Inputs of Terrestrial Dissolved Organic Matter Enhance Bacterial Production and Methylmercury Formation in Oxic Coastal Water

Uncovering mercury accumulation and the potential for bacterial bioremediation in response to contamination in the Singalila National Park

Several recent investigations into montane regions have reported on excess mercury accumulation in high-altitude forest ecosystems. This study explored the Singalila National Park, located on the Singalila ridge of the Eastern Himalayas, revealing substantial mercury contamination. Particular focus was on Sandakphu (3636 m), the highest peak in West Bengal, India. It harboured 6.77 ± 0.01 mg/kg of total mercury in its topsoil. Further evidence was provided by accumulation in leaves (0.040 ± 0.01 mg/kg), and roots (0.150 ± 0.008 mg/kg) of local vegetation, litterfall (0.234 ± 0.019 mg/kg), mosses (0.367 ± 0.043 mg/kg), surface water from local lakes and waterbodies (0.010 ± 0.005 mg/l), fresh snow (0.014 ± 0.004 mg/l), and sleet (0.019 ± 0.009 mg/l). Samples from other points of varying elevation in the park also demonstrated contamination. The soil displayed a range of 0.068-5.28 mg/kg, while the mean concentration in leaves was 0.153 ± 0.105 mg/kg, roots was 0.106 ± 0.054 mg/kg, and leaf litter was 0.240 ± 0.112 mg/kg. Additionally, the microbial consortia isolated from the contaminated soil displayed a high tolerance to mercuric chloride, presumably gained through repeated and consistent exposure. Four high tolerance bacterial strains, MTS2C, MTS3A, MTS4B and MTS6A, were further characterized for potential use in bioremediation strategies. Their mercury removal capacities were determined to be 82.35%, 75.21%, 61.95%, and 37.47%, respectively. Overall, the findings presented provide evidence for a highly contaminated environment in the Singalila National Park, that poses significant ecological risk to the flora, fauna and local inhabitants of this biodiversity hotspot. This research also highlights the need for further exploration and monitoring of the Eastern Himalayas for its role as a sink for atmospheric mercury.

Microorganisms Involved in Methylmercury Demethylation and Mercury Reduction are Widely Distributed and Active in the Bathypelagic Deep Ocean Waters

The ocean's mercury (Hg) content has tripled due to anthropogenic activities, and although the dark ocean (>200 m) has become an important Hg reservoir, concentrations of the toxic and bioaccumulative methylmercury (MeHg) are low and therefore very difficult to measure. As a consequence, the current understanding of the Hg cycle in the deep ocean is severely data-limited, and the factors controlling MeHg, as well as its transformation rates, remain largely unknown. By analyzing 52 globally distributed bathypelagic deep-ocean metagenomes and 26 new metatranscriptomes from the Malaspina Expedition, our study reveals the widespread distribution and expression of bacterial-coding genes merA and merB in the global bathypelagic ocean (∼4000 m depth). These genes, associated with HgII reduction and MeHg demethylation, respectively, are particularly prevalent within the particle-attached fraction. Moreover, our results indicate that water mass age and the organic matter composition shaped the structure of the communities harboring merA and merB genes living in different particle size fractions, their abundance, and their expression levels. Members of the orders Corynebacteriales, Rhodobacterales, Alteromonadales, Oceanospirillales, Moraxellales, and Flavobacteriales were the main taxonomic players containing merA and merB genes in the deep ocean. These findings, together with our previous results of pure culture isolates of the deep bathypelagic ocean possessing the metabolic capacity to degrade MeHg, indicated that both methylmercury demethylation and HgII reduction likely occur in the global dark ocean, the largest biome in the biosphere.

Soil organic matter degradation and methylmercury dynamics in Hg-contaminated soils: Relationships and driving factors

Biodegradation of soil organic matter (SOM), which involves greenhouse gas (GHG) emissions, plays an essential role in the global carbon cycle. Over the past few decades, this has become an important research focus, particularly in natural ecosystems. SOM biodegradation significantly affects contaminants in the environment, such as mercury (Hg) methylation, producing highly toxic methylmercury (MeHg). However, the potential link between GHG production from SOM turnover in contaminated soils and biogeochemical processes involving contaminants remains unclear. In this study, we investigated the dynamics of GHG, MeHg production, and the relationship between biogeochemical processes in soils from two typical Hg mining sites. The two contaminated soils have different pathways, explaining the significant variations in GHG and MeHg production. The divergence of the microbial communities in these two biogeochemical processes is essential. In addition to the microbial role, abiotic factors such as Hg species can significantly affect MeHg production. On the other hand, we found an inverse relationship between CH4 and MeHg, suggesting that carbon emission reduction policies and management could inadvertently increase the MeHg levels. This highlights the need for an eclectic approach to organic carbon sequestration and contaminant containment. These findings suggest that it is difficult to establish a general pattern to describe and explain the SOM degradation and MeHg production in contaminated soils within the specific scenarios. However, this study provides a case study and helpful insights for further understanding the links between environmental risks and carbon turnover in Hg mining areas.

Impacts of autochthonous dissolved organic matter on the accumulation of methylmercury by phytoplankton and zooplankton in a eutrophic coastal ecosystem

The bioaccumulation of methylmercury (MeHg) within the pelagic food webs is a crucial determinant of the MeHg concentration in the organisms at higher trophic levels. Dissolved organic matter (DOM) is recognized for its influence on mercury (Hg) cycling in the aquatic environment because of its role in providing metabolic substrate for heterotrophic organism and serving as a strong ligand for MeHg. However, the impact of DOM on MeHg bioaccumulation in pelagic food chains remain controversial. Here, we explored MeHg bioaccumulation within a pelagic food web in China, in the eutrophic Bohai Sea and adjacent seas, covering a range of DOM concentrations and compositions. Our findings show that elevated concentrations of dissolved organic carbon (DOC) and phytoplankton biomass may contribute to a reduction in MeHg uptake by phytoplankton. Moreover, we observe that a higher level of autochthonous DOM in the water may result in more significant MeHg biomagnification in zooplankton. This can be explained by alterations in the structure of pelagic food webs and/or an increase in the direct consumption of DOM and particulate organic matter (POM) containing MeHg. Our study offers direct field monitoring evidence of dual roles played by DOM in regulating MeHg transfers from water to phytoplankton and zooplankton in coastal pelagic food webs. A thorough understanding of the intricate interactions is essential for a more comprehensive evaluation of ecological risks associated with MeHg exposure in coastal ecosystems.

Interaction of Naturally Occurring Phytoplankton with the Biogeochemical Cycling of Mercury in Aquatic Environments and Its Effects on Global Hg Pollution and Public Health

The biogeochemical cycling of mercury in aquatic environments is a complex process driven by various factors, such as ambient temperature, seasonal variations, methylating bacteria activity, dissolved oxygen levels, and Hg interaction with dissolved organic matter (DOM). As a consequence, part of the Hg contamination from anthropogenic activity that was buried in sediments is reinserted into water columns mainly in highly toxic organic Hg forms (methylmercury, dimethylmercury, etc.). This is especially prominent in the coastal shallow waters of industrial regions worldwide. The main entrance point of these highly toxic Hg forms in the aquatic food web is the naturally occurring phytoplankton. Hg availability, intake, effect on population size, cell toxicity, eventual biotransformation, and intracellular stability in phytoplankton are of the greatest importance for human health, having in mind that such Hg incorporated inside the phytoplankton cells due to biomagnification effects eventually ends up in aquatic wildlife, fish, seafood, and in the human diet. This review summarizes recent findings on the topic of organic Hg form interaction with natural phytoplankton and offers new insight into the matter with possible directions of future research for the prevention of Hg biomagnification in the scope of climate change and global pollution increase scenarios.