Cadmium-induced toxicity to the mite, Oppia nitens C.L. Koch, 1836 (Acari: Oribatida): Maternal transfer, bioenergetics, and the influence of habitat quality on the mite’s response to cadmium toxicity

The mite, Oppia nitens, is a true soil dweller, one of many organisms that perform vital functions to support ecosystem services. Environment and Climate Change Canada and International Organization for Standardization recently completed a standardized protocol for Oppia nitens. Therefore, O. nitens is among the battery of soil invertebrates for toxicity testing, but its applicability is limited by the dearth of information on its responses to and interactions with contaminants in soil, and how soil affects its biology and ecology. The main objective of this study was to assess the responses of O. nitens to cadmium (Cd), a model chemical that is potentially toxic to this species, and to understand how habitat quality influences O. nitens’ reproduction and bioenergetics upon exposure to Cd in soil. Firstly, a critical review of the literature on the biology and ecology of O. nitens with notes on its response to metals and pesticides in soil was done. Also, the possible mechanisms on how O. nitens could respond to cadmium was proposed. This study, for the first time, gave detailed information on the bionomics (biology and ecology) of O. nitens, thus supporting existing knowledge on the applicability of O. nitens as test organisms in soil ecotoxicology. The toxicity and uptake of cadmium, as cadmium oxide (CdO) in standard soil was assessed on adult O. nitens and maternal transfer of Cd from adult to juvenile mites (tritonymphs) was estimated. According to the results, Cd as an oxide caused low toxicity compared to Cd as salts for both survival (LC50 = > 700 mg Cd kg-1) and reproduction (EC50 = 392 mg Cd kg-1 and EC25 = 215 mg Cd kg-1). The uptake of Cd by adult and juvenile mites was via the ingestion of total Cd and not via dermal adsorption of dissolved Cd in pore water. Adult O. nitens maternally transferred about 39 to 52 % (average of 46 %) of their Cd body burden to juveniles while the maternally acquired Cd in the juveniles accounted for 41 % of their Cd body burden. Finally, the influence of habitat quality on the reproduction and bioenergetics of O. nitens upon exposure to Cd was investigated. Mites raised in high and low habitat quality soils were exposed to Cd in neutral (artificial) soil for 28 days to assess their reproduction and energy reserves, including the activities of glucose metabolism enzymes, glucose-6-phosphate dehydrogenase and pyruvate kinases. Cd was found to alter the carbohydrate reserves of the mites that were exposed to 0–700 mg Cd kg-1 and reduced energy production by inhibiting the activities of glucose metabolism enzymes. Upon exposure of the mites from low and high habitat quality to concentrations equivalent to EC25 and EC50 of Cd, we found habitat quality to directly influence mite’s reproduction but not bioenergetics. This study, thus, supports previous knowledge of how habitat quality can modulate metal-induced toxicity on O. nitens. The findings from this research thus, suggest the incorporation of maternal transfer in setting soil quality guidelines for soil invertebrates and also the inclusion of habitat characterization in procedures for ecological risk assessment of contaminated sites

More than just a substrate for mites : Moss-dominated biological soil crust protected population of the oribatid mite, Oppia nitens against cadmium toxicity in soil

Metal-impacted sites often need aggressive ecorestoration strategies to restore a functional plant-soil system. The use of biological soil crusts for soil stabilization, moisture retention and C and N input in disturbed and contaminated soils is becoming a more common ecorestoration practice. Biological soil crusts comprise cyanobacteria, fungi, lichens, and bryophytes (mostly moss). Moss-dominated BSCs provide significant N mineralization rate in most terrestrial ecosystems. Oribatid mites or moss mites dominate moss-dominated BSCs and provide essential ecosystem services such as decomposition and nutrient cycling. We hypothesized that moss-dominated BSCs would create a high-quality habitat niche for O. nitens to resist Cd-induced toxicity. Adult mites were exposed to Cd for 28 days in soil with or without BSCs that were aged for eight months. Cadmium toxicity to mites in soil without BSCs was 1.7 and 5.4times greater than in soil with BSCs, respectively for the mites reproduction and instantaneous population growth rate (PGRi). The moss-dominated BSC did not reduce Cd bioavailability in the mites but increased the mite's resilience to Cd toxicity, likely mediated by the trophic transfer of calcium from the BSC to the mites. Our work identifies a second mechanistic avenue by which BSCs are useful for ecorestoration, i.e., the improvement of soil invertebrate physiology to resist metal stress