Figure 1: Zinc sulfide spherule precipitated on the surface of what may have been a bacterium.

Mentions: Total sulfur content in the Penn Mine sediment was determined to be 6.06 ± 0.12 wt%. Approximately 7% of the total sulfur was extracted in a dilute bicarbonate-carbonate buffer (pH 10, extraction carried out under anoxic conditions), a sulfur fraction associated with soluble sulfates. Total organic carbon concentration was 0.85 ± 0.05 wt%. Powder X-ray diffraction indicated the dominant presence of quartz and muscovite with minor amounts of pyrite in the mine sediments. Based on the identification of pyrite in the diffraction patterns it is likely that most of the total sulfur content is associated with sulfides, pyrite in particular. SEM analysis of the sediments (Figure 1) revealed the presence of spheroidal ZnS precipitates. An hypothesis for the formation of similar ZnS spheroids in an underground mine environment with near-neutral pH (a carbonate-hosted Pb-Zn deposit in Wisconsin, USA) was presented by Labrenz et al. [50] and Labrenz and Banfield [51], who considered nanometer-sized ZnS grains in bacterial slime of SRB to be produced by aerotolerant microbes as a mechanism for H2S regulation. Concentric banding within larger (μm-scale) spheroidal aggregates of ZnS grains was noted by Moreau et al. [52]. Geochemical models for formation of low-temperature Cu-Pb-Zn deposits have been proposed, based on metal fixation of H2S produced by SRB [53].

Church CD, Wilkin RT, Alpers CN, Rye RO, McCleskey RB - Geochem. Trans. (2007)

Bottom Line: Here we document multiple, independent analyses and show evidence that sulfate reduction and associated metal attenuation are occurring in the pH-4 mine environment.Water-chemistry analyses of the mine water reveal: (1) preferential complexation and precipitation by H2S of Cu and Cd, relative to Zn; (2) stable isotope ratios of 34S/32S and 18O/16O in dissolved SO4 that are 2-3 per thousand heavier in the mine water, relative to those in surface waters; (3) reduction/oxidation conditions and dissolved gas concentrations consistent with conditions to support anaerobic processes such as sulfate reduction.Phospholipid fatty acid (PLFA) and denaturing gradient gel electrophoresis (DGGE) analyses of Penn Mine sediment show a high biomass level with a moderately diverse community structure composed primarily of iron- and sulfate-reducing bacteria.

Affiliation: US Geological Survey, California Water Science Center, 4165 Spruance Road, San Diego, CA 92101, USA. clinton.church@ars.usda.gov

Abstract: Sediments recovered from the flooded mine workings of the Penn Mine, a Cu-Zn mine abandoned since the early 1960s, were cultured for anaerobic bacteria over a range of pH (4.0 to 7.5). The molecular biology of sediments and cultures was studied to determine whether sulfate-reducing bacteria (SRB) were active in moderately acidic conditions present in the underground mine workings. Here we document multiple, independent analyses and show evidence that sulfate reduction and associated metal attenuation are occurring in the pH-4 mine environment. Water-chemistry analyses of the mine water reveal: (1) preferential complexation and precipitation by H2S of Cu and Cd, relative to Zn; (2) stable isotope ratios of 34S/32S and 18O/16O in dissolved SO4 that are 2-3 per thousand heavier in the mine water, relative to those in surface waters; (3) reduction/oxidation conditions and dissolved gas concentrations consistent with conditions to support anaerobic processes such as sulfate reduction. Scanning electron microscope (SEM) analyses of sediment show 1.5-micrometer, spherical ZnS precipitates. Phospholipid fatty acid (PLFA) and denaturing gradient gel electrophoresis (DGGE) analyses of Penn Mine sediment show a high biomass level with a moderately diverse community structure composed primarily of iron- and sulfate-reducing bacteria. Cultures of sediment from the mine produced dissolved sulfide at pH values near 7 and near 4, forming precipitates of either iron sulfide or elemental sulfur. DGGE coupled with sequence and phylogenetic analysis of 16S rDNA gene segments showed populations of Desulfosporosinus and Desulfitobacterium in Penn Mine sediment and laboratory cultures.