TRIOCTAHEDRAL FE(II) SMECTITES ARE DOMINANT BASALT ALTERATION PRODUCTS UNDER ANOXIC CONDITIONS. ON MODERN EARTH THE TOP 600 M OF OCEANIC CRUST CONTAINS 6-10 VOL.% FE(II) SMECTITES WITH SHALLOW PORTIONS PARTIALLY OXIDIZED THROUGH SEAWATER INTERACTION. PRIOR TO OXYGENATION OF EARTH'S ATMOSPHERE AND OCEANS SUCH CLAYS WOULD ALSO HAVE OCCURRED IN SOILS ON LAND AND AS DETRITAL MINERALS IN RIVERINE LACUSTRINE AND MARINE SETTINGS. IN MARINE ENVIRONMENTS ALONE THE ESTIMATED CRUSTAL POOL OF FE(II) IN SMECTITES IS TWO ORDERS OF MAGNITUDE LARGER THAN THE DISSOLVED FE(II) POOL PREDICTED FOR ARCHAEAN SEAWATER. FE(II) SMECTITES HAVE RECENTLY BEEN IDENTIFIED ON MARS IN ~3.5 GA MUDSTONES DEPOSITED IN A REDOX-STRATIFIED LAKE IN GALE CRATER. IN ADDITION THESE LIKELY OCCUR THROUGHOUT THE SOLAR SYSTEM WHEREVER WATER ALTERS MAFIC ROCKS INCLUDING ON SUBSURFACE OCEANS ON ICY MOONS LIKE EUROPA AND ENCELADUS. THE ROLE THAT FE(II) SMECTITES PLAY IN BIOGEOCHEMICAL IRON CYCLING HAS BEEN LARGELY UNEXPLORED TO DATE BECAUSE THEIR MODERN OCCURRENCE IS RESTRICTED TO SUBSURFACE ANOXIC SETTINGS. TRIOCTAHEDRAL FE(II) SMECTITES ARE THUS A MASSIVE YET UNDER-EXAMINED FE(II) POOL POTENTIALLY AVAILABLE FOR USE AS ELECTRON DONORS IN EARLY MICROBIAL METABOLISMS AND ON OTHER HABITABLE PLANETARY BODIES. IT REMAINS UNCLEAR WHETHER CHEMOLITHOTROPHIC OR PHOTOFERROTROPHIC MICROORGANISMS ARE CAPABLE OF USING TRIOCTAHEDRAL FE(II) SMECTITES AS ELECTRON DONORS WHAT ENZYMATIC MECHANISMS ARE INVOLVED IN THIS OXIDATION HOW THE CHEMICAL AND STRUCTURAL PROPERTIES OF THESE SMECTITES AFFECT THEIR VIABILITY AS ELECTRON DONORS AND WHETHER SUCH PROCESSES LEAVE SIGNATURES IN THE ROCK RECORD. WE PROPOSE TO ASSESS THE ABILITY OF IRONOXIDIZING BACTERIA TO USE TRIOCTAHEDRAL FE(II) SMECTITES AS ELECTRON DONORS AND TO IDENTIFY THE MINERALOGICAL AND CHEMICAL SIGNATURES OF SUCH METABOLIC ACTIVITY. SPECIFIC OBJECTIVES INCLUDE TO: (1) CONSTRAIN THE CHEMICAL AND BIOLOGICAL MECHANISMS OF TRIOCTAHEDRAL FE(II) SMECTITE OXIDATION BY CHEMOLITHOTROPHS AND PHOTOFERROTROPHS; (2) IDENTIFY THE MINERALOGICAL PRODUCTS OF MICROBIAL AND ABIOTIC FE(II) SMECTITE OXIDATION; AND (3) ASSESS TRACE METAL SIGNATURES OF MICROBIAL OXIDATION OF FE(II) SMECTITES. A SERIES OF FE(II) SMECTITES WILL BE SYNTHESIZED WITH A SUBSET CONTAINING OXIDIZABLE [V(III) MN(II) CO(II)] AND NON-OXIDIZABLE [NI(II) CU(II) ZN(II)] TRACE METALS. KNOWN CULTURES OF CHEMOLITHOTROPHIC AND PHOTOFERROTROPHIC IRON OXIDIZING BACTERIA WILL BE USED TO INVESTIGATE MICROBIAL OXIDATION OF FE(II) SMECTITES. DIFFRACTION IMAGING AND SPECTROSCOPIC TECHNIQUES WILL DETERMINE HOW FE OXIDATION STATE AND SMECTITE STRUCTURE IS ALTERED BY MICROBIAL AND ABIOTIC OXIDATION. TRANSCRIPTOMIC AND RT-QPCR EXPERIMENTS WILL ASSESS THE ENZYMATIC MECHANISMS INVOLVED IN AQUEOUS FE(II) VERSUS SMECTITE FE(II) OXIDATION. COLLECTIVELY THESE MEASUREMENTS WILL PROVIDE AN UNDERSTANDING OF THE BIOLOGICAL MECHANISTIC AND STRUCTURAL CONTROLS ON MICROBIAL FE(II) SMECTITE OXIDATION. THE PRODUCTS OF SMECTITE OXIDATION AND THE EFFECT OF MICROBIAL OXIDATION ON THE DISTRIBUTION AND OXIDATION STATE OF STRUCTURALLY-BOUND TRACE METALS WILL BE DETERMINED TO ASSESS POTENTIAL MINERALOGICAL AND CHEMICAL SIGNATURES. THE PROPOSED RESEARCH WILL REVEAL WHETHER AN ABUNDANT MINERAL HOST OF FE(II) ON THE SURFACE OF THE EARLY EARTH THAT ALSO OCCURS WIDELY ON OTHER PLANETARY BODIES CAN BE USED AS AN ELECTRON DONOR IN DIVERSE METABOLISMS INVOLVED IN IRON CYCLING. THE PROPOSED RESEARCH ADDRESSES THE GOAL OF THE EXOBIOLOGY PROGRAM IN THE FOCUS AREA "EARLY EVOLUTION OF LIFE AND THE BIOSPHERE" BY EXAMINING BIOLOGICAL UTILIZATION OF MINERAL FORMS OF THE REDOX-ACTIVE ELEMENT IRON EXPECTED TO HAVE BEEN COMMON ON THE EARLY EARTH. THE PLANNED PROJECT ALSO ADDRESSES THE GOALS OF THE FOCUS AREA "BIOSIGNATURES AND LIFE ELSEWHERE" BY CHARACTERIZING DETECTABLE MINERALOGICAL AND COMPOSITIONAL SIGNATURES ASSOCIATED WITH MICROBIAL OXIDATION OF CLAYS ON THE EARLY EARTH AND OTHER PLANETARY BODIES.