES THAT ARE RELEASED TO THE ENVIRONMENT SULFATE REDUCTION HAS ONLY ONE PRODUCT: SULFIDE. THIS LEADS TO THE CONUNDRUM THAT TO PRODUCE NATIVE SULFUR FROM SULFATE SULFATE MUST BE FIRST REDUCED TO SULFIDE AND THEN OXIDIZED TO NATIVE SULFUR. IT IS PRESUMED THAT THIS OXIDATION REQUIRES AN EXTERNAL OXIDANT NAMELY ATMOSPHERIC OXYGEN WHICH IS PROBLEMATIC AS ATMOSPHERIC OXYGEN IS AN AGENT THAT INHIBITS SULFATE REDUCTION INACTIVATING THE PROCESS THAT WOULD GENERATE SULFIDE. BASED ON MY GEOCHEMICAL OBSERVATIONS I CHALLENGE THE CONCEPT THAT AN EXTERNAL OXIDANT IS INVOLVED IN THE GENESIS OF MASSIVE NATIVE SULFUR DEPOSITS. I HYPOTHESIZE THAT SULFIDE STRESS INDUCES MICROBIAL COMMUNITIES TO SHIFT FROM THE PRODUCTION OF SULFIDE TO THE PRODUCTION OF NATIVE SULFUR. IN THE LITERATURE THERE IS EVIDENCE FOR THE POTENTIAL EXISTENCE OF SUCH MECHANISMS WHICH INCLUDE THE OBSERVATION OF NATIVE SULFUR IN ANAEROBIC METHANE OXIDIZING ARCHAEA UNEXPECTED SULFUR CYCLING PATTERNS IN SULFIDIC HYPERSALINE ENVIRONMENTS AND THAT CLASSICAL SULFATE REDUCTION MAY 'LEAK' INTERMEDIATES WHEN EXPOSED TO ENVIRONMENTAL STRESS. SURPRISINGLY THIS EVIDENCE HAS NOT BEEN FURTHER INVESTIGATED. THE ADVENT OF NEW STABLE ISOTOPIC TOOLS SUCH AS CLUMPED ISOTOPE THERMOMETRY AND HIGH SPATIAL RESOLUTION SULFUR ISOTOPE ANALYSES OFFER AN UNPRECEDENTED OPPORTUNITY TO TACKLE THIS CHALLENGE. VALIDATING MY HYPOTHESIS REQUIRES TO 1) DEMONSTRATE THAT THAT MICROBIAL LIFE WAS INVOLVED IN THE FORMATION OF LARGE NATIVE SULFUR DEPOSITS; 2) ESTABLISH BIOCHEMICAL TOOLS TO DETECT NOVEL SULFUR METABOLIC PATHWAYS; AND 3) TEST IF SULFIDE STRESS INDUCES A CHANGE IN METABOLIC PATHWAY UTILIZATION. I WILL UTILIZE CLUMPED CARBON-OXYGEN ISOTOPE THERMOMETRY TO ASSESS THE TEMPERATURE AT WHICH NATIVE SULFUR WAS GENERATED AND HIGH-RESOLUTION SULFUR ISOTOPE ANALYSES AS BIOSIGNATURES TO INFER PRESENCE AND ACTIVITY OF SULFUR METABOLIZING ORGANISMS AND ESTABLISH A NEW ISOTOPIC TOOL - SULFUR-OXYGEN ISOTOPE FRACTIONATION FINGERPRINTING TO TEST IF MICROBES CHANGE METABOLIC PATHWAYS IN RESPONSE TO SULFIDE STRESS. FINALLY I WILL USE MIXED-CULTURE INCUBATION EXPERIMENTS TO TEST IF HIGH SULFIDE LEVELS INDUCE A CHANGE IN THE METABOLIC FUNCTION OF THE MICROBIAL COMMUNITIES AND IF THESE CHANGES RESULT IN THE ABSENCE OF OXYGEN IN THE FORMATION OF NATIVE SULFUR FROM SULFATE. MY PROJECT AIMS PERFECTLY OVERLAP WITH KEY ASTROBIOLOGICAL QUESTIONS SUCH AS WHERE LIFE COULD HAVE EXISTED OR COULD EXIST TODAY THE RESULTS OF MY RESEARCH WILL GUIDE OUR SEARCH FOR LIFE ELSEWHERE AND PROVIDE INFORMATION ABOUT THE POTENTIAL OF LIFE TO ADAPT TO DIFFERENT ENVIRONMENTS THEREBY CONTRIBUTING TO INSIGHTS ON THE IMPLICATIONS FOR LIFE ELSEWHERE