Photodegradation of methylmercury in Jialing River of Chongqing, China.

Photodegradation (PD) of methylmercury (MMHg) is a key process of mercury (Hg) cycling in water systems, maintaining MMHg at a low level in water systems. However, we possess little knowledge of this important process in the Jialing River of Chongqing, China. In situ incubation experiments were thus performed to measure temporal patterns and influencing factors of MMHg PD in this river. The results showed that MMHg underwent a net demethylation process under solar radiation in the water column, which predominantly occurred in surface waters. For surface water, the highest PD rate constants were observed in spring (12×10(-3)±1.5×10(-3) m2/E), followed by summer (9.0×10(-3)±1.2×10(-3) m2/E), autumn (1.4×10(-3)±0.12×10(-3) m2/E), and winter (0.78×10(-3)±0.11×10(-3) m2/E). UV-A radiation (320-400 nm), UV-B radiation (280-320 nm), and photosynthetically active radiation (PAR, 400-700 nm) accounted for 43%-64%, 14%-31%, and 16%-45% of MMHg PD, respectively. PD rate constants varied substantially with the treatments that filtered the river water and amended it with chemicals (i.e., Cl-, NO3-, dissolved organic matter (DOM), Fe(III)), which reveals that suspended particulate matter and water components are important factors in affecting the PD process. For the entire water column, the PD rate constant determined for each wavelength range decreased rapidly with water depth. UV-A, UV-B, and PAR contributed 27%-46%, 6.2%-12%, and 42%-65% to the PD process, respectively. PD flux was estimated to be 4.7 µg/(m2·year) in the study site. Our results are very important to understand the cycling characteristics of MMHg in the Jialing River of Chongqing, China.

Light-induced degradation of dimethylmercury in different natural waters.

Photo-induced degradation of dimethylmercury (DMHg) is considered to be an important source for the generation of methylmercury (MMHg). However, studies on DMHg photodegradation are scarce, and it is even debatable about whether DMHg can be degraded in natural waters. Herein, we found that both DMHg and MMHg could be photodegraded in three natural waters collected from the Yellow River Delta, while in pure water only DMHg photodegradation occurred under visible light irradiation. The effects of different environmental factors on DMHg photodegradation were investigated, and the underlying mechanisms were elucidated by density functional theory calculations and a series of control experiments. Our findings revealed that the DMHg degradation rate was higher in the tidal creek water compared to Yellow River, Yan Lake, and purified water. NO3-, NO2-, and DOM could promote the photodegradation with DOM and NO3- showing particularly strong positive effects. Different light sources were employed, and UV light was found to be more effective in DMHg photodegradation. Moreover, MMHg was detected during the photodegradation of DMHg, confirming that the photochemical demethylation of DMHg is a source of MMHg in sunlit water. This work may provide a novel mechanistic insight into the DMHg photodegradation in natural waters and enrich the study of the global biogeochemical cycle of Hg.

A new pathway of monomethylmercury photodegradation mediated by singlet oxygen on the interface of sediment soil and water.

Photodegradation is an important pathway for monomethylmercury (MeHg) degradation in aquatic ecosystems. In this process, dissolved organic matter (DOM) plays an essential role. However, little information is available regarding the photo-transformation of MeHg in shallow aquatic environments, where a significant portion of MeHg is associated with soil suspensions. In this study, 14 soils sampled from different sites in China were used to simulate these conditions. Our results clearly demonstrated that soil organic matter (SOM) was the most important factor controlling the MeHg photodegradation in suspension. Degradation in this heterogeneous aqueous system was shown to be mediated by the 1O2 produced by organic matter on the surface of the soil particles rather than by DOM. This was confirmed by the strong correlation between the kinetics rate constant of MeHg degradation and steady state concentrations of 1O2 (R2 = 0.81). Our results propose a new pathway of MeHg induced by sediment soils under sunlight irradiation. Identification of this pathway may improve the estimates of potential ecological risk of Hg in shallow field ecosystems.

Probing the DOM-mediated photodegradation of methylmercury by using organic ligands with different molecular structures as the DOM model.

Photodegradation is the main depletion pathway for methylmercury (MeHg) in surface water. The formation of MeHg-dissolved organic matter (DOM) complexes has been found to be a key step in MeHg photodegradation. However, the major functional groups involved in the DOM-mediated process have yet to be clearly resolved. In this work, we systematically investigated the effects of DOM molecular structures on MeHg photodegradation by using a variety of organic ligands with different functional groups (e.g., thiosalicylate, thiophenol, and thioaniline). The results showed that thiol and phenyl groups may be the major functional groups governing DOM-mediated MeHg photodegradation, with photodegradation rates also dependent on the type (carboxyl, hydroxyl, and amino group) and position (ortho-, meta-, and para-) of other chemical substituents. The addition of "non-photochemically active" thiol ligands (e.g., mercaptoethanol and dithiothreitol) and high concentrations of Cl- can significantly inhibit the o-thiosalicylate-induced MeHg photodegradation, indicating that complexation of MeHg with these ligands is necessary for MeHg photodegradation. Sparging with O2 had a negligible effect on MeHg photodegradation, while sparging with N2 significantly enhanced MeHg photodegradation. This finding suggests that MeHg photodegradation may be a reductive process, which was further supported by identification of the degradation products of MeHg. A possible protonolysis mechanism of MeHg photodegradation in the presence of o-thiosalicylate was then proposed based on the findings of this study.

Effects of photodemethylation on the methylmercury budget of boreal Norwegian lakes.

Methylmercury (MeHg) concentrations in freshwater fish from southeastern Norway continue to increase, highlighting the need for a comprehensive understanding of MeHg sources, cycling, and degradation in the aquatic environment. The authors assessed the importance of photodemethylation in the MeHg budget of 4 Norwegian lakes. Photodemethylation rates were determined using incubation experiments with MeHg-spiked natural lake water. The authors determined full-spectrum exposure rates at all study sites and waveband-specific rates (photosynthetically active radiation, ultraviolet-A radiation, and ultraviolet-B radiation) at 1 clear-water (Sognsvann) and 1 humic (Langtjern) site. No significant differences in photodemethylation rates between the sites were found, and the authors' observed rates agreed with available literature for lake and wetland waters. The authors paired experimentally derived photodemethylation rates with lake-specific incident irradiation, light attenuation, and MeHg concentrations to estimate MeHg loss through photodemethylation for the study sites. For Langtjern, losses through photodemethylation equalled 27% of total annual inputs, highlighting the importance of photodemethylation in the MeHg budget. Furthermore, the authors assessed how changes in terrestrial dissolved organic carbon (DOC) exported to freshwaters and climate-driven reductions in ice cover duration may affect MeHg losses through photodemethylation. Results suggest that future increases in DOC may lead to higher aqueous MeHg concentrations in boreal lakes due to increased DOC-associated MeHg inputs paired with significant decreases in the loss of MeHg through photodemethylation due to increased light attenuation.

Photodecomposition of methylmercury in an Arctic Alaskan lake.

Sunlight-induced decomposition of monomethylmercury (MMHg) reduces its availability for accumulation in aquatic food webs. We examined MMHg degradation in epilimnetic waters of Toolik Lake (68 degrees 38' N, 149 degrees 36' W) in arctic Alaska, a region illuminated by sunlight almost continuously during the summer. MMHg decomposition in surface water of Toolik Lake is exclusively abiotic and mediated by sunlight; comparable rates of MMHg decomposition were observed in filter-sterilized and unfiltered surface waters incubated under in situ sunlight and temperature conditions, and no MMHg was degraded in unfiltered aliquots incubated in the dark. Rates of photodecomposition are first order with respect to both MMHg concentration and the intensity of photosynthetically active radiation (PAR), except at the lake surface where rates of photochemical degradation are enhanced relative to PAR intensity and may be attributed to an additional influence of ultraviolet light. The estimated annual loss of MMHg to photodecomposition in Toolik Lake, though limited to a 100-d ice-free season, accounts for about 80% of the MMHg mobilized annually from in situ sedimentary production, the primary source in Toolik Lake. These results suggest that greater light attenuation in lacustrine surface waters, a potential result of increased loadings of dissolved organic matter due to continued warming in the Arctic, may result in less photodecomposition and subsequently greater availability of MMHg for bioaccumulation.

Biogeochemical cycling of methylmercury in lakes and tundra watersheds of Arctic Alaska.

The fate of atmospherically deposited and environmentally active Hg is uncertain in the Arctic, and of greatest toxicological concern is the transformation to monometh-ylmercury (MMHg). Lake/watershed mass balances were developed to examine MMHg cycling in four northern Alaska lakes near the ecological research station at Toolik Lake (68 degrees 38' N, 149 degrees 36' W). Primary features of the cycle are watershed runoff, sedimentary production and mobilization, burial, and photodecomposition in the water column. The principal source of MMHg is in situ benthic production with 80-91% of total inputs provided by diffusion from sediments. The production and contribution of MMHg from tundra watersheds is modest. Photodecomposition, though confined to a short ice-free season, provides the primary control for MMHg (66-88% of total inputs) and greatly attenuates bioaccumulation. Solid-phase MMHg and gross potential rates of Hg methylation, assayed with an isotopic tracer, vary positively with the level of inorganic Hg in filtered pore water, indicating that MMHg production is Hg-limited in these lakes. Moreover, sediment-waterfluxes of MMHg (i.e., net production at steady state) are related to sediment Hg loadings from the atmosphere. These results suggest that loadings of Hg derived from atmospheric deposition are a major factor affecting MMHg cycling in arctic ecosystems. However, environmental changes associated with warming of the Arctic (e.g., increased weathering, temperature, productivity, and organic loadings) may enhance MMHg bioaccumulation by stimulating Hg methylation and inhibiting photodecomposition.

Methyl mercury toxicity in plant cultures: modification of resistance and demethylation by light and/or 2,4-dichlorophenoxyacetic acid.

Cultures of Daucus carota, Ca-68-10, and Lactuca sativa, Le-67, were grown at increasing methyl mercury (MeHg) concentrations ranging from initial doses of 0.05 to 5.0 micrograms/ml per day for 4 days with or without 0.15 microgram/ml 2,4-dichlorophenoxyacetic acid (2,4-D) in the presence or absence of light. The presence of 2,4-D interacted with light synergistically in the expression of MeHg toxicity within the whole range of concentrations. Demethylation patterns increased or decreased depending on the species, the 2,4-D concentration in the medium, and methyl mercury concentration used in the treatment. Lettuce was more sensitive to this interaction than carrot. In lettuce, the presence of 2,4-D in the light lowered the concentration of total Hg (or MeHg) required to reduce growth by 50%, about 13 times relative to that in the dark (i.e., it sensitized the cells). In the absence of 2,4-D the pattern was reversed. In carrot the pattern was similar but less pronounced. This suggests that, in these cell populations, MeHg toxicity is partly a hormone-mediated and light-sensitive event.

Degradation of methyl and ethyl mercury by singlet oxygen generated from sea water exposed to sunlight or ultraviolet light.

Photodegradation of methyl mercury (MeHg) and ethyl Hg (EtHg) in sea water was studied by sunlight or ultraviolet (UV) light exposure, and by determining inorganic Hg produced by degradation. Sea water containing 1 microM MeHg or EtHg was exposed to sunlight or UV light. N-Acetyl-L-cysteine was added to the solution for preventing Hg loss during the light exposure. MeHg and EtHg in sea water were degraded by sunlight (> 280 nm), UV light A (320-400 nm) and UV light B (280-320 nm), though the amounts of inorganic Hg produced from MeHg were 1/6th to 1/12th those from EtHg. Inorganic Hg production was greater with increasing concentration of sea water. Degradation of MeHg and EtHg by the UV light A exposure was inhibited by singlet oxygen (1O2) trappers such as NaN3, 1,4-diazabicyclo[2,2,2]octane, histidine, methionine and 2,5-dimethylfuran. On the other hand, inhibitors or scavengers of superoxide anion, hydrogen peroxide or hydroxyl radical did not inhibit the photodegradation of alkyl Hg. These results suggested that 1O2 generated from sea water exposed to sunlight, UV light A or UV light B was the reactive oxygen species mainly responsible for the degradation of MeHg and EtHg.