Effects of mixtures containing chlordecone and a dechlorinated by-product on hydra regeneration capacity: use of experimental design to evaluate the toxicity risk associated with remediation programs in long-lasting polluted areas

Chlordecone (CLD), an obsolete insecticide, used in the French West Indies between 1972 and 1993, is persistent in the environment but can be dechlorinated either chemically or under the action of microorganisms. Therefore, if soil remediation programs based on these processes are implemented in areas still contaminated today, those will see their concentrations of dechlorinated derivatives increase and these compounds will be also found in freshwater by streaming, leaching and erosion processes. The purpose of the present study was to evaluate, at environmental concentrations, the toxic effects of mixtures of chlordecone and a three-chlorine substituted byproduct. A hydra clone, which has been confirmed to be Hydra vulgaris Pallas, 1766 has been retained for bioassays where the toxicity has been evaluated by regeneration capacity during exposure. Exposure to mixtures is complex to investigate by classical methods, therefore, an experimental design associated to a mathematical model has been used to predict the effects of all the mixtures and to detect the toxic influence of each compound. The predictive model is discussed regarding the stochastic “endocrine disruptor effect” of CLD. At probable environmental concentrations of the compounds in the mixture, results show that impairment of regeneration capacity is explained mostly by the presence of CLD in the mixtures and support the implementation of remediation programs aimed at dechlorination of this persistent organochlorine pesticide.

Environmental concentrations as ratios of random variables

Human-induced environmental change increasingly threatens the stability of socio-ecological systems. Careful statistical characterization of environmental concentrations is critical to quantify and predict the consequences of such changes on human and ecosystems conditions. However, while concentrations are naturally defined as the ratio between solute mass and solvent volume, they have rarely been treated as such, typically limiting the analysis to familiar distributions generically used for any other environmental variable. To address this gap, we propose a more general framework that leverages their definition explicitly as ratios of random variables. We show that the resulting models accurately describe the behavior of nitrate plus nitrite in US rivers and salt concentration in estuaries in the Everglades by accounting for heavy tails potentially emerging when the water volume fluctuates around low values. Models that preclude the presence of heavy tails and the related high probability of extreme concentrations could significantly undermine the accuracy of diagnostic frameworks and the effectiveness of mitigation interventions, especially for soil contamination characterized by a water volume (i.e., soil moisture) frequently approaching zero.

Contrasting Effects of Environmental Concentrations of Sulfonamides on Microbial Heterotrophic Activities in Freshwater Sediments

The sulfonamide antibiotics sulfamethoxazole (SMX) and sulfamethazine (SMZ) are regularly detected in surface sediments of contaminated hydrosystems, with maximum concentrations that can reach tens of μg kg–1 in stream and river sediments. Little is known about the resulting effects on the exposed benthic organisms. Here we investigated the functional response of stream sediment microbial communities exposed for 4 weeks to two levels of environmentally relevant concentrations of SMX and SMZ, tested individually. To this end, we developed a laboratory channel experiment where natural stream sediments were immersed in water contaminated with nominal environmental concentrations of 500 and 5,000 ng L–1 of SMX or SMZ, causing their accumulation in surface sediments. The mean maximum concentrations measured in the sediment (about 2.1 μg SMX kg–1 dw and 4.5 μg SMZ kg–1 dw) were consistent with those reported in contaminated rivers. The resulting chronic exposure had various effects on the functional potential of the sediment microbial communities, according to the substance (SMX or SMZ), the type of treatment (high or low) and the measured activity, with a strong influence of temporal dynamics. Whereas the SMZ treatments resulted in only transient effects on the five microbial activities investigated, we observed a significant stimulation of the β-glucosidase activity over the 28 days in the communities exposed to the high concentration of SMX. Together with the stimulation of aerobic respiration at low SMX concentrations and the reduced concentration observed in the last days, our results suggest a potential biodegradation of sulfonamides by microbial communities from sediments. Given the key functional role of surface sediment microbial communities in streams and rivers, our findings suggest that the frequently reported contamination of sediments by sulfonamides is likely to affect biogeochemical cycles, with possible impact on ecosystem functioning.