Uptake and subcellular distributions of cadmium and selenium in transplanted aquatic insect larvae

Role of Toxicokinetic and Toxicodynamic Parameters in Explaining the Sensitivity of Zebrafish Larvae to Four Metals

Given the persistence and toxic potencies of metal contaminants in ecosystems, animals, and human beings, they are considered to be hazardous global pollutants. While the lethality of metal toxicities (e.g., LC₅₀) can significantly vary, even within the same species, the underlying mechanisms are less well-understood. In this study, we developed a subcellular two-compartment toxicokinetic-toxicodynamic (TK-TD) model for zebrafish larvae when exposed to four metals (cadmium, lead, copper, and zinc) to reveal whether differences in metal toxicity (LC₅₀ values) were dominated by the TK or TD processes. Results showed that the subcellular TK and TD parameters of the four metals were significantly different, and the bioconcentration factor (BCF) value of copper was higher than those of the other metals. We also found that the TD parameter internal threshold concentration (C_IT) was significantly positively correlated to the LC₅₀ values (R² = 0.7), suggesting a dominant role of TD processes in metal toxicity. Furthermore, the combined parameter C_IT/BCF for a metal-sensitive fraction (BCF_MSF), which linked exposure to effects through the TK-TD approach, explained up to 89% of the variation in toxicity to the four metals. The present study suggests that the observed variation in toxicity of these four metals was mainly determined by TD processes but that TK processes should not be ignored, especially for copper.

Uptake, translocation and metabolism of di-n-butyl phthalate in alfalfa (Medicago sativa)

Uptake and metabolism by plants are important biotransformation processes of organic pollutants in ecosystems. However, very limited information is currently available on the metabolism of phthalic acid esters (PAEs) in plants. In this study, alfalfa, highly efficient in phytoremediation of PAE contaminated soil, was chosen as the model to understand the fate of di-n-butyl phthalate (DnBP) in remediation plant. The results of hydroponic experiments indicated that DnBP accumulated mainly in alfalfa roots and adsorption to root epidermis might be the primary uptake mechanism. A large proportion of DnBP was subjected to apparent metabolism. De-esterification could be specified to be the predominant metabolism pathway. Mono-n-butyl phthalate (MnBP) and phthalic acid (PA) were detected as DnBP metabolites in all alfalfa roots and shoots throughout the entire exposure period. Around >90% of MnBP were distributed in cell soluble components and organelles, and MnBP gradually transferred from organelles and cell walls to soluble components as the exposure time extended. Similar to MnBP, PA located mainly in soluble components and organelles as well, while no PA existed in alfalfa cell walls. Exposure to DnBP ultimately resulted in the coexistence of DnBP and MnBP for a long term in interior plants, raising concerns on their combined potential toxicity on plant health or even ecosystem.

Trophic transfer and biotransformation of selenium in the mosquito (Aedes albopictus) and interactive effects with hexavalent chromium

As an essential micronutrient for animals with a narrow range between essentiality and toxicity, selenium (Se) usually coexists with chromium (Cr) in contaminated aquatic environments. This study investigated effects of three diets (Microcystis aeruginosa, Chlorella vulgaris and biofilms) exposed to Se or/and Cr on Aedes albopictus as a vector for the aquatic-terrestrial transfer of Se and Cr. Se(IV)-exposed mosquitoes concentrated Se up to 66-fold faster than Se(VI)-exposed ones, corresponding to the greater Se enrichment in Se(IV)-treated diets. Analysis using synchrotron-based X-ray absorption spectroscopy (XAS) showed that Se(0) (61.9-74.6%) dominated Se(VI)-exposed mosquitoes except for the C. vulgaris-fed larvae (organo-Se, 94.0%), while organo-Se accounted for 93.3-100.0% in Se(IV)-exposed mosquitoes. Cr accumulation in larvae (56.40-87.24 μg Cr/g DW) or adults (19.41-50.77 μg Cr/g DW) was not significantly different among all Cr(VI) treatments, despite varying diet Cr levels. With Cr(0) being dominant (57.7-94.0%), Cr(VI)-exposed mosquitoes posed little threat to predators. Although mosquitoes exposed to Se or Cr had shorter wings, adults supplied with C. vulgaris or biofilms co-exposed to Se(VI) and Cr(VI) had wings significantly (1.1-1.2 fold) longer than Se(VI) only exposed ones. Overall, our study reveals the role of Ae. albopictus in transferring waterborne Se and Cr from the contaminated aquatic ecosystem to the terrestrial ecosystem with the resulting eco-risks to wildlife in both ecosystems.

Insects for Food and Feed-Safety Aspects Related to Mycotoxins and Metals

Edible insects as an alternative source of protein are discussed as an important contribution to future feed and food security. However, it has to be ensured that the consumption is non-hazardous. This systematic review summarizes findings concerning contaminations of insects with mycotoxins and heavy metal ions (SciFinder, Pubmed, until 26 June 2019). Both kinds of contaminants were reported to reduce growth performance and increase mortality in insects. There was no evidence for accumulation of various mycotoxins analyzed in distinct insect species. However, further research is necessary due to limitation of data. Since the gut content contributes relevantly to the total body burden of contaminants in insects, a starving period before harvesting is recommended. Contrary, accumulation of heavy metal ions occurred to a varying extent dependent on metal type, insect species, and developmental stage. Examples are the accumulation of cadmium (black soldier fly) and arsenic (yellow mealworm). The reported species-specific accumulation and metabolism patterns of contaminants emphasize the importance to assess potential safety hazards in a case-by-case approach. Subject to regular monitoring of contaminants, the general ban in the European Union to use waste in animal feed should also be questioned regarding insect farming.

Habitat use strategy influences the tissue signature of trace elements including rare earth elements in an urban-adapted omnivorous bird

Concentrations of trace elements vary naturally between geological environments and as a result of emissions from anthropogenic activities. Habitat use strategy is an important determinant of trace element concentrations in tissues and eggs of wild birds. However, a scarcity of studies have documented the relationships between individual bird movements related to foraging activities and exposure to contaminants including trace elements. The objective of the present study was to investigate the influence of habitat use strategy, determined using GPS-based telemetry, on the liver concentrations of selected trace elements including rare earth elements (REEs) as well as lead (Pb) isotope ratios in an urban-adapted omnivorous bird, the ring-billed gull (Larus delawarensis). Male (n = 20) and female (n = 18) ring-billed gulls breeding near Montreal (QC, Canada) in one of the largest colony in North America were tracked using miniature GPS devices to characterize their movements over a 10-days period. The time spent foraging by both male and female gulls in landfills and wastewater basins positively correlated with liver Pb concentrations. A positive correlation was also found between the time spent foraging in agricultural fields and liver concentrations of yttrium (Y) in male and female ring-billed gulls. Heavy REE concentrations were significantly greater in female gull liver relative to those of males, although this was not associated with the time spent in any foraging habitats. Pb isotope ratios (²⁰⁸Pb/²⁰⁶Pb and ²⁰⁶Pb/²⁰⁷Pb) in the liver of male ring-billed gulls tended to be lower in individuals that foraged in landfills, thus indicating that they may have been exposed to different Pb sources relative to birds that visited other sites. This study provided valuable information on the potential sources of trace elements at the landscape level in free-ranging birds spanning urbanized environments.

Mercury speciation and subcellular distribution in experimentally dosed and wild birds

Many bird species are exposed to methylmercury (MeHg) at levels shown to cause sublethal effects. Although MeHg sensitivity and assimilation can vary among species and developmental stages, the underlying reasons (such as MeHg toxicokinetics) are poorly understood. We investigated Hg distribution at the tissue and cellular levels in birds by examining Hg speciation in blood, brain, and liver and Hg subcellular distribution in liver. We used MeHg egg injection of white leghorn chicken (Gallus gallus domesticus), sampled at 3 early developmental stages, and embryonic ring-billed gulls (Larus delawarensis) exposed to maternally deposited MeHg. The percentage of MeHg (relative to total Hg [THg]) in blood, brain, and liver ranged from 94 to 121%, indicating little MeHg demethylation. A liver subcellular partitioning procedure was used to determine how THg was distributed between potentially sensitive and detoxified compartments. The distributions of THg among subcellular fractions were similar among chicken time points, and between embryonic chicken and ring-billed gulls. A greater proportion of THg was associated with metal-sensitive fractions than detoxified fractions. Within the sensitive compartment, THg was found predominately in heat-denatured proteins (∼42-46%), followed by mitochondria (∼15-18%). A low rate of MeHg demethylation and high proportion of THg in metal-sensitive subcellular fractions further indicates that embryonic and hatchling time points are Hg-sensitive developmental stages, although further work is needed across a range of additional species and life stages. Environ Toxicol Chem 2017;36:3289-3298. © 2017 SETAC.

Influences of chromium and cadmium on the development of black soldier fly larvae

The black soldier fly Hermetia illucens is a good candidate for waste management. The harvested insects are rich in protein and have the potential to be used in animal feed. However, people are wary about heavy metals in waste. Therefore, it is necessary to understand how the uptake of heavy metals could affect H. illucens and where and to what extent metals are accumulated by the black soldier fly. Based on these considerations, developmental parameters were investigated in the different life stages of H. illucens fed an increasing concentration gradient of cadmium (Cd) and chromium (Cr); additionally, Cd and Cr distribution in the body parts of H. illucens at the different life stages was monitored. We found that Cd and Cr have no effects on larvae survival and eclosion rate, but they do have effects on larvae duration and pupation rate. Both Cd and Cr were transferred into larvae, prepupae, and pupae. While the concentrations of Cd in larvae and prepupae were much higher than that in their diets, the opposite case was observed with Cr. The concentrations of Cd and Cr in H. illucens decreased in later development stages. In individual larva and prepupa, Cd and Cr were mainly included in the body and not in the integument. In the pupa, the puparium contained higher Cd and Cr concentrations than the pupa body. The distribution of Cd and Cr in the different life stages and body parts may present a potential strategy for how H. illucens tolerate and remove heavy metal stress.

Subcellular distribution of trace elements and liver histology of landlocked Arctic char (Salvelinus alpinus) sampled along a mercury contamination gradient

We sampled landlocked Arctic char (Salvelinus alpinus) from four lakes (Small, 9-Mile, North, Amituk) in the Canadian High Arctic that span a gradient of mercury contamination. Metals (Hg, Se, Tl, and Fe) were measured in char tissues to determine their relationships with health indices (relative condition factor and hepatosomatic index), stable nitrogen isotope ratios, and liver histology. A subcellular partitioning procedure was employed to determine how metals were distributed between potentially sensitive and detoxified compartments of Arctic char livers from a low- and high-mercury lake (Small Lake and Amituk Lake, respectively). Differences in health indices and metal concentrations among char populations were likely related to differences in feeding ecology. Concentrations of Hg, Se, and Tl were highest in the livers of Amituk char, whereas concentrations of Fe were highest in Small and 9-Mile char. At the subcellular level we found that although Amituk char had higher concentrations of Tl in whole liver than Small Lake char, they maintained a greater proportion of this metal in detoxified fractions, suggesting an attempt at detoxification. Mercury was found mainly in potentially sensitive fractions of both Small and Amituk Lake char, indicating that Arctic char are not effectively detoxifying this metal. Histological changes in char livers, mainly in the form of melano-macrophage aggregates and hepatic fibrosis, could be linked to the concentrations and subcellular distributions of essential or non-essential metals.