Sulfide oxidation and (nano)particle formation along redox gradients in the marine environment

Findlay, Alyssa J.L.
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University of Delaware
Recent thermodynamic calculations and experimental work have demonstrated that the oxidation of sulfide to elemental sulfur by oxygen is slow. Moreover, additional studies have shown that nanoparticles are a widespread component of many environments. Here, the oxidation of sulfide to form elemental sulfur and the presence of metal sulfide and elemental sulfur nanoparticles were investigated along redox gradients in the water column of the Chesapeake Bay and in buoyant hydrothermal vent plumes (< 1.5 meters from the orifice) along the Mid-Atlantic Ridge. The partitioning of trace metals into sulfide phases was found to differ between the bouyant plumes of three vent sites along the Mid-Atlantic Ridge due to differences in the metal to sulfide ratio of the vent fluid. Significant concentrations of HNO3 -extractable metals were found in the < 0.2 μm fraction at all three vent sites, indicating that these metals were incorporated into nanoparticulate pyrite. Elemental sulfur nanoparticles (< 0.2 μm) were found to be a significant percentage of total S0 in both the Chesapeake Bay water column and hydrothermal vent plumes. In the Chesapeake Bay, elemental sulfur is formed by both abiotic and biotic sulfide oxidation. Manganese oxides are the dominant available chemical oxidant for sulfide, and a strain of phototrophic sulfide oxidizing bacteria (PSOB), CB11, that was enriched from the Chesapeake Bay was shown to produce elemental sulfur as a product of sulfide oxidation. In buoyant vent plumes, sulfide oxidation is abiotic, and the oxidation of sulfide through an iron catalytic cycle accounts for all elemental sulfur formed. These results indicate that nanoparticulate elemental sulfur should be a common component of a variety of different types of environments in which sulfur is cycled along an oxic/anoxic interface. The oxidation of sulfide in the Chesapeake Bay was further investigated through incubation experiments that monitored sulfide loss in natural water samples and in cultures of CB11. Small increases in light intensity as low as 0.1 μEi were found to significantly affect sulfide loss in both sets of experiments, indicating the activity of PSOB in the Chesapeake Bay water column. PSOB need only comprise about 5 % of the total microbial community in order to account for all observed light-dependent sulfide loss. In order to explore the impact of variability in the water column redox structure on sulfide oxidation by PSOB, a one-dimensional diffusion-reaction model of the Chesapeake Bay water column was developed using kinetic parameters determined from the incubation experiments. The model simulations demonstrate that the contribution of PSOB to sulfide oxidation is highly variable and dependent upon the location of the oxic/anoxic interface in the water column.