Mercury in a stream-lake network of Andean Patagonia (Southern Volcanic Zone): Partitioning and interaction with dissolved organic matter

Spatiotemporal variation in methylmercury and related water quality variables in a temperate river under highly dynamic hydrologic conditions

The understanding of the essential environmental factors affecting the spatiotemporal variation in methylmercury (MeHg) in river water is limited to date, despite its importance for predicting the effect of ongoing climate change on MeHg accumulation in freshwater ecosystems. This study aimed to explore the variation in MeHg concentration and related environmental factors in the downstream zone of the Yeongsan River under highly dynamic hydrologic conditions by using water quality and hydrologic data collected from 1997 to 2022, and Hg and MeHg data collected from 2017 to 2022. The mean concentration of unfiltered MeHg was 35.7 ± 13.7 pg L-1 (n = 24) in summer and 26.7 ± 7.43 pg L-1 (n = 24) in fall. Dissolved oxygen (DO), conductivity, nitrate, and dissolved organic carbon (DOC) were determined to be the most influential variables in terms of MeHg variation based on the partial least squares regression model, and their effects on the MeHg concentration were negative, except for DOC. Heatmaps representing the similarity distances between temporal trends of hydrologic and water quality variables were constructed to determine fundamental factors related to the time-based variations in DO, conductivity, nitrate, and DOC using a dynamic time warping algorithm. The heatmap cluster analysis showed that the temporal trends of these variables were closely related to rainfall variation rather than irradiance or water temperature. Overall, biogeochemical factors directly related to in situ methylation rate of Hg(II)-rather than transport of Hg(II) and MeHg from external sources-mainly control the spatiotemporal variation of MeHg in the downstream zone of the Yeongsan River.

Particles-involved photochemical processes: A review for the case of mercury reduction in relation to aquatic mercury cycling

Mercury (Hg) is one of the toxic metals of global and environmental concern, with aquatic Hg cycling being central in determining the production of highly toxic methylmercury and the air-water Hg exchange influencing the long-range intercontinental atmospheric Hg transport. Both inorganic and organic forms of Hg can be bound by suspended particles, including inorganic minerals (in particular metal oxides/sulfides) and particulate organic matter. Photochemical transformation is a critical process in surface water, and the role of suspended particles in Hg redox photoreactions has increasingly emerged, albeit in limited studies in comparison to extensive studies on aqueous (homogeneous) photoreactions of Hg. The lack of understanding of what roles suspended particles play might result in inaccurate estimation of how Hg species transform and/or cycle in the environment. In view of this gap, this paper critically reviews and synthesizes information on the studies conducted on different natural surface waters with respect to the potential roles of suspended particles on Hg photo-redox reactions. It robustly discusses the various possible pathways and/or mechanisms of particle-mediated Hg (II) reduction, in enhancing or lowering the production of dissolved gaseous mercury. These processes include photo hole-electron pair formation and reactive oxygen species generation from particle excitation and their involvement in Hg photoreduction, in addition to the light attenuation effect of particles. This paper highlights the necessity of future studies exploiting these particles-mediated Hg photoreactions pathways and the implications of including these heterogeneous photoreactions (together with particulate elemental Hg species) on the air-water Hg exchange estimation.

A synthesis of mercury research in the Southern Hemisphere, part 1: Natural processes

Recent studies demonstrate a short 3-6-month atmospheric lifetime for mercury (Hg). This implies Hg emissions are predominantly deposited within the same hemisphere in which they are emitted, thus placing increasing importance on considering Hg sources, sinks and impacts from a hemispheric perspective. In the absence of comprehensive Hg data from the Southern Hemisphere (SH), estimates and inventories for the SH have been drawn from data collected in the NH, with the assumption that the NH data are broadly applicable. In this paper, we centre the uniqueness of the SH in the context of natural biogeochemical Hg cycling, with focus on the midlatitudes and tropics. Due to its uniqueness, Antarctica warrants an exclusive review of its contribution to the biogeochemical cycling of Hg and is therefore excluded from this review. We identify and describe five key natural differences between the hemispheres that affect the biogeochemical cycling of Hg: biome heterogeneity, vegetation type, ocean area, methylation hotspot zones and occurence of volcanic activities. We review the current state of knowledge of SH Hg cycling within the context of each difference, as well as the key gaps that impede our understanding of natural Hg cycling in the SH. The differences demonstrate the limitations in using NH data to infer Hg processes and emissions in the SH.

Dynamics of mercury in the plankton of a hydroelectric reservoir, Western Amazon

The energy transfer in the aquatic food chain is an important way for mercury (Hg) to enter other trophic levels. The objective of this work was to evaluate the Hg concentrations in plankton upstream and downstream of the Samuel Hydroelectric Reservoir, Rondônia, Brazil. Phytoplankton and zooplankton samples were collected with 20-μm and 68-μm nylon nets. An aliquot was removed for taxonomic analysis and another for total mercury determination, performed by cold vapor atomic absorption spectroscopy. Water physical-chemical parameters were also measured. The Hg concentrations in total plankton (phytoplankton and zooplankton samples) obtained at the three sampling upstream stations showed the same behavior, with the highest values registered in June 2005 (232 μg kg^-1, 118 μg kg^-1, 128 μg kg^-1). The lowest values at stations J1 and M1 were recorded in November 2005 (4 μg kg^-1 and 22 μg kg^-1, respectively), while the lowest values at stations M4 and M8 were recorded in October 2005 (22 μg kg^-1 and 5 μg kg^-1, respectively). The Hg results found in the plankton in this study corroborate the results of other recent studies in the same region. The statistical analyses revealed that Hg concentrations in plankton do not explain the distribution of these organisms at the four sampling stations of Samuel Reservoir. Graphical Abstract.

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.

Organic amendments affect dissolved organic matter composition and mercury dissolution in pore waters of mercury-polluted paddy soil

Organic amendments (OA) have been applied in many mercury (Hg)-polluted paddy soils to meet increasing food demands with scarce land resources. However, little is known on the effects of different OAs on Hg dissolution and the composition of dissolved organic matter (DOM) in soil pore waters, both of which may be associated with Hg mobility. Consequently, DOM composition and Hg release levels were investigated in soil pore waters after applying food waste compost (FC), fulvic acids (FA) and humic acids (HA) to Hg-polluted paddy soils. FA and HA treatments promoted increased abundances of humic- and fulvic-like substances in pore water DOM while FC amendment increased soluble microbial by-products. FA amendment and high levels of both HA and FC amendments greatly promoted Hg dissolution in pore waters that could be attributed to the complexation of Hg with different DOM components. However, among all DOM components, only UVA fulvic and visible humic-like substances were positively correlated with Hg release levels and total organic carbon. These results indicate that discrepant DOM compositions are induced by different OA. Further, these differences may be associated with differential Hg dissolution in pore waters. Consequently, FA amendment and high level of FC or HA amendments should be limited to reduce potential Hg release into pore waters.

The microbial mercury link in oligotrophic lakes: Bioaccumulation by picocyanobacteria in natural gradients of dissolved organic matter

Andean Patagonian lakes are oligotrophic systems characterized by low dissolved organic carbon (DOC) levels and moderate to high Hg concentration that determine naturally high Hg/DOC ratios and bioavailability. In these lakes, microbial food webs are extremely important in Hg trophodynamics, being that the picophytoplankton fraction is a major entrance path of Hg²⁺ into pelagic food webs. This study analyzed the bioaccumulation of Hg²⁺ by the picocyanobacteria Synechococcus sp. using the radiotracer ¹⁹⁷Hg²⁺ and water from four Andean Patagonian lakes presenting a natural gradient of DOM concentration and quality. Hg²⁺ bioaccumulation by Synechococcus was calculated as the uptake of Hg²⁺ per biovolume unit (volume concentration factor VCF; pL μm^-3). Hg uptake showed a wide variation (13 < VCF< 300 pL μm^-3) in the natural DOC gradient tested (0.7-4 mg L^-1; Hg²⁺/DOC ratio: 1.8-14 ng mg^-1). The bioaccumulation of Hg²⁺ in Synechococcus decreased exponentially with DOC concentration. Differences in the quality of dissolved organic matter (DOM) among lake water influenced also Hg²⁺ bioaccumulation. Naturally degraded DOM, with low molecular weight/size, promoted higher Hg uptakes in Synechococcus compared to humic DOM, rich in high molecular weight/size aromatic compounds, that retained Hg in the dissolved phase. In Andean Patagonian lakes picocyanobacteria are pivotal organisms in the Hg cycling, taking dissolved Hg²⁺ and transferring it to pelagic food webs, as well as fueling the benthic Hg pathway through sedimentation.

Purification of water contaminated with Hg using horizontal subsurface constructed wetlands

As a global pollutant, Hg (Hg) since the turn of the last century has received increased attention. Decreasing the emission of Hg into the food chain and the atmosphere is an effective way to reduce the Hg damage. The current study provided information about pilot-scale horizontal subsurface flow (HSSF) constructed wetlands (CWs) to remove different Hg species in polluted water. Synthetic wastewater was fed to two HSSF CWs, one was planted with Acorus calamus L and the other was unplanted as a control. The total Hg (THg), dissolved Hg (DHg), and particulate Hg (PHg) from five sites along the HSSF CWs were analyzed to describe the process of Hg removal. Results show that the CWs have high removal efficiency of Hg which is more than 90%. The removal efficiencies of THg and DHg from the unplanted CW were 92.1 ± 3.6% and 72.4 ± 13.1%, respectively. While, the removal efficiencies of THg and DHg in planted CW were 95.9 ± 7.5% and 94.9 ± 4.9%, which were higher than that in blank CW. The PHg was mainly removed in the first quarter of the CWs, which was also revealed by the partition coefficient K_d. To a certain extent, the effect of plants depends on the hydraulic retention time (HRT). The results in the current study show the potential of the HSSF-CWs for restoration from Hg-contaminated water.

Natural levels and photo-production rates of hydrogen peroxide (H2O2) in Andean Patagonian aquatic systems: Influence of the dissolved organic matter pool

In aquatic environments the reactive oxygen species hydrogen peroxide (H₂O₂) is produced through photochemical reactions involving chromophoric dissolved organic matter (CDOM). Andean Patagonian freshwaters experience challenging underwater UV levels, which promote high levels of photochemical weathering. In this investigation, we study natural H₂O₂ levels and experimentally address the photochemical formation of H₂O₂ in stream and lake water with a range of dissolved organic matter (DOM) concentrations and quality. The screening of different pristine aquatic systems of Patagonia revealed that H₂O₂ concentration fluctuates between 8 and 60 nM. Laboratory incubation of different water types in PAR + UV showed photo-production of H₂O₂. The H₂O₂ formation rate increased linearly with dissolved organic carbon (DOC) in streams (13.5-20.5 nM h^-1) and shallow lakes (25.7-37.8 nM h^-1). In contrast, the H₂O₂ formation rate in deep lakes was much lower (2.1-7.1 nM h^-1), and decreased with DOC. The natural potential for H₂O₂ formation was related to the concentration and quality of the DOM pool. At higher DOC levels, such as those present in shallow lakes, H₂O₂ production was directly related to DOC, whereas at low DOC levels in deep lakes and streams, two patterns were distinguished in relation to their DOM pool quality. Stream DOM, composed of high molecular weight/size humic compounds, proved to be a reactive substrate, as reflected by their high H₂O₂ formation rates. On the other hand, deep lake DOM, with its higher relative contribution of small and more processed compounds, was found to be a less reactive substrate, affording lower H₂O₂ formation rates.

Inorganic mercury (Hg2+) accumulation in autotrophic and mixotrophic planktonic protists: Implications for Hg trophodynamics in ultraoligotrophic Andean Patagonian lakes

Microbial assemblages are typical of deep ultraoligotrophic Andean Patagonian lakes and comprise picoplankton and protists (phytoflagellates and mixotrophic ciliates), having a central role in the C cycle, primary production and in the incorporation of dissolved inorganic mercury (Hg²⁺) into lake food webs. In this study we evaluated the mechanisms of Hg²⁺ incorporation in hetero- and autotrophic bacteria, in the autotrophic dinoflagellate (Gymnodinium paradoxum) and in two mixotrophic ciliates (Stentor araucanus and Ophrydium naumanni) dominating the planktonic microbial assemblage. The radioisotope ¹⁹⁷Hg was used to trace the Hg²⁺ incorporation in microbiota. Hg uptake was analyzed as a function of cell abundance (BCF: bioconcentration factor), cell surface (SCF: surface concentration factor) and cell volume (VCF: volume concentration factor). Overall, the results obtained showed that these organisms incorporate substantial amounts of dissolved Hg²⁺ passively (adsorption) and actively (bacteria consumption or attachment), displaying different Hg internalization and therefore, varying potential for Hg transfer. Surface area and quality, and surface:volume ratio (S:V) control the passive uptake in all the organisms. Active incorporation depends on bacteria consumption in the mixotrophic ciliates, or on bacteria association to surface in the autotrophic dinoflagellate. Hg bioaccumulated by pelagic protists can be transferred to higher trophic levels through plankton and fish feeding, regenerated to the dissolved phase by excretion, and/or transferred to the sediments by particle sinking. In ultraoligotrophic Andean Patagonian lakes, picoplankton and planktonic protists are key components of lake food webs, linking the pelagic and benthic Hg pathways, and thereby playing a central role in Hg trophodynamics.