Microbially enhanced dissolution of HgS in an acid mine drainage system in the California Coast Range

Abstract

Mercury sulfides (cinnabar and metacinnabar) are the main ores of Hg and are relatively stable under oxic conditions (Ksp = 10⁻⁵⁴ and 10⁻⁵² , respectively). However, until now their stability in the presence of micro-organisms inhabiting acid mine drainage (AMD) systems was unknown. We tested the effects of the AMD microbial community from the inoperative Hg mine at New Idria, CA, present in sediments of an AMD settling pond adjacent to the main waste pile and in a microbial biofilm on the surface of this pond, on the solubility of crystalline HgS. A 16S rRNA gene clone library revealed that the AMD microbial community was dominated by Fe-oxidizing (orders Ferritrophicales and Gallionellas) and S-oxidizing bacteria (Thiomonas sp.), with smaller amounts (≤ 6%) being comprised of the orders Xanthomondales and Rhodospirillales. Though the order Ferritrophicales dominate the 16S rRNA clones (>60%), qPCR results of the microbial community indicate that the Thiomonas sp. represents ~55% of the total micro-organisms in the top 1 cm of the AMD microbial community. Although supersaturated with respect to cinnabar and metacinnabar, microcosms inoculated with the AMD microbial community were capable of releasing significantly more Hg into solution compared to inactivated or abiotic controls. Four different Hg-containing materials were tested for bacterially enhanced HgS dissolution: pure cinnabar, pure metacinnabar, mine tailings, and calcine material (processed ore). In the microcosm with metacinnabar, the presence of the AMD microbial community resulted in an increase of dissolved Hg concentrations up to 500 μg L ⁻¹during the first 30 days of incubation. In abiotic control microcosms, dissolved Hg concentrations did not increase above 100 ng L⁻¹ . When Hg concentrations were below 50 μg L⁻¹ , the Fe-oxidizing bacteria in the AMD microbial community were still capable of oxidizing Fe(II) to Fe(III) in the AMD solution, whereas concentrations above 50 μg L⁻¹ resulted in inhibition of microbial iron oxidation. Our experiments show that the AMD microbial community contributes to the dissolution of mercury sulfide minerals. These findings have major implications for risk assessment and future management of inoperative Hg mines worldwide.

Efficient Low-pH Iron Removal by a Microbial Iron Oxide Mound Ecosystem at Scalp Level Run

“…Other sequences closely related to Ferritrophicum radicicola have been retrieved from the Yunfu sulfide mine (31) and Yinshan lead-zinc mine, China (32), the Carnoulès acid mine drainage, France (33), and La Zarza, Spain (34). Ferritrophicum species are not abundant in AMD sites studied to date, with few exceptions (35).…”

Mercury transformation and release differs with depth and time in a contaminated riparian soil during simulated flooding

“…The formation of Hg 0 (aq) under the given experimental conditions is not easily explained by known processes assuming that Hg(II) was released from β-HgS and subsequently reduced to Hg 0 (aq) , and there is no evidence in the literature for reduction of Hg(II) in bulk mercuric sulfides to Hg 0 (aq) (Ravichandran et al, 1998;Brandon et al, 2001). Mercury mobilization was observed under anoxic, sulfide-limited conditions (≤ 10 -6 M); therefore, Hg(II) release from β-HgS did not occur through oxidative dissolution by molecular oxygen (Barnett et al, 2001) or redox-active DOM functional groups (Waples et al, 2005), ligand-promoted dissolution by sulfide (reported for sulfide ≥ 10 -4 M) (Paquette and Helz, 1995;Ravichandran et al, 1998), or interactions with aerobic sulfur-oxidizing bacteria (Jew et al, 2014). Likewise, the formation of aqueous Hg(II)-polysulfide complexes (e.g., HgS x SH -), proposed to increase the solubility of β-HgS (Paquette and Helz, 1997;Jay et al, 2000), does not explain mercury mobilization.…”

Genome-resolved metagenomics and detailed geochemical speciation analyses yield new insights into microbial mercury cycling in geothermal springs

“…For example, Hg methylation is impeded in ecosystems with elevated aqueous sulfide concentrations (56). Conversely, the availability of soluble Hg to microorganisms is limited by the solubility and/or microbially-mediated dissolution of cinnabar/metacinnabar (57). To elucidate these interactions, the Cub and Tiger Bath metagenomes were searched for genes related to dissimilatory sulfite/sulfate reduction using dsrAB genes that encode subunits A and B of the dissimilatory bi(sulfite) reductase enzyme (DsrAB); (58), and the sox pathway genes ( soxR , soxXA, soxYZ , soxB, sox(CD) 2 , soxG ) that encode enzymes used in hydrogen sulfide and elemental sulfur oxidation (59).…”

“…The observed physicochemical conditions in Tiger and Cub Baths were on the cusp of HgS (s) formation/dissolution, at pH 2-3 and pH > 4 (76). Thus the bioavailability of Hg(II) in the acid warm springs of the NGF reflected Hg solubility in the context of sulfur speciation and acidic conditions (56, 57). In aerobic sediments, sulfur-and iron-oxidizing bacteria and archaea may enhance the dissolution of cinnabar and increase Hg(II) bioavailability.…”