Soil urease functional stability to Hg pollution: An ecotoxicological perspective

Mercury (Hg) is a persistent soil pollutant, and its toxicity can be evaluated using soil enzyme indicators. However, a thorough understanding of how the enzyme resists and remains resilient to Hg stress is essential, as it significantly impacts the accuracy of toxicity assessments. Therefore, it is worthwhile to understand the functional stability of urease in soil under Hg pollution. This study compares the effects of Hg at different concentrations and exposure times on soil urease. Results indicate that soil urease activity was enhanced in the first two hours under low levels of Hg pollution, decreased after six hours of acute Hg pollution, and reached its maximum reduction in 24 hours. The urease in fluvo-aquic soil, with higher soil organic matter showed higher resistance to Hg acute pollution than that in red soil. Over a longer aging process, soil urease activity gradually recovered with time. Hormesis effects were observed in red soil under high Hg stress after 30 days, showing the strong resilience of urease enzyme function to Hg pollution. The ecological dose, ED10, (the Hg concentration causing a 10% reduction in soil urease activity) ranged from 0.09 to 0.59 mg kg-1 under short-term exposure, and was lower than that under a longer aging process (0.28 to 2.71 mg kg-1). Further, aging reduced the Hg ecotoxicity due to decreased Hg availability and the resilience of soil urease activity. This indicates that the risk of Hg pollution estimated by soil urease as an indicator depends on exposure time and enzyme stability. These factors need consideration in heavy metal pollution assessments using soil enzymes.

Bacterial Metal-Scavengers Newly Isolated from Indonesian Gold Mine-Impacted Area: Bacillus altitudinis MIM12 as Novel Tools for Bio-Transformation of Mercury

Selikat river, located in the north part of Bengkulu Province, Indonesia, has critical environmental and ecological issues of contamination by mercury due to artisanal small-scale gold mining (ASGM) activities. The present study focused on the identification and bioremediation efficiency of the mercury-resistant bacteria (MRB) isolated from ASGM-impacted areas in Lebong Tambang village, Bengkulu Province, and analyzed their merA gene function in transforming Hg2+ to Hg0. Thirty-four MRB isolates were isolated, and four out of the 34 isolates exhibited not only the highest degree of resistance to Hg (up to 200 ppm) but also to cadmium (Cd), chromium (Cr), copper (Cu), and lead (Pb). Further analysis shows that all four selected isolates harbor a merA operon-encoded mercuric ion (Hg2+) reductase enzyme, with the Hg bioremediation efficiency varying from 71.60 to 91.30%. Additionally, the bioremediation efficiency for Cd, Cr, Cu, and Pb ranged from 54.36 to 98.37%. Among the 34, two isolates identified as Bacillus altitudinis possess effective and superior multi-metal degrading capacity up to 91.30% for Hg, 98.07% for Cu, and 54.36% for Cr. A pilot-scale study exhibited significant in situ bioremediation of Hg from gold mine tailings of 82.10 and 95.16% at 4- and 8-day intervals, respectively. Interestingly, translated nucleotide blast against bacteria and Bacilli merA sequence databases suggested that B. altitudinis harbor merA gene is the first case among Bacilli with the possibility exhibits a novel mechanism of bioremediation, considering our new finding. This study is the first to report the structural and functional Hg-resistant bacterial diversity of unexplored ASGM-impacted areas, emphasizing their biotechnological potential as novel tools for the biological transformation and adsorption of mercury and other toxic metals.

Mechanisms and biological effects of organic amendments on mercury speciation in soil-rice systems: A review

Mercury (Hg) pollution is a well-recognized global environmental and health issue and exhibits distinctive persistence, neurotoxicity, bioaccumulation, and biomagnification effects. As the largest global Hg reservoir, the Hg cumulatively stored in soils has reached as high as 250-1000 Gg. Even more concerning is that global soil-rice systems distributed in many countries have become central to the global Hg cycle because they are both a major food source for more than 3 billion people worldwide and the central bridge linking atmospheric and soil Hg circulation. In this review, we discuss the form distribution, transformation, and bioavailability of Hg in soil-rice systems by focusing on the Hg methylation and demethylation pathways and distribution, uptake, and accumulation in rice plants and the effects of Hg on the community structure and ecological functions of microorganisms in soil-rice systems. In addition, we clarify the mechanisms through which commonly used humus and biochar organic amendments influence Hg and its environmental effects in soil-rice systems. The review also elaborates on the advantages of sulfur-modified biochars and their critical role in controlling Hg migration and bioavailability in soils. Finally, we provide key information about Hg pollution in soil-rice systems, which is of great significance for developing appropriate strategies and mitigation planning to limit Hg bioconcentration in rice crops and achieving key global sustainable development goals, such as the guarantee of food security and the promotion of sustainable agriculture.

Efficient treatment of mercury(Ⅱ)-containing wastewater in aerated constructed wetland microcosms packed with biochar

Effective removal of mercury (Hg) pollutants from contaminated water/wastewater to prevent severe environmental pollution is of great significance due to the extremely high toxicity of Hg. In this study, granular biochar and gravel (control) were packed into intermittently aerated constructed wetland (CW) microcosms to treat Hg(Ⅱ)-containing wastewater over 100 d. The results showed that the biochar-filled CWs exhibited notably better Hg(Ⅱ) removal than the gravel systems by facilitating chemical and microbial Hg(Ⅱ) reduction and volatilization and promoting plant growth and Hg assimilation. More than ten times more Hg was absorbed by the plants (L. salicaria) in biochar CWs than in the gravel systems, with the roots acting as the major sink. In contrast, substrate binding in a predominantly oxidizable fraction was the dominant pathway for Hg removal in the gravel CWs. Biochar substrates also exhibited higher levels of COD, N and P removal, and Hg(Ⅱ) import impacted the removal of these pollutants only slightly. Filling material played a more crucial role than Hg input in shaping the microbial communities in the CWs. The proportions of some dominant genera, including Arenimonas, Lysobacter, Micropruina and Hydrogenophaga, increased in the presence of Hg, implying their tolerance to Hg toxicity and potential roles in Hg detoxification in the CWs. Granular biochar-based CW has high potential for treating Hg(Ⅱ)-contaminated wastewater.

Interactions between Hg and soil microbes: microbial diversity and mechanisms, with an emphasis on fungal processes

Mercury (Hg) is a highly toxic metal with no known biological function, and it can be highly bioavailable in terrestrial ecosystems. Although fungi are important contributors to a number of soil processes including plant nutrient uptake and decomposition, little is known about the effect of Hg on fungi. Fungi accumulate the largest amount of Hg and are the organisms capable of the highest bioaccumulation of Hg. While referring to detailed mechanisms in bacteria, this mini-review emphasizes the progress made recently on this topic and represents the first step towards a better understanding of the mechanisms underlying Hg tolerance and accumulation in fungal species and hence on the role of fungi within the Hg cycle at Hg-contaminated sites. KEY POINTS: • The fungal communities are more resilient than bacterial communities to Hg exposure. • The exposure to Hg is a threat to microbial soil functions involved in both C and nutrient cycles. • Fungal (hyper)accumulation of Hg may be important for the Hg cycle in terrestrial environments. • Understanding Hg tolerance and accumulation by fungi may lead to new remediation biotechnologies.

Water-soluble mercury induced by organic amendments affected microbial community assemblage in mercury-polluted paddy soil

Mercury (Hg) pollution or organic amendments (OA) may individually induce changes in the microbial community of paddy soils. However, little is known regarding the interaction of Hg and OA and the effect of different OA applications on the microbial community assemblage in Hg-polluted paddy soil. A soil incubation experiment was performed by applying three organic amendments (OA), namely a food-waste compost (FC), and its HA and FA, into an Hg-polluted paddy soil to examine the changes in the microbial community and merA/merB gene abundance. The results showed that the OA treatments promoted total (SOC) and dissolved organic carbon (DOC) in soils, which may harbor copiotrophic bacteria. The HA and FA treatments decreased microbial diversity and richness along with an increase of water-soluble Hg (WHg) through the complexation of DOC to Hg, which may be mainly attributed to the enhanced Hg biotoxicity to soil microbiome induced by the increased WHg under these two treatments. Additionally, the WHg enhancement also contributed to the increase of Hg-resistant bacteria and merA/merB gene abundance, and consequently, induced changes in the microbial community. These results indicated the interaction of Hg and different OA induced the variation of WHg fraction in paddy soil, which played a fundamental role in the distinct responses of the microbial community assemblage. Collectively, the application of FA and HA to Hg-polluted soil should be limited considering Hg risk to microbiome, and FC can be an alternative.

Oligotrophic wetland sediments susceptible to shifts in microbiomes and mercury cycling with dissolved organic matter addition

Recent advances have allowed for greater investigation into microbial regulation of mercury toxicity in the environment. In wetlands in particular, dissolved organic matter (DOM) may influence methylmercury (MeHg) production both through chemical interactions and through substrate effects on microbiomes. We conducted microcosm experiments in two disparate wetland environments (oligotrophic unvegetated and high-C vegetated sediments) to examine the impacts of plant leachate and inorganic mercury loadings (20 mg/L HgCl₂) on microbiomes and MeHg production in the St. Louis River Estuary. Our research reveals the greater relative capacity for mercury methylation in vegetated over unvegetated sediments. Further, our work shows how mercury cycling in oligotrophic unvegetated sediments may be susceptible to DOM inputs in the St. Louis River Estuary: unvegetated microcosms receiving leachate produced substantially more MeHg than unamended microcosms. We also demonstrate (1) changes in microbiome structure towards Clostridia, (2) metagenomic shifts toward fermentation, and (3) degradation of complex DOM; all of which coincide with elevated net MeHg production in unvegetated microcosms receiving leachate. Together, our work shows the influence of wetland vegetation in controlling MeHg production in the Great Lakes region and provides evidence that this may be due to both enhanced microbial activity as well as differences in microbiome composition.

Comparative Investigation of Bacterial, Fungal, and Archaeal Community Structures in Soils in a Typical Oilfield in Jianghan, China

Agricultural soils in oilfields have high risk for polycyclic aromatic hydrocarbon (PAH) pollution. In this study, from the Jianghan Oilfield (Hubei Province, China) with a history of >50 years, 7 soil samples (OS-1 to OS-7) were collected. Subsequently, the bacterial, archaeal, and fungal community structures were investigated by Illumina MiSeq sequencing, and the relationship between microbial community structure and soil PAH content was analyzed. The results indicated that bacterial and archaeal Chao 1 indices showed a significantly negative relationship with soil PAH content, and only the bacterial Shannon index had a significantly negative relationship with soil PAH content. Moreover, the community structure of bacteria (r ² = 0.9001, p = 0.013) showed a stronger correlation with PAH content than that of fungi (r ² = 0.7357, p = 0.045), and no significant relationship was found between archaeal community structure (r ² = 0.4553, p = 0.262) and soil PAH content. In addition, the relative greater abundances of some bacterial genus belonging to Actinobacteria (Mycobacterium and Micromonospora) and Proteobacteria (Pseudomonas, Lysobacter, Idiomarina, Oxalobacteraceae, and Massilia), fungal genus belonging to Ascomycota (Sordariales and Pleosporales), and archaeal phylum (Euryarchaeota) were detected in the soil samples (OS-3 and OS-5) with greater PAH content. In summary, soil PAHs showed an obvious influence and selectivity on the soil microbiota. Furthermore, compared with fungi and archaea, bacteria was more sensitive to soil PAH pollution, and the diversity indices and community structure of bacteria both might be suitable indicators for assessment of soil PAH stress on the soil ecosystem.

Assessment of heavy metal contamination and Hg-resistant bacteria in surface water from different regions of Delhi, India

The present study aims to monitor the surface water quality of different regions in Delhi (India). With many physical and chemical properties, all samples had a high load of pollution in which Najafgarh drain (Nd) exhibited maximum and laboratory tap water (Ltw) minimum contamination. Water samples contained notable amounts of heavy metals including Cr, Cd, As, Cu, Pb and Hg. A total of 88 Hg-resistant bacteria were isolated from all the regions except Ltw. Among all the samples, the density of Hg-resistant bacteria was highest in sample of Nd and their morphotype heterogeneity was highest in sample collected from river Yamuna nearby Kashmiri gate (Kg). Different strains showed different patterns of resistance to different heavy metals and antibiotics. Multiple antibiotic resistance (MAR) indices were high in two samples, the highest reported in a sample taken from river Yamuna nearby Majnu ka tila (Mkt) (0.34). The 12.5% and 24.45% isolates showed β- and α-hemolytic natures, respectively that might be of pathogenic concern. In this account, high concentrations of heavy metals and their resistant bacteria in surface water have severely damaged the quality of water and their resources and produced high risk to the associated life forms.

Applications of organic and inorganic amendments induce changes in the mobility of mercury and macro- and micronutrients of soils

Both soil organic matter and sulfur (S) can reduce or even suppress mercury (Hg) mobility and bioavailability in soil. A batch incubation experiment was conducted with a Chernozem and a Luvisol artificially contaminated by 440 mg·kg⁻¹ Hg showing wide differences in their physicochemical properties and available nutrients. The individual treatments were (i) digestate from the anaerobic fermentation of biowaste; (ii) fly ash from wood chip combustion; and (iii) ammonium sulfate, and every treatment was added with the same amount of S. The mobile Hg portion in Chernozem was highly reduced by adding digestate, even after 1 day of incubation, compared to control. Meanwhile, the outcome of these treatments was a decrease of mobile Hg forms as a function of incubation time whereas the contents of magnesium (Mg), potassium (K), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), and phosphorus (P) were stimulated by the addition of digestate in both soils. The available calcium (Ca) contents were not affected by the digestate addition. The experiment proved digestate application as the efficient measure for fast reduction of mobile Hg at extremely contaminated soils. Moreover, the decrease of the mobile mercury portion was followed by improvement of the nutrient status of the soils.

Enzymes as useful tools for environmental purposes

In the environment enzymes may play important and different roles at least in three cases: as main agents (as isolated, cell-bound or immobilized enzymes) in charge of either the transformation and/or degradation of compounds polluting the environment and the restoration of the polluted environment; as reliable and sensitive tools to detect and measure the amount and concentration of pollutants before, during and after the restoration process; as reliable, easy and sensitive indicators of quality and health status of the environment subjected to the restoration process. To our knowledge papers or reviews integrating findings on these three functions of enzymes are missing in literature. Therefore the main scope of the present paper is to briefly encompass general and specific concepts about roles of enzymes as decontaminating agents, pollutant assaying agents and indicators of environment safety. Examples chosen among those published very recently, supporting and confirming peculiarities, features, and performance of enzymatic agents will be illustrated.

Mercury-resistant bacteria from salt marsh of Tagus Estuary: the influence of plants presence and mercury contamination levels

Mercury (Hg) contamination of aquatic systems has been recognized as a global, serious problem affecting both wildlife and humans. High levels of Hg, in particular methylmercury (MeHg), were detected in surface sediments of Tagus Estuary. MeHg is neurotoxic and its concentration in aquatic systems is dependent upon the relative efficiency of reduction, methylation, and demethylation processes, which are mediated predominantly by the microbial community, in particular mercury-resistant (HgR) bacteria. Plants in contaminated ecosystems are known to take up Hg via plant roots. Therefore, the aims of this study were to (1) isolate and characterize HgR bacteria from a salt marsh of Tagus Estuary (Rosário) and (2) determine HgR bacteria levels in the rhizosphere and, consequently, their influence in metal cycling. To accomplish this objective, sediments samples were collected during the spring season in an area colonized by Sacocornia fruticosa and Spartina maritima and compared with sediments without plants. From these samples, 13 aerobic HgR bacteria were isolated and characterized morphologically, biochemically, and genetically, and susceptibility to Hg compounds, Hg(2+), and MeHg was assessed by determination of minimal inhibitory concentration (MIC). Genetically, the mer operon was searched by polymerase chain reaction (PCR) and 16S rRNA sequencing was used for bacterial identification. Results showed that the isolates were capable of growing in the presence of high Hg concentration with MIC values for HgCl2 and MeHgCl in the ranges of 1.7-4.2 μg/ml and 0.1-0.9 μg/ml, respectively. The isolates from sediments colonized with Sacocornia fruticosa displayed higher resistance levels compared to ones colonized with Spartina maritima. Bacteria isolates showed different capacity of Hg accumulation but all displayed Hg volatilization capabilities (20-50%). Mer operon was found in two isolates, which genetically confirmed their capability to convert Hg compounds by reducing them to Hg(0). Thus, these results are the first evidence of the relevance of interaction between bacteria and plants in Hg cycling in Tagus Estuary.

Isolation and characterization of mercury-resistant bacteria from sediments of Tagus Estuary (Portugal): implications for environmental and human health risk assessment

Mercury (Hg) contamination of aquatic systems has been recognized as a global and serious problem affecting both human and environmental health. In the aquatic ecosystems, mercurial compounds are microbiologically transformed with methylation responsible for generation of methylmercury (MeHg) and subsequent biomagnification in food chain, consequently increasing the risk of poisoning for humans and wildlife. High levels of Hg, especially MeHg, are known to exist in Tagus Estuary as a result of past industrial activities. The aim of this study was to isolate and characterize Hg-resistant bacteria from Tagus Estuary. Mercury-resistant (Hg-R) bacteria were isolated from sediments of two hotspots (Barreiro and North Channel) and one reserve area (Alcochete). Mercury contamination in these areas was examined and bacterial susceptibility to Hg compounds evaluated by determination of minimal inhibitory concentrations (MIC). The isolates characterization was based on morphological observation and biochemical testing. Bacteria characteristics, distribution, and Hg resistance levels were compared with metal levels. Barreiro and North Channel were highly contaminated with Hg, containing 126 and 18 μg/g total Hg, respectively, and in Alcochete, contamination was lower at 0.87 μg/g total Hg. Among the isolates there were aerobic and anaerobic bacteria, namely, sulfate-reducing bacteria, and Hg resistance levels ranged from 0.16 to 140 μg/ml for Hg(2+) and from 0.02 to 50.1 μg/ml for MeHg. The distribution of these bacteria and the resistance levels were consistent with Hg contamination along the depth of the sediments. Overall, results show the importance of the characterization of Tagus Estuary bacteria for ecological and human health risk assessment.

Responses of activities, abundances and community structures of soil denitrifiers to short-term mercury stress

The responses of activities, abundances and community structures of soil denitrifiers to mercury (Hg) stress were investigated through a short-term incubation experiment. Four soil treatments with different concentrations of Hg (CK, Hg25, Hg50, and Hg 100, denoted as 0, 25, 50, and 100 mg Hg/kg dry soil, respectively) were incubated for 28 days. Soil denitrification enzyme activity (DEA) was measured at day 3, 7 and 28. The abundances and community structures of two denitrification concerning genes, nirS (cd(1)-nitrite reductase gene) and nosZ (nitrous oxide reductase gene), were analyzed using real-time PCR and denaturing gradient gel electrophoresis (DGGE). Results showed that soil DEA was significantly stimulated in the treatments of Hg25 and Hg50 compared with others at day 7. Meanwhile, no difference in the abundances of soil nirS and nosZ was found between Hg spiked treatments and CK, except the lower abundance of nirS (P < 0.05) in the Hg added treatments compared with that in the CK at day 28. The community structures of denitrifiers based on nirS gene presented obvious change at day 7 along with the Hg additions, however, no variation was found in all treatments based on the nosZ gene. The results indicated that Hg (Hg25 and Hg50) had a strongly short-term stimulation on soil DEA, and nirS gene is more sensitive than nosZ gene to Hg stress.