Sulfur Isotopic Fractionation of Carbonyl Sulfide during Degradation by Soil Bacteria

We performed laboratory incubation experiments on the degradation of gaseous phase carbonyl sulfide (OCS) by soil bacteria to determine its sulfur isotopic fractionation constants ((34)epsilon). Incubation experiments were conducted using strains belonging to the genera Mycobacterium, Williamsia, and Cupriavidus isolated from natural soil environments. The (34)epsilon values determined were -3.67 +/- 0.33 parts per thousand, -3.99 +/- 0.19 parts per thousand, -3.57 +/- 0.22 parts per thousand, and -3.56 +/- 0.23 parts per thousand for Mycobacterium spp. strains THI401, THI402, THI404, and THI405; -3.74 +/- 0.29 parts per thousand for Williamsia sp. strain THI410; and -2.09 +/- 0.07 parts per thousand and -2.38 +/- 0.35 parts per thousand for Cupriavidus spp. strains THI414 and THI415. Although OCS degradation rates divided by cell numbers (cell specific activity) were different among strains of the same genus, the (34)epsilon values for same genus showed no significant differences. Even though the numbers of bacterial species examined- were limited, our results suggest that (34)epsilon values for OCS bacterial degradation depend not on cell-specific activities, but on genus-level biological differences, suggesting that (34)epsilon values are dependent on enzymatic and/or membrane properties. Taking our (34)epsilon values as representative for bacterial OCS degradation, the expected atmospheric changes in delta S-34 values of OCS range from 0.5 parts per thousand to 0.9 parts per thousand,-based on previously reported decreases in OCS, concentrations at Mt. Fuji, Japan. Consequently, tropospheric observation of delta S-34 values for OCS coupled with (34)epsilon values for OCS bacterial degradation can potentially be used to investigate soil as an OCS sink.