THE ASTROBIOLOGY COMMUNITY IS CURRENTLY ON THE VERGE OF THE FIRST DETAILED CHARACTERIZATION OF HABITABLE EXTRASOLAR PLANETS. LARGE SPACE-BASED TELESCOPE MISSIONS CURRENTLY IN THEIR SCIENCE DEFINITION PHASE WILL DIRECTLY IMAGE TERRESTRIAL EXOPLANETS AT UV TO NEAR- IR WAVELENGTHS WHILE FUTURE GROUND-BASED OBSERVATORIES WILL ALSO BE ABLE TO SPECTRALLY CHARACTERIZE THE ATMOSPHERES OF SMALL PLANETS AROUND THE VERY NEAREST STARS. INDEED FOR THE FORESEEABLE FUTURE OUR ONLY ACCESSIBLE METHOD FOR DETECTING LIFE OR EVEN FULLY CHARACTERIZING HABITABILITY BEYOND EARTH WILL LIKELY BE DECIPHERING THE CHEMISTRY OF EXOPLANETARY ATMOSPHERES. AS WE DEVELOP A SEARCH STRATEGY FOR LIFE BEYOND EARTH THE EVOLUTIONARY HISTORY OF OUR OWN PLANET PROVIDES A POWERFUL NATURAL LAB FOR EXAMINING THE PROCESSES THAT PROMOTE THE EMERGENCE AND MAINTENANCE OF ATMOSPHERIC BIOSIGNATURES. HOWEVER THE CHEMISTRY OF EARTH S OCEAN-ATMOSPHERE SYSTEM IS REGULATED BY A VAST NETWORK OF MICROBIAL METABOLISMS LINKED BY COMPETITION FOR SUBSTRATES SYNTROPHIC COOPERATION AND PRODUCTION OF TOXIC WASTE PRODUCTS. IN ADDITION THE ARRAY OF BASIC METABOLIC PATHWAYS AND MAJOR PROCESSES STRUCTURING THE NATURE OF THEIR CONNECTIVITY HAVE NOT BEEN STATIC THROUGH EARTH S HISTORY NOR ARE THEY EXPECTED TO BE GENERALLY APPLICABLE TO HABITABLE EARTH-LIKE PLANETS. FOR EXAMPLE THE PROCESSES INVOLVED IN THE DIRECT AND INDIRECT METABOLIC CONSUMPTION OF PHOTOSYNTHETICALLY PRODUCED O2 FORM A CRITICAL NETWORK REGULATING THE MAJOR BIOSIGNATURE GASES IN EARTH S ATMOSPHERE. MORE GENERALLY HABITABLE PLANETS HOSTING BACTERIAL BIOSPHERES THAT DO NOT PRODUCE OR CONSUME MOLECULAR OXYGEN (O2) REDUCING OR ANOXIC WORLDS MAY BE COMMON THROUGHOUT THE UNIVERSE. INDEED THIS PLANETARY STATE MAY HAVE DOMINATED MUCH OF EARTH S EARLIEST HISTORY. OUR MOTIVATION THOUGH MULTI-FACETED IN PRACTICE CAN BE CONCEPTUALLY DISTILLED TO A SINGLE BASIC QUESTION: WHAT METABOLIC NETWORKS PROMOTE THE EMERGENCE AND MAINTENANCE OF ATMOSPHERIC BIOSIGNATURES ON PRIMITIVELY REDUCING PLANETS? WE WILL PROVIDE FUNDAMENTAL NEW CONSTRAINTS ON THIS QUESTION BY DEVELOPING A NEW ECOPHYSIOLOGICAL MODULE FOR USE IN LARGE-SCALE PLANETARY SYSTEM MODELS. THIS MODULE WILL CONTAIN A SUITE OF PRIMITIVE AND DERIVED MICROBIAL METABOLISMS RELEVANT FOR A WIDE RANGE OF HABITABLE PLANETS AND WILL BE DESIGNED FOR FLEXIBLE INTEGRATION ACROSS A RANGE OF PLATFORMS. AS PROOF OF CONCEPT WE WILL EMBED THIS MODULE IN A SIMPLE COUPLED OCEAN-ATMOSPHERE PHOTOCHEMICAL MODEL (ATMOS) AND IN AN EARTH SYSTEM MODEL OF INTERMEDIATE COMPLEXITY (CGENIE) AND PROVIDE THE FIRST DETAILED RECONSTRUCTION OF EARTH S UPSIDE-DOWN BIOSPHERE DURING THE HADEAN AND EARLY ARCHEAN EONS.ACCORDING TO PROGRAM ELEMENT C.5 OF NASA S RESEARCH OPPORTUNITIES IN SPACE AND EARTH SCIENCES (ROSES) 2015 THE OVERARCHING GOAL OF THE EXOBIOLOGY PROGRAM IS: [T]O UNDERSTAND THE ORIGIN EVOLUTION DISTRIBUTION AND FUTURE OF LIFE IN THE UNIVERSE [ ] IN THE CONTEXT OF NASA S ONGOING EXPLORATION OF OUR STELLAR NEIGHBORHOOD AND THE IDENTIFICATION OF BIOSIGNATURES FOR IN SITU AND REMOTE SENSING APPLICATIONS . THE RESEARCH PROPOSED HERE WILL BE DESIGNED TO PROVIDE SIGNIFICANT STEPS FORWARD IN OUR PREDICTIVE UNDERSTANDING OF THE LINKS BETWEEN MICROBIAL METABOLISM AND ATMOSPHERIC BIOSIGNATURES RENDERING IT GERMANE TO BOTH THE OVERARCHING GOALS OF THE EXOBIOLOGY PROGRAM AND MANY OF THE MAJOR FOCI DELINEATED IN THE MOST RECENT NASA ASTROBIOLOGY STRATEGIC PLAN IN PARTICULAR SECTION 3.4 (EARLY LIFE AND INCREASING COMPLEXITY) SECTION 4.4 (CO-EVOLUTION OF LIFE AND THE PHYSICAL ENVIRONMENT) AND SECTION 5.4 (IDENTIFYING EXPLORING AND CHARACTERIZING ENVIRONMENTS FOR HABITABILITY AND BIOSIGNATURES).