No oxygen isotope exchange between water and APS-sulfate at surface temperature: Evidence from quantum chemical modeling and triple-oxygen isotope experiments

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In both laboratory experiments and natural environments where microbial dissimilatory sulfate reduction (MDSR) occurs in a closed system, the δ34SSO4 (( 34S/ 32S) sample/( 34S/ 32S) standard-1) for dissolved SO 42- has been found to follow a typical Rayleigh-Distillation path. In contrast, the corresponding δ18OSO4 (( 18O/ 16O) sample/( 18O/ 16O) standard)-1) is seen to plateau with an apparent enrichment of between 23‰ and 29‰ relative to that of ambient water under surface conditions. This apparent steady-state in the observed difference between δ18OSO4 and δ18OH2O can be attributed to any of these three steps: (1) the formation of adenosine-5'-phosphosulfate (APS) from ATP and SO 42-, (2) oxygen exchange between sulfite (or other downstream sulfoxy-anions) and water later in the MDSR reaction chain and its back reaction to APS and sulfate, and (3) the re-oxidation of produced H 2S or precursor sulfoxy-anions to sulfate in environments containing Fe(III) or O 2. This study examines the first step as a potential pathway for water oxygen incorporation into sulfate. We examined the structures and process of APS formation using B3LYP/6-31G(d,p) hybrid density functional theory, implemented in the Gaussian-03 program suite, to predict the potential for oxygen exchange. We conducted a set of in vitro, enzyme-catalyzed, APS formation experiments (with no further reduction to sulfite) to determine the degree of oxygen isotope exchange between the APS-sulfate and water. Triple-oxygen-isotope labeled water was used in the reactor solutions to monitor oxygen isotope exchange between water and APS sulfate. The formation and hydrolysis of APS were identified as potential steps for oxygen exchange with water to occur. Quantum chemical modeling indicates that the combination of sulfate with ATP has effects on bond strength and symmetry of the sulfate. However, these small effects impart little influence on the integrity of the SO 42- tetrahedron due to the high activation energy required for hydrolysis of SO 42- (48.94kcal/mol). Modeling also indicates that APS dissociation via hydrolysis is achieved through cleavage of the P-O bond instead of S-O bond, further supporting the lack of APS-H 2O-oxygen exchange. The formation of APS in our in vitro experiments was verified by HPLC fluorescence spectroscopy, and triple-oxygen isotope data of the APS-sulfate indicate no oxygen isotope exchange occurred between APS-sulfate and water at 30°C for an experimental duration ranging from 2 to 120h. The study excludes APS formation as one of the causes for sulfate-oxygen isotope exchange with water during MDSR. © 2012 Elsevier Ltd.

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Geochimica et Cosmochimica Acta

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