Temporal and spatial variation in the mechanisms used by microorganisms to form methylmercury in the water column of Changshou Lake

Microbial diversity and abundance of Hg related genes from water, sediment and soil the Colombian amazon ecosystems impacted by artisanal and small-scale gold mining

The Amazon region abounds in precious mineral resources including gold, copper, iron, and coltan. Artisanal and small-scale gold mining (ASGM) poses a severe risk in this area due to considerable mercury release into the surrounding ecosystems. Nonetheless, the impact of mercury on both the overall microbiota and the microbial populations involved in mercury transformation is not well understood. In this study we evaluated microbial diversity in samples of soil, sediment and water potentially associated with mercury contamination in two localities (Taraira and Tarapacá) in the Colombian Amazon Forest. To this end, we characterized the bacterial community structure and mercury-related functions in samples from sites with a chronic history of mercury contamination which today have different levels of total mercury content. We also determined mercury bioavailability and mobility in the samples with the highest THg and MeHg levels (up to 43.34 and 0.049 mg kg-1, respectively, in Taraira). Our analysis of mercury speciation showed that the immobile form of mercury predominated in soils and sediments, probably rendering it unavailable to microorganisms. Despite its long-term presence, mercury did not appear to alter the microbial community structure or composition, which was primarily shaped by environmental and physicochemical factors. However, an increase in the relative abundance of merA genes was detected in polluted sediments from Taraira. Several Hg-responsive taxa in soil and sediments were detected in sites with high levels of THg, including members of the Proteobacteria, Acidobacteria, Actinobacteria, Firmicutes and Chloroflexi phyla. The results suggest that mercury contamination at the two locations sampled may select mercury-adapted bacteria carrying the merA gene that could be used in bioremediation processes for the region.

Responses of the structure and function of microbes in Yellow River Estuary sediments to different levels of mercury

The health and stability of the estuary of the Yellow River ecosystem have come under increasing pressure from land-based inputs of heavy metals. While it is known that heavy metals affect the function and health of the microbial community, there remains little knowledge on the responses of the microbial community to heavy metals, particularly highly toxic mercury. The research aimed to characterize the responses of the sediment microbial community of the estuary of the Yellow River to different levels of mercury stress. Estuary sediment samples were collected for microbial community analysis, measurement of mercury [including total mercury (THg) and methylmercury (MeHg)], and measurement of other physicochemical factors, including pH, total organic carbon (TOC), sulfide, iron ratio (Fe3+/Fe2+), ammonium salt (NH4+), and biochemical oxygen demand (BOD). The application of 16S rRNA sequencing identified 60 phyla of bacteria, dominated by Proteobacteria, Firmicutes, and Bacteroidetes. Stations with higher THg or MeHg and lower microbial abundance and diversity were generally distributed further outside of the estuary. Besides mercury, the measured physicochemical factors had impacts on microbial diversities and distribution. Metagenomics assessment of three stations, representative of low, moderate, and high mercury concentrations and measured physicochemical factors, revealed the abundances and functions of predicted genes. The most abundant genes regulating the metabolic pathways were categorized as metabolic, environmental information processing, and genetic information processing, genes. At stations with high levels of mercury, the dominant genes were related to energy metabolism, signal transport, and membrane transport. Functional genes with a mercury-resistance function were generally in the mer system (merA, merC, merT, merR), alkylmercury lyase, and metal-transporting ATPase. These results offer insight into the microbial community structure of the sediments in the Yellow River Estuary and the microbial function of mercury resistance under mercury stress.

Water microecology is affected by seasons but not sediments: A spatiotemporal dynamics survey of bacterial community composition in Lake Changshou-The largest artificial lake in southwest China

This study aimed to evaluate the correlation between microecology of sediments and water as well as their spatial-temporal variations in Changshou Lake. The results demonstrated that microecology in the lake exhibits spatiotemporal heterogeneity, and microbial diversity of sediments was significantly higher than that of water body. Further, it was found that there was statistically insignificant positive correlation between microecology of sediments and that of water body. PCoA and community structure analysis revealed that the predominant phyla which exhibited significant spatial differences in sediments were Proteobacteria, Actinobacteria and Planctomycetes. While, the distribution of dominant bacteria Actinobacteria and Verrucomicrobia in water body showed significant seasonal differences. Microbial networks analysis indicated that there was a cooperative symbiotic relationship between lake microbial communities. Notably, the same bacterial genus had no significant positive correlation in sediment and water, which suggested that bacteria transport between sediment-water interface does not influence the microecological functions of lake water.

Trends in mercury concentrations and methylation in Minamata Bay, Japan, between 2014 and 2018

Methylmercury concentrations in Minamata Bay are high, but the cause is unclear. We conducted a basic study on the behavior of methylmercury in Minamata Bay seawater; the findings suggest that mercury methylation may occur throughout the year in Minamata Bay. Seawater temperature, salinity, and concentrations of dissolved organic carbon were the environmental factors that affected methylation, and the degree of methylation was closely related to bacterial community structure. The concentration of methylmercury in suspended particulate matter was highest 10 m below the surface and decreased with greater depths. We did not observe a correlation between methylmercury concentrations in suspended particulate matter and concentrations of dissolved methylmercury.

Risk assessment for and microbial community changes in Farmland soil contaminated with heavy metals and metalloids

Food security and human health can be seriously affected by heavy metal and metalloid (HM) pollution of soil. In this study, the risks posed by HMs and microbial community responses to HM pollution of agricultural soil in southwestern China were investigated. The C, N, P, and S (nutrients) concentrations were 12040.7-15912.7, 1298.06-1832.01, 750.91-2050.35, and 269.17-2115.52 mg/kg, respectively. The As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn concentrations were 3.11-8.20, 1.85-6.56, 22.83-43.96, 11.21-23.30, 0.08-0.81, 11.02-22.97, 24.07-42.96, and 193.63-698.39 mg/kg, respectively. Interpolation analysis indicated that the nutrient and HM concentrations varied spatially rather strongly. The concentrations of all of the elements were higher in soil from the northern sampling sites than in soil from the other sites. HMs in soil were found to pose high levels of risk (RI 898.85, i.e., >600). Cd contributed more than the other HMs to the risk assessment values (Er_Cd 293.72-1031.94), so was the most serious contaminant. Microbial diversity decreased over time in soil with high HM concentrations (plot S2) and was lower than in soil with low HM concentrations (plot S8). The nutrient and HM concentrations correlated with the microbial community characteristics. Proteobacteria, Acidobacteria, and Chloroflexi were (in decreasing order) the dominant bacterial phyla. We speculate that these phyla may be strongly resistant to HMs. The fourth most common phylum was Actinobacteria. Bacteria in this phylum could be used as biological indicators of the HM pollution status. Soil micro-ecosystems can self-regulate. HM stress will affect the evolution of soil microorganisms and relevant functional genes. The spatiotemporal variability in the microbial community responses to HMs and the spatial analysis and ecological risk assessment results will be useful reference data for the remediation of HM-polluted soil.

Removal of inorganic mercury by selective extraction and coprecipitation for determination of methylmercury in mercury-contaminated soils by chemical vapor generation inductively coupled plasma mass spectrometry (CVG-ICP-MS)

A procedure is developed for selective extraction of methylmercury (CH₃Hg⁺) from heavily Hg-contaminated soils and sediments for determination by chemical vapor generation inductively coupled plasma mass spectrometry (CVG-ICP-MS). Soils artificially contaminated with 40 μg g^-1 inorganic mercury (Hg²⁺) or methylmercury chloride (CH₃HgCl) were agitated by shaking or exposing to ultrasounds in dilute hydrochloric acid (HCl) or nitric acid (HNO₃) solutions at room temperature. Extractions in HCl (5 or 10% v/v) resulted in substantial leaching of Hg²⁺ from soils, whereas 5% (v/v) HNO₃ provided selectivity for quantitative extraction of CH₃Hg⁺ with minimum Hg²⁺ leaching. Agitation with ultrasounds in 5% (v/v) HNO₃ for about 3 min was sufficient for extraction of all CH₃Hg⁺ from soils. Coprecipitations with Fe(OH)₃, Bi(OH)₃ and HgS were investigated for removal of residual Hg²⁺ in soil extracts. Hydroxide precipitations were not effective. Thiourea or l-cysteine added to soil extracts prior to hydroxide precipitation improved precipitation of Hg²⁺, but also resulted in removal of CH₃Hg⁺. HgS precipitation was made with dilute ammonium sulfide solution, (NH₄)₂S. Adding 30 μL of 0.35 mol L^-1 (NH₄)₂S to soil extracts in 5% (v/v) HNO₃ resulted in removal of all residual Hg²⁺ without impacting CH₃Hg⁺ levels. Vapor generation was carried out by reacting Hg²⁺-free soil extracts with 1% (m/v) NaBH₄. No significant interferences were observed from (NH₄)₂S on the vapor generation from CH₃Hg⁺. The slopes of the calibration curves for CH₃HgCl standard solutions in 5% (v/v) HNO₃ with and without (NH₄)₂S were similar. Limits of detection (LOD, 3s method) were around 0.08 μg L^-1 for 5% (v/v) HNO₃ blanks (n = 10) and 0.10 μg L^-1 for 5% (v/v) HNO₃ + 0.005 mol L^-1 (NH₄)₂S blanks (n = 10). Percent relative standard deviation (%RSD) for five replicate measurements varied between 3.1% and 6.4% at 1.0 CH₃HgCl level. The method is validated by analysis of two certified reference materials (CRM); purely Methylmercury sediment (SQC1238, 10.00 ± 0.291 ng g^-1 CH₃Hg⁺) and Hg-contaminated Estuarine sediment (ERM - CC580, 75 ± 4 ng g^-1 CH₃Hg⁺ and 132 ± 3 μg g^-1 total Hg). CH₃Hg⁺ values for SQC1238 were between 13.0 and 13.2 ng g^-1, and 79 and 81 ng g^-1 for ERM - CC580. Hg-contaminated soils (57-96 μg g^-1 total Hg) collected from the floodplains of Oak Ridge, TN were analyzed for CH₃Hg⁺ using the procedure by CVG-ICPMS. CH₃Hg⁺ levels ranged from 30 to 51 ng g^-1 and did not correlate with total Hg levels (R² = 0.01).