Toxicity assessment of diesel- and metal-contaminated soils through elutriate and solid phase assays with the slime mold Dictyostelium discoideum.

A suite of organisms from different taxonomical and ecological positions is needed to assess environmentally relevant soil toxicity. A new bioassay based on Dictyostelium is presented that is aimed at integrating slime molds into such a testing framework. Toxicity tests on elutriates and the solid phase developmental cycle assay were successfully applied to a soil spiked with a mixture of Zn, Cd, and diesel fuel freshly prepared (recently contaminated) and after 2 yr of aging. The elutriates of both soils provoked toxic effects, but toxicity was markedly lower in the aged soil. In the D. discoideum developmental cycle assay, both soils affected amoeba viability and aggregation, with fewer multicellular units, smaller fruiting bodies and, overall, inhibition of fruiting body formation. This assay is quick and requires small amounts of test soil, which might facilitate its incorporation into a multispecies multiple-endpoint toxicity bioassay battery suitable for environmental risk assessment in soils. Environ Toxicol Chem 2016;35:1413-1421. © 2015 SETAC.

Dictyostelium discoideum developmental cycle (DDDC) assay: a tool for Hg toxicity assessment and soil health screening.

The social amoeba Dictyostelium discoideum has been proposed for assessing stress responses to pollutants in soil and it has already been successfully employed in the aquatic environment. Presently, we developed the DDDC assay (D. discoideum developmental cycle assay) for both soil toxicity assessment and soil health screening. The DDDC assay is primarily aimed at determining the capacity of D. discoideum to undergo its developmental programme forming a fruiting body, measured in terms of fruiting body formation inhibition and fruiting body size factor, which may be considered an indication of its ecological fitness (potential for spore dispersal). A second objective of the solid phase DDDC assay is to identify potential mechanisms of toxic action on the developmental cycle, for which three checkpoints are examined: (a) aggregation arrest, (b) migration arrest, and (c) culmination arrest. Presently, conditions for the DDDC assay such as soil texture, soil water content, soil pH, food availability and incubation time were investigated and optimized. In addition, both solid and liquid phase variants of the DDDC assay were applied to assess the toxicity of Hg, at regulatory concentrations. The developmental cycle and ecological fitness were affected from the exposure to 0.3 mg Hg/kg dry-wt soil onwards. The DDDC assay has been shown to be a high sensitivity test.

Towards an integrative soil health assessment strategy: a three tier (integrative biomarker response) approach with Eisenia fetida applied to soils subjected to chronic metal pollution.

This is a pilot study for assessing soil ecosystem health in chronically polluted sites on the basis of a 3-tier approach (screening+scoring+understanding) designed to be cost-effective and scientifically based, and to provide straightforward advice and support to managers and stakeholders involved in environmental protection. For the initial screening (Tier 1), the use of a highly sensitive, low-cost biomarker such as neutral red uptake (NRU) in earthworm coelomocytes is proposed. In sites where an alteration in NRU has been established, the stress level may be further assessed by utilising a suite of low-cost and rapid biomarkers of effect integrated in an integrative biological response (IBR) index to obtain an objective (scored) assessment of the induced stress syndrome (Tier 2). The IBR/n index is based on the integration of biomarkers at different levels of biological organisation. Acyl-CoA oxidase activity (AOX), catalase activity (CAT), lipofuscin optical density (LOD%), NRU and the mean epithelial thickness (MET) have been used to calculate the IBR/n index. Biomarkers are determined in earthworms, Eisenia fetida, exposed ex situ to real soils (three mining sites and a reference) for 3, 10 and 17d. The 3d NRU (Tier 1) provided signal of stress. After 3d, PCA, based on the suite of biomarkers (Tier 2), discriminated reference and polluted sites according to toxicity profiles and at 17d, the most polluted site is segregated from less polluted and reference sites. Soils were classified as harmful, unhealthy (not apparently toxic) or healthy. Soils were investigated by microarray transcriptomics (Tier 3), to understand the causes (aetiology) and consequences (prognosis) of health impairment. Tier 3 discriminates, according to stress syndrome traits, soils that did not fall into the category of highly stressed and revealed the main agent causing toxicity at each site by identifying the toxicity mechanisms and biological responses.