Abstract
Mercury (Hg) is a toxic heavy metal that poses significant environmental and human health risks. Soils and sediments, where Hg can exist as the Hg sulfide mineral metacinnabar (β-HgS), represent major Hg reservoirs in aquatic environments. Metacinnabar has historically been considered a sink for Hg in all but severely acidic environments, and thus disregarded as a potential source of Hg back to aqueous or gaseous pools. Here, we conducted a combination of field and laboratory incubations to identify the potential for metacinnabar as a source of dissolved Hg within near neutral pH environments and the underpinning (a)biotic mechanisms at play. We show that the abundant and widespread sulfur-oxidizing bacteria of the genus Thiobacillus extensively colonized metacinnabar chips incubated within aerobic, near neutral pH creek sediments. Laboratory incubations of axenic Thiobacillus thioparus cultures led to the release of metacinnabar-hosted Hg(II) and subsequent volatilization to Hg(0). This dissolution and volatilization was greatly enhanced in the presence of thiosulfate, which served a dual role by enhancing HgS dissolution through Hg complexation and providing an additional metabolic substrate for Thiobacillus. These findings reveal a new coupled abiotic-biotic pathway for the transformation of metacinnabar-bound Hg(II) to Hg(0), while expanding the sulfide substrates available for neutrophilic chemosynthetic bacteria to Hg-laden sulfides. They also point to mineral-hosted Hg as an underappreciated source of gaseous elemental Hg to the environment.
Highlights
Mercury (Hg) is a toxic heavy metal that poses significant environmental and human health risks (Wiener et al, 2002; Clarkson and Magos, 2006)
Culture-independent analysis of the microbial communities showed that the metacinnabar surfaces were extensively colonized by chemosynthetic bacteria, which accounted for a striking majority (>92%) of the taxonomically classified microbial community (Figure 1A, Supplementary Table S2)
The metacinnabar surface enriched for these sulfur-oxidizing organisms as previous characterization of these creek sediments revealed a microbial community that was phylogenetically diverse and taxonomically distinct from those found on the mineral incubations (Vishnivetskaya et al, 2011)
Summary
Mercury (Hg) is a toxic heavy metal that poses significant environmental and human health risks (Wiener et al, 2002; Clarkson and Magos, 2006). Under acidic pH conditions (pH∼3), HgS dissolution can be enhanced in the presence of mine tailing-derived microbial cultures (Jew et al, 2014). Under these acidic conditions, bacteria of the genus Acidithiobacillus have been implicated in inducing cinnabar dissolution and volatilization of released Hg (Baldi and Olson, 1987). Under non-acidic conditions (pH > 4), both HgS phases are considered to be poorly reactive and stable due to their low solubility (e.g., logKso ∼ -39) (Dyrssen, 1989) and very slow abiotic dissolution kinetics (Barnett et al, 2001; Holley et al, 2007). HgS minerals are still largely considered as stable Hg phases and microbial-assisted dissolution of HgS at near neutral pH conditions is assumed to be negligible
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