Ecotoxicity evaluation using the avoidance response of the oribatid mite Oppia nitens (Acari: Oribatida) in bioplastics, microplastics, and contaminated Superfund field sites.

Bioplastics are considered sustainable alternatives to conventional microplastics which are recognized as a threat to terrestrial ecosystems. However, little is known about the potential ecotoxicological effects of bioplastics on soil fauna and ecosystems. The present study assessed the toxicity of microplastics [Polystyrene (PS), Polyethylene (PE)] and bioplastics [Polyvinyl alcohol (PVA), Sodium polyacrylate (NaPa) on a key soil fauna Oppia nitens, a soil oribatid mite, and investigated the ecological relevance of O. nitens avoidance response as a valuable tool for the risk assessment of contaminated soils such as the Superfund sites. Findings showed that the mites' net response indicated avoidance behavior such that in most cases as concentrations of micro- and bioplastics increased, so did the avoidance responses. The avoidance EC50 endpoints showed PS < PE < PVA < NaPa, indicating higher deleterious effects of microplastics. High toxicity of PS in soils to O. nitens at EC50 of 165 (±25) mg/kg compared to bioplastics and other known contaminants poses an enormous threat to soil. For bioplastics in this study, there were no significant avoidances at concentrations up to 16,200 mg/kg compared to PS and PE which showed avoidance responses at 300 and 9000 mg/kg respectively, implying that bioplastics might be relatively safer to soil mites compared to conventional microplastics. Also, results indicated that long-term heavy metal pollution such as in contaminated Superfund sites decreased microbial biomass; a useful bioindicator of soil pollution. Furthermore, O. nitens avoidance of heavy metals contaminated sites demonstrated the ecological relevance of avoidance response test when assessing the habitat integrity of contaminated soil. The present study further supports the inclusion of the oribatid mite, O. nitens in the ecological risk assessment of contaminants in soil.

Integument colour change: Tracking delayed growth of Oppia nitens as a sub-lethal indicator of soil toxicity.

Growth is an important toxicity end-point in ecotoxicology but is rarely used in soil ecotoxicological studies. Here, we assessed the growth change of Oppia nitens when exposed to reference and heavy metal toxicants. To assess mite growth, we developed an image analysis methodology to measure colour spectrum changes of the mite integument at the final developmental stage, as a proxy for growth change. We linked the values of red, green, blue, key-black, and light colour of mites to different growth stages. Based on this concept, we assessed the growth change of mites exposed to cadmium, copper, zinc, lead, boric acid, or phenanthrene at sublethal concentrations in LUFA 2.2 soil for 14 days. Sublethal effects were detected after 7 days of exposure. The growth of O. nitens was more sensitive than survival and reproduction when exposed to copper (EC50growth = 1360 mg/kg compared to EC50reproduction = 2896 mg/kg). Mite growth sensitivity was within the same order of magnitude to mite reproduction when exposed to zinc (EC50growth = 1785; EC50reproduction = 1562 mg/kg). At least 25% of sublethal effects of boric acid and phenanthrene were detected in the mites but growth was not impacted when O. nitens were exposed to lead. Consistent with previous studies, cadmium was the most toxic metal to O. nitens. The mite growth pattern was comparable to mite survival and reproduction from previous studies. Mite growth is a sensitive toxicity endpoint, ecologically relevant, fast, easy to detect, and can be assessed in a non-invasive fashion, thereby complimenting existing O. nitens testing protocols.

Single metal and metal mixture toxicity of five metals to Oppia nitens in five different Canadian soils.

Metal mixture toxicity across soil types is a daunting challenge to risk assessment. Here, we evaluated metal mixture toxicity in Oppia nitens, using ten fixed metal mixture ratios in five Canadian soils that closely matched some of the EU PNEC reference soils. Soils were dosed with five metals (Cu, Zn, Pb, Co, Ni) as single metals (ten concentrations) and as mixtures (eight concentrations). Synchronized adult mites were exposed to metals, with survival and reproduction assessed after 28 days. We found out that (i) the differences among soils in mite sensitivity and single metals were not consistent when mites were exposed to metal mixtures, (ii) assuming concentration addition, the mixture interaction factor (MIF) showed that single metal low effect levels excessively underestimated low level metal mixture effects (iii) Zn emerged as a protective metal in most mixtures, and (iv) Soil properties such as CEC, independent of effects on metal speciation, explained more of the variation than measured metals. This study suggests that metal risk assessment should be done on a case by case basis. Further work is needed to ensure that by protecting soil-dwelling organisms from single metals, the risk from metal mixtures is appropriately protected for.

Lethal and Sublethal Toxicity of Thiamethoxam and Clothianidin Commercial Formulations to Soil Invertebrates in a Natural Soil.

The use of neonicotinoids in agriculture is a critical environmental protection issue. Although there has been considerable research on pollinator exposure and aquatic toxicological effects, few studies have investigated the chronic impacts on soil-dwelling species. Given the application of neonicotinoids into soil systems, there is the potential for risk to soil invertebrates. The toxicity of 2 commercial formulations containing the active ingredients (a.i.) thiamethoxam (Actara® 240SC) or clothianidin (Titan™) was investigated using 3 soil invertebrate species: Oppia nitens, Eisenia andrei, and Folsomia candida. No adverse effects were observed for O. nitens at the highest tested concentrations (≥92 mg a.i./kg dry soil) after a 28-d exposure. Exposure to clothianidin resulted in a 28-d median inhibitory concentration (IC50) of 0.069 (95% confidence limits: 0.039-0.12) mg/kg dry soil for F. candida, and a 56-d IC50 of 0.26 (0.22-3.2) mg a.i./kg dry soil for E. andrei. Exposure to thiamethoxam was less toxic, with IC50s of 0.36 (0.19-0.66) and 3.0 (2.2-4.0) mg a.i./kg dry soil for F. candida and E. andrei reproduction, respectively. The observed toxicity for F. candida adult survival and reproduction and for E. andrei reproduction occurred at environmentally relevant concentrations. However, because clothianidin is a degradation product of thiamethoxam, and detection of clothianidin rose to levels of concern in the thiamethoxam-amended soils over time, the observed toxicity may be partly attributed to the presence of clothianidin. Environ Toxicol Chem 2019;38:2111-2120. © 2019 Crown in the right of Canada. Published by Wiley Periodicals Inc. on behalf of SETAC.

The forgotten role of toxicodynamics: How habitat quality alters the mite, Oppia nitens, susceptibility to zinc, independent of toxicokinetics.

Soil habitat quality is thought to influence metal toxicity via changes in speciation and thereby toxicokinetics. Here, we assessed the toxicokinetic and toxicodynamic effects of habitat quality on mite, Oppia nitens when exposed to zinc (Zn) contaminated soils. Forty-seven soils were ranked into three habitat qualities; high, medium, and low based on biological reproduction of Folsomia candida, Enchytraeus crypticus, and Elymus lanceolatus. From the 47 soils, eighteen soils (comprising of six soils from each habitat quality) were randomly selected and dosed with field relevant concentrations of Zn. Mite survival and reproduction were assessed after 28 days. Total Zn, bioaccessible Zn, Zn bioavailability, Zn body burden, lactate dehydrogenase activity (LDH) and glucose-6-phosphate dehydrogenase (G6PDH) activities of the mites were determined. Zinc toxicity and potency were much less in the high compared to low quality soils and the mites in the high habitat quality soils tolerated higher zinc body burdens (2040 ±â€¯130 µg/g b.w) than the lower habitat quality (1180 ±â€¯310 µg/g b.w). Lower LDH activity (20 ±â€¯2 µU mg-1) in the high quality soils compared to lower quality soils (50 ±â€¯8 µU mg-1) suggested that there was less stress in the high habitat quality mites. Despite changes in speciation across habitat qualities, bioavailability of zinc was similar (∼20%) irrespective of habitat quality. Our results suggest that the influence of soil properties on survival is modulated by toxicodynamics rather than toxicokinetics. Restoring habitat quality may be more important for soil invertebrate protection than metal concentration at contaminated sites.

Multigenerational exposure of populations of Oppia nitens to zinc under pulse and continuous exposure scenarios.

Current soil remediation guidelines for metals reflect single-generation laboratory studies, but in the field, organisms are exposed to metals for more than one generation. The present study assessed the multigenerational effect of zinc (Zn) on Oppia nitens under a pulse or continuous exposure scenario. Synchronized adult mites (parents) were exposed to 6 concentrations of Zn in a field soil. For the pulse exposure, juveniles of parent mites from 3 of the 6 concentrations (105, 158, 237, 335, 553, and 800 mg/kg) were kept in clean media and reared until the third generation. At every generation, the sensitivity of the mites to Zn was tested in a dose-response manner. For the continuous exposure, the mites produced from the parents were re-exposed to the same concentration as their parents. According to critical-level estimates like the median effect concentration, all populations of the F2 and F3 generation mites in the pulse exposure were less sensitive to Zn than the parents and were protected at 250 mg/kg of Zn (Canadian Council of Ministers of the Environment [2018] soil quality guideline). However, the mite generations of the continuous exposure remained as sensitive as the parent generation and were not protected by the Zn guideline level. The Zn niche width narrowed considerably for all continuously exposed mite populations, indicating that they were more sensitive than the parent. Our results show that Zn has a deleterious multigenerational effect on continuously exposed populations of mites. Environ Toxicol Chem 2019;38:896-904. © 2019 SETAC.

Perfluorooctane sulfonate in surface soils: Effects on reproduction in the collembolan, Folsomia candida, and the oribatid mite, Oppia nitens.

Perfluorooctane sulfonate (PFOS) is a persistent organic pollutant, which has been detected at significant concentrations in soils at sites used for fire-fighting training operations. Recent ecotoxicological research has mainly focused on earthworms to assess the toxicity of PFOS in soil. However, the inclusion of other soil taxonomic groups allow for a more holistic estimate of contaminant risk, including the derivation of more comprehensive soil quality guidelines. The present study assessed the toxicity of PFOS using the collembolan, Folsomia candida, and the oribatid mite, Oppia nitens, in two types of soil: a coarse-textured sandy loam (VSL) and fine-textured clay loam (NRS). As a standard O. nitens reproduction test is being formalized, the results of the study were also used to compare sensitivity across test species. Effects were soil dependent, with test species being 2-4 times more susceptible to PFOS in VSL, relative to NRS, likely due to differences in organic matter and clay content. Oppia nitens was significantly more sensitive to PFOS, regardless of soil type, in comparison to F. candida. The IC50s for reproduction for O. nitens were 23 mg kg-1 (95% confidence interval: 17-32 mg kg-1) in the VSL and 95 mg kg-1 (69-134 mg kg-1) in the NRS, and for F. candida were 94 mg kg-1 (72-122 mg kg-1) in the VSL and 233 mg kg-1 (177-306 mg kg-1) in the NRS. The present study demonstrates the application and inclusion of the oribatid mite, O. nitens, for the risk assessment of contaminants in soil.

The potentiation of zinc toxicity by soil moisture in a boreal forest ecosystem.

Northern boreal forests often experience forest dieback as a result of metal ore mining and smelting. The common solution is to lime the soil, which increases pH, reducing metal toxicity and encouraging recovery. In certain situations, however, such as in Flin Flon, Manitoba, Canada, liming has yielded only moderate benefits, with some locations responding well to liming and other locations not at all. In an effort to increase the effectiveness of the ecorestoration strategy, the authors investigated if these differences in liming responsiveness were linked to differences in toxicity. Toxicity of metal-impacted Flin Flon soils on the oribatid mite Oppia nitens and the collembolan Folsomia candida was assessed, with a view toward identifying the metal of concern in the area. The effects of moisture content on metal sorption, uptake, and toxicity to the invertebrates were also investigated. Toxicity tests with the invertebrates were conducted using either Flin Flon soils or artificial soils with moisture content adjusted to 30%, 45%, 60%, or 75% of the maximum water-holding capacity of the soil samples. The Relative to Cd Toxicity Model identified Zn as the metal of concern in the area, and this was confirmed using validation tests with field contaminated soils. Furthermore, increasing the moisture content in soils increased the amount of mobile Zn available for uptake with the ion exchange resin. Survival and reproduction of both invertebrates were reduced under Zn exposure as moisture level increased. Thus, moisture-collecting landforms, which are often also associated with high Zn concentrations at Flin Flon, have, as a result, higher Zn toxicity to the soil ecosystem because of increases in soil moisture.