G8 (II) Mercury bioaccumulation and trophic transfer

Mercury contamination has become a very important subject in the scientific community, due its diverse damages functions on the environment and human health. This work proposes the investigation of mercury ecotoxicity and its bioavailability in three tropical soil classes (yellow ferralsol, red ferralsol and chernosol), artificially contaminated by divalent mercury, using acute bioassays with earthworms (Eisenia andrei). The acute bioassays (14 days) were conducted according to ASTM (2004) recommendations. The lethal concentration on 50% of the organisms (LC50) was estimated using the software TRIMMED. The soils were contaminated using a standard Merck solution. Total mercury determination was performed with LUMEX, a portable atomic absorption. Before being submitted to such procedure, the survival worms were frozen and lyophilized. Mercury bioavailability was evaluated by calculating bioconcentration factors (BCF), a ratio between the Hg total content in the organisms and the total Hg concentration in the soil. Results revealed higher mortality levels according to the following sequence: yellow ferralsol (LC50 ~ 7mg/kg) > chernosol (LC50 ~ 15mg/kg) > red ferralsol (LC50 ~ 20mg/kg). Such observation suggests that soil properties played a crucial role in the bioavailability processes. High levels of organic matter in the red ferralsol seem to form stable complexes with mercury, decreasing its content in the soil solution and its ecotoxicity. In the chernosol, the occurrence of expansive clay minerals suggests the existence of more effective mercury adsorption processes, reducing its bioavailability in the soil solution. Furthermore, the chernosols present high contents of nutrients, which represent more availability of food for the earthworms. Thus, healthier animals tend to support higher levels of mercury in the soil. On the other hand, yellow ferralsols have a very low fertility and are predominantly composed by kaolinite (1:1 clay mineral), thus becoming mercury more available in the soil solution. Mercury determination in the survival worms indicated BCFs closer and higher than one unit (between 0.7 and 3.0 units), indicating that organisms not only absorbed the contaminant, but also bioaccumulated it. For all soil classes, the BCFs were inversely proportional to the increase of mercury content in the soil, suggesting the existence of saturation mechanisms by the organisms. It is expected that those results can generate important data for the future establishment of toxic reference values for tropical pedofauna, supporting decision-making in environmental control programs and establishment of sustainable indicators.

Consumption of rice is the primary pathway of methylmercury (MeHg) exposure to the population in mercury (Hg) mining areas, SW China. Mechanistic information on MeHg accumulation in rice are however to date very scarce. MeHg dynamics in irrigated rice paddy was investigated in the rice growing season. The diffusive gradient in thin films technique (DGT) for MeHg measurements was applied at three sites in Guizhou province, SW China disparately impacted by mercury pollution. Elevated MeHg concentrations were observed in porewater of rice paddies in Hg mining areas compared to those obtained from the control site, which attributed to the Hg contamination of soil compartments by the historical large-scale Hg mining/smelting and ongoing artisanal Hg smelting activities. At same site different MeHg concentrations were also observed in porewater at different stages in the rice growing season, which was consist with the MeHg concentrations in rice plants (Oryza sativa L.). Our observations showed that MeHg in paddy soil was an important source to rice.

Consuming fish, fish products and marine mammals is currently considered as the main pathway of human exposure to Hg and especially methylmercury (MeHg) posing a worldwide human health threat. However, recent studies showed that rice consumption can be an important pathway of methylmercury (MeHg) exposure to humans in Hg mining areas and also in certain inland areas in Southwestern China. The rice seed (brown rice) has the highest ability to accumulate MeHg compared to the other tissues. In order to investigate the distribution of inorganic mercury (IHg) and MeHg in polished (white rice, bran removed) and unpolished (brown rice) rice grains, ninety-five rice grains were collected during regular harvest periods at different typical mercury (Hg) contaminated sites. The rice grains were divided into bran and polished rice (bran removed). The mass of different parts of rice grains was recorded. Totol mercury (THg) and MeHg were measured in white rice, brown rice, and bran according to US EPA Method 1631 and Method 1630. The concentration of IHg in samples was calculated by the difference between the concentration of THg and MeHg. Our results showed that both the concentrations of IHg and MeHg in bran were significantly higher than those in white rice and brown rice (K-S test, P < 0.001). Even the concentrations of IHg and MeHg in bran were much higher than those in white rice; most of the IHg (64±10 %) was located in the bran. However, the white rice accumulated the majority of MeHg (82±7.5 %) in the whole rice grains. Synchrotron-based X-ray fluorescence (S-XRF) was utilized to locate Hg in brown rice grains. In brown rice, Hg was found to be preferentially localized at the surface, in the region corresponding to the pericarp and aleurone layer.

It is well known that selenium (Se) shows protective effects against mercury (Hg) bioaccumulation and toxicity, but the underlying effects of Se chemical species, concentration, and administration method are poorly known. In this study, we conducted laboratory studies on a marine fish Terapon jurbua to explain why Hg accumulation is reduced in the presence of Se observed in field studies. When Se and Hg were administrated concurrently in the fish diets, different Se species including selenite, selenate, seleno-DL-cystine (SeCys), seleno-DL-methionine (SeMet) affected Hg bioaccumulation differently. At high concentration in fish diet (20 µg g^-1 normally), selenate and SeCys significantly reduced the dietary Hg(II) assimilation efficiency (AE) from 38% to 26%. After the fish were pre-exposed to dietary selenite or SeMet (7 µg g^-1 normally) for 22 days with significantly elevated Se body concentrations, the Hg(II) AEs were pronouncedly reduced (from 41% to 15-26%), whereas the dissolved uptake rate and elimination rate were less affected. In contrast to Hg(II), all the MeHg biokinetic parameters remained relatively constant whether Se was administrated simultaneously with the fish diet or when the fish were pre-exposed to Se with elevated body concentrations. Basic biokinetic measurements thus revealed that Se had direct interaction with Hg(II) during dietary assimilation rather than with MeHg, and that different Se species had variable effects on Hg assimilation.