Genome-Resolved Metagenomics and Detailed Geochemical Speciation Analyses Yield New Insights into Microbial Mercury Cycling in Geothermal Springs

ABSTRACT Geothermal systems emit substantial amounts of aqueous, gaseous, and methylated mercury, but little is known about microbial influences on mercury speciation. Here, we report results from genome-resolved metagenomics and mercury speciation analysis of acidic warm springs in the Ngawha Geothermal Field (<55°C, pH <4.5), Northland Region, Aotearoa New Zealand. Our aim was to identify the microorganisms genetically equipped for mercury methylation, demethylation, or Hg(II) reduction to volatile Hg(0) in these springs. Dissolved total and methylated mercury concentrations in two adjacent springs with different mercury speciation ranked among the highest reported from natural sources (250 to 16,000 ng liter−1 and 0.5 to 13.9 ng liter−1, respectively). Total solid mercury concentrations in spring sediments ranged from 1,274 to 7,000 μg g−1. In the context of such ultrahigh mercury levels, the geothermal microbiome was unexpectedly diverse and dominated by acidophilic and mesophilic sulfur- and iron-cycling bacteria, mercury- and arsenic-resistant bacteria, and thermophilic and acidophilic archaea. By integrating microbiome structure and metagenomic potential with geochemical constraints, we constructed a conceptual model for biogeochemical mercury cycling in geothermal springs. The model includes abiotic and biotic controls on mercury speciation and illustrates how geothermal mercury cycling may couple to microbial community dynamics and sulfur and iron biogeochemistry. IMPORTANCE Little is currently known about biogeochemical mercury cycling in geothermal systems. The manuscript presents a new conceptual model, supported by genome-resolved metagenomic analysis and detailed geochemical measurements. The model illustrates environmental factors that influence mercury cycling in acidic springs, including transitions between solid (mineral) and aqueous phases of mercury, as well as the interconnections among mercury, sulfur, and iron cycles. This work provides a framework for studying natural geothermal mercury emissions globally. Specifically, our findings have implications for mercury speciation in wastewaters from geothermal power plants and the potential environmental impacts of microbially and abiotically formed mercury species, particularly where they are mobilized in spring waters that mix with surface or groundwaters. Furthermore, in the context of thermophilic origins for microbial mercury volatilization, this report yields new insights into how such processes may have evolved alongside microbial mercury methylation/demethylation and the environmental constraints imposed by the geochemistry and mineralogy of geothermal systems.

Total Atmospheric Mercury Deposition in Forest Areas in Korea

Atmospheric mercury dry and wet deposition, mercury in throughfall and litterfall, and mercury volatilization from soil were measured during August 2008 to February 2010 in a temperate deciduous forest in Korea. The yearly estimated mercury budget was calculated using two input approaches. For this location the annual mercury accumulation was estimated to be 6.8 μg m-2 yr-1 or 3.9 μg m-2 yr-1 depending on the approach used. Cumulative wet and throughfall fluxes were 4.3 and 6.7 μg m-2 yr-1, respectively. The annual litterfall flux was 4.6 μg m-2 yr-1 and was highest from October to December due to the increased litter production during that period. The annual Hg emission flux from soil was 6.8 μg m-2 yr-1. The overall ratio of wet deposition, throughfall, and litterfall was 1 : 1.6 : 1.1. Cumulative dry deposition fluxes of gaseous oxidized mercury (GOM) were highest in spring 2009 (10.0 ± 2.0 μg m-2 yr-1), followed by summer 2009 (5.8 ± 4.2μg m-2 yr-1), winter 2008 (5.1 ± 5.0 μg m-2 yr-1), winter 2009 (4.6 ± 5.7 μg m-2 yr-1), fall 2008 (1.9 ± 1.0 μg m-2 yr-1) and fall 2009 (1.2 ± 1.4 μg m-2 yr-1) while dry deposition fluxes for particulate bound mercury (PBM) were highest in summer 2009 (9.6 ± 9.0 μg m-2 yr-1), followed by winter 2009 (5.3 ± 5.9 μg m-2 yr-1), winter 2008 (3.8 ± 2.0 μg m-2 yr-1), spring 2009 (3.3 ± 2.6 μg m-2 yr-1), fall 2008 (3.0 ± 1.7 μg m-2 yr-1) and fall 2009 (1.2 ± 0.4 μg m-2 yr-1). The VWM TM concentration in throughfall (14.4 ± 7.1 ng L-1) was about two times higher than that in wet deposition (5.9 ± 3.8 ng L-1). Wet deposition and throughfall fluxes were higher in summer than those in other seasons possibly due to a high precipitation depth.

Volatilization of elemental mercury from fresh blast furnace sludge mixed with basic oxygen furnace sludge under different temperatures

Mercury volatilization from blast furnace sludge mixed with basic oxygen furnace sludge was shown for the first time.