The bioaccumulation and effects of selenium in the oligochaete Lumbriculus variegatus via dissolved and dietary exposure routes

Comparative effects of different metals on the Japanese medaka embryos and larvae

Rapid evaluation of the toxicity of metals using fish embryo acute toxicity is facilitative to ecological risk assessment of aquatic organisms. However, this approach has seldom been utilized for the comparative study on the effects of different metals to fish. In this study, acute and sub-chronic tests were used to compare the toxicity of Se(IV) and Cd in the embryos and larvae of Japanese medaka (Oryzias latipes). The embryos with different levels of dechorionation and/or pre-exposure were also exposed to Se(IV) and Cd at various concentrations. The results showed that the LC50-144 h of Cd was 1.3-5.2 folds higher than that of Se(IV) for the embryos. In contrast, LC50-96 h of Se(IV) were 200-400 folds higher than that of Cd for the larvae. Meanwhile, dechorionated embryos were more sensitive to both Se and Cd than the intact embryos. At elevated concentrations, both Se and Cd caused mortality and deformity in the embryos and larvae. In addition, pre-exposure to Cd at the embryonic stages enhanced the resistance to Cd in the larvae. However, pre-exposure to Se(IV) at the embryonic stages did not affect the toxicity of Se(IV) to the larvae. This study has distinguished the nuance differences in effects between Se(IV) and Cd after acute and sub-chronic exposures with/without chorion. The approach might have a potential in the comparative toxicology of metals (or other pollutants) and in the assessment of their risks to aquatic ecosystems.

Mechanistic characterization of waterborne selenite uptake in the water flea, Daphnia magna, indicates water chemistry affects toxicity in coal mine-impacted waters

Concentrations of selenium that exceed regulatory guidelines have been associated with coal mining activities and have been linked to detrimental effects on aquatic ecosystems and the organisms therein. Although the major route of selenium uptake in macroinvertebrates is via the diet, the uptake of waterborne selenite (HSeO3-), the prominent form at circumneutral pH, can be an important contributor to selenium body burden and thus selenium toxicity. In the current study, radiolabelled selenite (Se75) was used to characterize the mechanism of selenite uptake in the water flea, Daphnia magna. The concentration dependence (1-32 μM) of selenite uptake was determined in 1-hour uptake assays in artificial waters that independently varied in bicarbonate, chloride, sulphate, phosphate and selenate concentrations. At concentrations representative of those found in highly contaminated waters, selenite uptake was phosphate-dependent and inhibited by foscarnet, a phosphate transport inhibitor. At higher concentrations, selenite uptake was dependent on waterborne bicarbonate concentration and inhibited by the bicarbonate transporter inhibitor DIDS (4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid). These findings suggest that concentrations of phosphate in coal mining-affected waters could alter selenite uptake in aquatic organisms and could ultimately affect the toxic impacts of selenium in such waters.

Transfer of Se from sediments to the western mosquitofish Gambusia affinis: Tissue distribution, accumulation, and effects on the antioxidant physiology

Selenium (Se) has been shown to cause various toxicities in predatory species (i.e., fish and birds) in Se-contaminated aquatic environments. However, trophic transfer of Se from abiotic environments to freshwater fish has been relatively less addressed. In this study, 2-month-old mosquitofish (Gambusia affinis) were fed Se-enriched oligochaete (Lumbriculus variegatus, exposed to different concentrations of Se(IV) at 0.0, 3.0, 10.0, and 30.0 µg/g dry weight for 7 days) for 45 days. Tissue distribution, Se speciation, and effects on the antioxidant physiology in G. affinis were assessed. The results showed Se was rapidly accumulated in the oligochaete, with 6.30 ± 1.20, 16.20 ± 2.10, and 34.50 ± 2.40 µg/g dw of total Se levels in the worms exposed to 3.0, 10.0, and 30.0 µg/g of Se(IV), respectively. Total Se levels were increased in a dose-dependent manner in fish tissues and Se(IV) from sediments was maternally transferred to the fish embryos. Se-Met-and Se-Cys-were the predominant Se species in the worm and fish tissues, accounting for a minimum of 91.01% of the total Se. Furthermore, increased lipid peroxidation and altered the activities of antioxidant enzymes and levels of GSH were noticed in G. affinis fed the Se-enriched L. variegatus. This study has demonstrated that Se(IV) is transferred from an abiotic vector to freshwater organisms, disturbing the antioxidant physiology in G. affinis and potentially their offspring. This study highlights the importance of dietary exposure on the accumulation and toxicity of Se in aquatic organisms.

Laboratory study of Se speciation in the sediment and oligochaete Lumbriculus variegatus from an aquatic environment

Sediments are the major sink for selenium (Se) in aquatic environments. Se speciation in sediments is crucial for its bioavailability and toxicity in benthos, but this is relatively understudied. In this study, the background levels of Se in the river sediments, fish flakes, and Lumbriculus variegatus were also detected. Then, the dynamic changes of selenium speciation and concentrations in sediments were investigated after adding selenite (Se(IV)) and seleno-L-methionine (Se-Met) in the sediments for 90 and 7 days, and the accumulation and depuration of Se(IV) and Se-Met for 7 days in the oligochaete L. variegatus were also explored. Without the presence of worms, the levels of Se(IV) in the sediments were relatively stable within 7 days but showed a decreasing trend during the 90 days of aging. In contrast, Se-Met in the sediments showed a sharp decrease within 3 days of aging. The LC₅₀-96 h values of Se(IV) and Se-Met in L. variegatus were 372.6 and 9.4 μg/g, respectively. Interestingly, the dominant Se species in Se(IV)- or Se-Met-treated L. variegatus was Se-Met, whose level was increased with time in 7 days of exposure. Se was barely depurated from L. variegatus during the 8 days of the depuration period. This study has provided indispensable data on the levels of total Se in the abiotic and biotic matrices and the biodynamics of Se in a representative benthos, which could better understand the ecological risk of Se to the freshwater benthic communities.

Synchrotron-Based Imaging Reveals the Fate of Selenium in Striped Marsh Frog Tadpoles

Synchrotron-based X-ray fluorescence microscopy (XFM) coupled with X-ray absorption near-edge structure (XANES) imaging was used to study selenium (Se) biodistribution and speciation in Limnodynastes peronii tadpoles. Tadpoles were exposed to dissolved Se (30 μg/L) as selenite (Se^IV) or selenate (Se^VI) for 7 days followed by 3 days of depuration. High-resolution elemental maps revealed that Se partitioned primarily in the eyes (specifically the eye lens, iris, and retinal pigmented epithelium), digestive and excretory organs of Se^IV-exposed tadpoles. Speciation analysis confirmed that the majority of accumulated Se was converted to organo-Se. Multielement analyses provided new information on Se colocalization and its impact on trace element homeostasis. New insights into the fate of Se on a whole organism scale contribute to our understanding of the mechanisms and risks associated with Se pollution.

Interactive effects of earthworm Eisenia fetida and bean plant Phaseolus vulgaris L on the fate of soil selenium

Selenium (Se) is an essential micronutrient for animals with a narrow margin between essentiality and toxicity. Se toxicity is largely related to inorganic forms of Se in soil, i.e., selenite and selenate that enter food chains through plant uptake, threatening higher trophic level organisms. This experiment investigated effects of earthworm activity on Se bioavailability in soil and the subsequent plant uptake, using earthworm Eisenia fetida and bean plant Phaseolus vulgaris L, both exposed to either selenite or selenate at 1 or 4 mg Se kg^-1 for 16 weeks. Plants took up selenate (up to 221-fold) faster than selenite, with up to 84% of the Se rapidly transported to shoots. In the presence of earthworms, Se accumulation obviously increased for selenate-supplied plants, leading to an up to 4% increase in Se translocation factor for all treatments except for 1 mg kg^-1 selenite treatment. Earthworms also concentrated Se faster in tissues (up to 274 mg kg^-1 DW) at exposure to selenate. For Se toxicity, Se speciation analysis was conducted on the plants and earthworms using XAS. Compared to worm-free treatments, the percentage of organo-Se, i.e., SeMet and CysSeSeCys, increased in beans (up to 34%) in the presence of earthworms for selenate, while the elemental Se portion was significantly reduced or absent, opposite to the results for selenite. Surprisingly, elemental Se (up to 65%) dominated earthworms, regardless of the form of Se supplied. In conclusion, earthworms clearly enhanced Se uptake and translocation in plants, leading to elevated Se levels in shoots. To prevent resulting hazards to humans and other animals, caution should be taken while consuming the shoots, particularly beans, harvested from the Se contaminated soil where earthworm activity is high. Finally, the significant reduction in soil Se suggests phytoextraction of Se from the soil could be improved using earthworms as an aid to plants.

Selenomethionine exposure affects chondrogenic differentiation and bone formation in Japanese medaka (Oryzias latipes)

Excess selenium entering the aquatic environment from anthropogenic activities has been associated with developmental abnormalities in fish including skeletal deformities of the head and spine. However, mechanisms of this developmental toxicity have not been well-characterized. In this study, Japanese medaka (Oryzias latipes) embryos were exposed to seleno-l-methionine (Se-Met) in a range of concentrations. Gene expression was evaluated for sex-determining region Y (SRY)-related box (Sox9a and Sox9b), runt-related transcription factor 2 (Runx2), and melatonin receptor (Mtr). Alterations in the length of Meckel's cartilage, tail curvature, and decreased calcification were observed in skeletal stains at 10- and 22-days post-fertilization (dpf). Embryonic exposure of Osterix-mCherry transgenic medaka resulted in fewer teeth. Sox9a and Sox9b were up-regulated, while Runx2 and Mtr were down-regulated by Se-Met prior to hatch. Whole mount in situ hybridization (WISH) localized gene expression to areas observed to be affected in vivo. In addition, Se-Met exposures of a Mtr morpholino (Mtr-MO) as well as Luzindole exposed embryos developed similar skeletal malformations, supporting involvement of Mtr. These findings demonstrate that Se-Met modulates expression of key genes involved in chondrogenic differentiation and bone formation during development.

Subchronic effects of dietary selenium yeast and selenite on growth performance and the immune and antioxidant systems in Nile tilapia Oreochromis niloticus

Selenium is an essential element but toxic at high levels in animals. The effects of Se on growth performance and the immune system in Nile tilapia remain inconclusive. In this study, Nile tilapia Oreochromis niloticus was fed on selenium yeast (Se(Y))- and selenite (Se(IV))-enriched feed at 0, 3, 6, and 12 μg/g (dry wt) for 45 and 90 d. The growth, bioaccumulation, biochemical markers related to antioxidant, immunological, nervous and digestive systems were evaluated in various fish tissues (liver, intestine, kidney, muscle, brain, spleen, gills). The results showed that the accumulation of Se(Y) was 1.3-2 folds of Se(IV) in most tissues. The growth of tilapia was enhanced by both Se(Y) and Se(IV) at 3 μg/g after 90 d, with Se(Y) better than Se(IV) in tilapia feed. After 45 d, the levels of lipid peroxidation, the activity of the antioxidant enzymes, and the transcriptional levels of the immune related genes (IL-1β, IFN-γ and TNF-α) and stress proteins (HSP70 and MT) were enhanced in all treatments, except that of MT in the 12 μg/g Se(Y) group. In addition, both Se species inhibited the activity of acetylcholinesterase (AChE) in the brain and one digestive enzyme α-glucosidase (α-Glu) in the intestine at 12 μg/g. However, after 90 d, the effects on most biochemical markers were less pronounced, implying a possible acclimation after prolonged duration. The results demonstrate Se is beneficial to O. niloticus at low levels and toxic at elevated levels. The immunostimulation by Se might be greatly weakened after long term feeding Se-enriched feed. This study helps to better understand the effects of Se on the antioxidant and immune systems and to establish the optimal Se levels in the feed and duration for O. niloticus.

The role of the freshwater oligochaete Limnodrilus hoffmeisteri in the distribution of Se in a water/sediment microcosm

Selenite(IV) and selenate(VI) are the major species of Se in the seleniferous aquatic ecosystem. The redistribution of Se in the water/sediment microcosm by bioturbation remains largely unknown. In this study, the redistribution of Se in the water/sediment microcosm by the benthic oligochaete Limnodrilus hoffmeisteri was assessed. The worms were exposed to 2-40 μg/g dry weight of Se(IV) or Se(VI) in the sediment (diet) for 2 months. The changes in the Se levels in different compartments of the microcosm (sediment, overlying water, and worms) were quantified after 2 weeks and 2 months. The subcellular distribution of Se in the worms were also evaluated. Finally, the volatilization of Se from the two Se sources was estimated. The results showed that Se concentration in the overlying water and Se bioaccumulation in the worms were increased with Se levels in the sediments. Approximately 1.6-9.8% of Se was volatilized in the absence of the worms and was intensified in the presence of the worms (2.1-25.7%). The subcellular distribution witnessed high levels of Se in the cell debris (>60%). Se(IV) and Se(VI) differ in their bioaccumulation, redistribution and the effects on the growth of the worms. Our results suggest that the bioturbation by benthos play an essential role in the redistribution of Se in the water/sediment microcosm.

Lumbriculus variegatus (Annelida) biological responses and sediment sequential extractions indicate ecotoxicity of lake sediments contaminated by biomining

We assessed potential ecotoxicity of lake sediments affected by biomining effluents in northeastern Finland. Growth, reproduction and behavior of the sediment-dwelling oligochaete Lumbriculus variegatus (Müller 1774) were used as ecotoxicity endpoints. Standardized chronic bioassays were used for growth and reproduction, and acute and chronic tests with Multispecies Freshwater Biomonitor (MFB) for behavior assessments. Sequential extractions were used to characterize metal bioavailability and exposure conditions in the sediments, which indicated mining-induced contamination gradients of S, Cu, Ni and U and also bioavailability gradients of S and Ni. Among the ecotoxicity endpoints, growth and reproduction responses of the standard bioassays appeared more sensitive than the behavioral responses at 21 d. In the two most mining-affected test sediments, mean number of worms and dry biomass decreased 35-42% and 46-51% in comparison to the reference sediment, respectively. The behavioral changes of worms, i.e. peristaltic and overall locomotory activity, decreased on average 20-70% and 2-61% at 21 d in the same sediments. However, these behavioral changes were observed at the onset of exposure indicating MFB technique is a suitable and rapid screening level ecotoxicity assessment tool.

Selenium accumulation and the effects on the liver of topmouth gudgeon Pseudorasbora parva exposed to dissolved inorganic selenium

Selenite(IV) and selenate(VI) are the major forms of Se in aquatic ecosystem. In this study, Pseudorasbora parva were exposed to 10, 200 and 1000 μg L^-1 selenite and selenate for 28 days. Selenium accumulation, antioxidant enzyme levels, glutathione concentrations, lipid peroxidation and histology were evaluated in livers following exposure. Our results showed that Se(IV) and Se(VI) caused different accumulation patterns in the liver, with a more rapid accumulation of Se with Se(IV) treatment. Both Se species increased hepatic lipid peroxidation after 14 and 28 d (~ 30%). Among the antioxidants examined, the activity of SOD (except day 28) and the cellular levels of GSH were induced by 72-137% at lower concentrations, while the activity of GST was at least 24% lower than that of the control at 200 and 1000 μg L^-1 for both Se species at all sampling points. Both forms of Se reduced the hepatosomatic index at 1000 μg L^-1 after 28 d. In addition, marked histopathological alterations (10-31%) were observed in the liver of P. parva after exposure to both Se species, with higher frequency in the Se(IV) exposed fish. Liver local necrosis was observed only in the liver of fish exposed to 1000 μg L^-1 of Se(IV) (~ 20%). Our results suggest that the ecological impacts of dissolved Se in this freshwater species may also contribute to overall toxicity.

The differences in bioaccumulation and effects between Se(IV) and Se(VI) in the topmouth gudgeon Pseudorasbora parva

Selenium (Se) might be protective against oxidative stress at nutritional levels, but elevated Se concentrations in the diet has been revealed as the main culprit for the extinction of natural fish populations in Se-contaminated lakes. Though Se predominate as waterborne selenite (IV) and selenate (VI) in the water, the differences in bioaccumulation, effects (e.g., oxidative stress, antioxidants etc.) and molecular mechanisms between Se(IV) and Se(VI) have been relatively understudied in wild fish. In this study, the P. parva were exposed to waterborne Se (10, 200 and 1000 μg/L of Se(IV) or Se(VI)) and sampled at 4, 14 and 28 days. Bioaccumulation, tissue distributions of Se and following effects in different tissues were evaluated. The results showed that the levels of Se in the gills and intestine were significantly elevated with a seemingly concentration-dependent pattern in the Se(IV) treatment, with respectively 173.3% and 57.2% increase after 28 days of exposure, relative to that of Se(VI) treatment. Additionally, significant accumulation of Se was also observed in the muscle of Se(IV) treated fish. Se exposure increased the MDA levels in the brain and gills in the Se(IV) treatment, but less apparent in the Se(VI) treatment. Meanwhile, Se exposure lowered (at least 56%) the activity of GST in the gills, but increased the activity of AChE in the muscle (~69%) and brain (~50%) after 28 d. Most importantly, after 28 d of exposure, Se exposure caused significant decrease in GSH levels in the gills (at least 35%) and in all tissues examined at the highest test concentration. In general, the results showed that Se(IV) led to faster accumulation of Se than Se(VI) in P. parva, and the resulted lipid peroxidation was closely related to the levels of antioxidants, especially GSH. Our results suggest that the ecotoxicological effects of waterborne selenite and selenate differ in this freshwater species in the field.

Characterisation of artisanal mine waste on Buru Island, Indonesia and toxicity to the brittle star Amphipholis squamata

Artisanal small-scale gold mining (ASGM) using mercury (Hg) amalgamation commenced on Buru Island, Indonesia, in 2012, but was halted in 2015 due to concerns of widespread Hg contamination. Much of the Hg used in the mining process is lost to trommel waste which is disposed of in settlement ponds that drain into adjacent waterways and into Kayeli Bay. Several thousand unmanaged trommel sites and associated tailing ponds exist on Buru Island. This study shows that waste from the Marloso trommel at the Gogrea site contained 203 mg/kg total Hg (THg), with a negligible proportion present as bioavailable methyl Hg (MeHg) and a low total organic carbon content. There are currently very few tools available for ecotoxicological risk assessment of mine tailings for tropical marine ecosystems, and we support the development of Tailings Toxicity Tests (TTTs) and describe laboratory toxicity test methods using the cosmopolitan benthic echinoderm Amphipholis squamata. Undiluted trommel waste caused 100% mortality of A. squamata within 48 h, and a 96-h LC50 of 6.7% w/w trommel waste (4 mg/kg THg) was estimated. Sub-lethal effects on the water vascular system of the brittle star were assessed by quantification of the Ability to Right Itself (ARI), and a 48-h EC50 of 7.3% w/w trommel waste (14.4 mg/kg THg) was estimated. The results show that trommel waste produced on Buru Island is highly contaminated with THg and is acutely toxic, raising serious concern for receiving ecosystems where Hg methylation to more toxic and bioavailable forms is likely.