Fine-scale tissue distribution of cadmium, inorganic mercury, and methylmercury in nymphs of the burrowing mayfly Hexagenia rigida studied by whole-body autoradiography

Response of α-glucosidase in gypsy moth larvae to acute and chronic dietary cadmium

We investigated the effects of acute and chronic treatments with cadmium at 10 μg Cd/g dry food and 30 μg Cd/g dry food on α-glucosidase activity of the 4th instar larvae of Lymantria dispar (Lepidoptera: Lymantriidae) as well as subsequent recovery. Enzyme inhibition was recorded during acute exposure to 30 μg Cd/g dry food and during chronic treatment at the lower metal concentration. After three days recovery from 10 μg Cd/g dry food, the α-glucosidase activity returned to the control level. One-way ANOVA showed that cadmium significantly influenced the activity of α-glucosidase during all treatments. The index of phenotypic plasticity was higher during chronic treatment at 10 μg Cd/g dry food than at 30 μg Cd/g as well as during the recovery. We detected four glucosidase isoforms by NATIVE PAGE. The activities and expressions of the isoforms depended on both larval genotype and cadmium treatment.

Mercury bioaccumulation in dragonflies (Odonata: Anisoptera): examination of life stages and body regions

Dragonflies (Odonata: Anisoptera) are an important component of both aquatic and terrestrial food webs and are vectors for methylmercury (MeHg) biomagnification. Variations in mercury content with life stage and body regions may affect the relative transfer of mercury to aquatic or terrestrial food webs; however, there has been little research on this subject. Also, little is known about mercury bioaccumulation in different body regions of dragonflies. To address these knowledge gaps, dragonfly naiads, adults, and exuviae were collected at 2 lakes in Kejimkujik National Park, Nova Scotia, Canada, and mercury concentrations in different life stages and body regions were quantified. Mean whole body concentrations of MeHg were substantial in naiads (232 ± 112 ng g(-1) dry wt, n = 66), emerging adults (236 ± 50 ng g(-1) dry wt, n = 10), and mature adults (231 ± 74 ng g(-1) dry wt, n = 20). Mean MeHg concentrations in exuviae (5.6 ± 4.3 ng g(-1), n = 32) were 40-fold lower than in naiads and adults. Emerging adults had 2-fold to 2.5-fold higher Hg(II) concentrations than naiads, mature adults, and exuviae. In body regions of both naiads and adults, some abdomens contained significantly higher concentrations of Hg(II) than heads or thoraces, and this trend was consistent across families. Across families, Aeshnidae had significantly higher concentrations of MeHg and total Hg than Gomphidae and Libellulidae, but not higher than Cordulidae. The Hg(II) concentrations were lower in Aeshnidae and Libellulidae than in Gomphidae and Cordulidae. Shedding of exuviae presents a possible mechanism for mercury detoxification, but mercury concentrations and burdens in exuviae are low in comparison with naiads and adults. Dragonfly adults retain a high potential for transferring substantial amounts of MeHg to their predators.

Cadmium exposure route affects antioxidant responses in the mayfly Centroptilum triangulifer

Aquatic organisms accumulate metals directly from water and from their diets. Exposure to metals is known to generate oxidative stress in living organisms and this stress may be ameliorated via activation of antioxidant enzymes and non-enzymatic antioxidants. To determine if antioxidant physiology is dependent on Cd exposure route in the mayfly Centroptilum triangulifer, we exposed larvae to environmentally relevant concentrations of Cd from isolated dissolved or dietary exposure routes to achieve comparable tissue concentrations. Dissolved Cd had no effect on the antioxidant enzymes examined. However, dietary Cd significantly suppressed catalase and superoxide dismutase activities, and decreased concentrations of the reduced (active) form of glutathione in C. triangulifer larvae. These findings suggest that dietary Cd is potentially more toxic than aqueously derived Cd in this mayfly. We further examined the effect of dietary Cd tissue loading rates on antioxidant enzyme suppression and found that absolute tissue load appeared more important than loading rate. These results may help explain why insects are routinely unresponsive to dissolved metal exposures in the laboratory, yet highly responsive to metal pollution in nature.

Effects of heavy metal contamination upon soil microbes: lead-induced changes in general and denitrifying microbial communities as evidenced by molecular markers

Lead (Pb) is a common environmental contaminant found in soils. Unlike other metals, Pb has no biological role, and is potentially toxic to microorganisms. Effects of low (1 ppm) and high (500–2000) levels of lead (Pb) upon the soil microbial community was investigated by the PCR/DGGE analysis of the 16S and nirK gene markers, indicative of general microbial community and denitrifying community, respectively. Community analysis by use of those markers had shown that Pb has detectable effects upon the community diversity even at the lowest concentration tested. Analysis of sample diversity and similarity between the samples suggested that there are several thresholds crossed as metal concentration increase, each causing a substantial change in microbial diversity. Preliminary data obtained in this study suggest that the denitrifying microbial community adapts to elevated levels of Pb by selecting for metal-resistant forms of nitrite reductases.

Silver accumulation in Daphnia magna in the presence of reactive sulfide

Previously, we demonstrated a higher silver body burden when Daphnia magna were exposed to silver in the presence of environmentally relevant concentrations (25 nM) of reactive sulfide, but the explanation was unclear. In the present study, D. magna were exposed to AgNO3 (0.93 microg Ag/L=8.6 nM as a mixture of cold Ag and (110m)Ag) in synthetic water in either the presence or absence of 25 nM sulfide as zinc sulfide clusters. After 1-h exposure, daphnids were transferred to clean water for up to 5-h depuration. At different times of Ag exposure and depuration, daphnids were randomly sampled for whole body silver burden. Also, after 1 h, daphnids were sampled for silver accumulation in "gills" (small organs on the thoracic appendages), digestive tract, and carcass. Other groups were exposed to the same silver and sulfide concentrations for 1 h and then sampled for whole-body autoradiography. Silver body burden was about two-fold higher in the presence of sulfide. A two-fold increase in silver burden in "gills" and digestive tract, but not in carcass, was also observed in the presence of sulfide. Absolute differences due to sulfide were greatest in digestive tract and explained most of the difference in whole body burden. Transfer to clean water caused a significant drop in silver concentration in whole body and all compartments to similar levels in the two groups after 5-h depuration. These results indicate that the higher silver body burden observed in the presence of sulfide is mainly due to sulfide-bound silver in the digestive tract of the daphnids. This conclusion is supported by autoradiography, which showed a high concentration of silver in the digestive tract of daphnids exposed to Ag/sulfide.

Histopathological evidence of inorganic mercury and methyl mercury toxicity in the arctic charr (Salvelinus alpinus)

In our sustained effort to understand mechanisms of mercury toxicity in fish, the histopathological effects of a 96-h acute exposure to water-borne inorganic mercury (inorganic Hg) (15 micro gL(-1)), as well as those of a single dietary dose of inorganic and methyl mercury (methyl Hg) (0.260.05 micro g.Hgg(-1) body weight), over 30 days were examined. Samples of gills, olfactory epithelium, kidneys, and liver of arctic charr, Salvelinus alpinus, were studied using light and electron microscopy. The distribution of dietary inorganic and methyl Hg in the intestinal epithelium was determined using 203Hg microautoradiography. Gills of fish exposed to water-borne inorganic Hg presented a severe disorganization of epithelial cells after 12h and modifications of cilia of ciliated olfactory cells appeared after 24h. Nevertheless, a partial recovery was seen in both tissues by the end of the 96-h exposure period. Liver was little affected by water-borne and single-trophic-dose contamination of inorganic Hg, but dietary methyl Hg had drastic effects, despite its low dosage, with severe necrosis and alterations of cytoplasmic organization. Microautoradiograms showed that inorganic Hg was distributed evenly in the intestinal epithelium, whereas methyl Hg was found at very specific locations on the epithelial surface.