Effects of an environmentally relevant temporal application scheme of low herbicide concentrations on larvae of two anuran species

Using mesocosms to evaluate the impacts of pasture intensification and pasture-sugarcane conversion on tadpoles in Brazil

This study evaluated the effects of environmental contamination caused by pasture intensification and pasture-sugarcane conversion on oxidative stress, biotransformation, esterase enzymes, and development of Scinax fuscovarious and Physalaemus nattereri. Tadpoles were exposed in mesocosms allocated in three treatments: (1) untreated extensive pasture (EP); (2) intensive-pasture conversion (IP) (2,4-D herbicide + fertilizers); and (3) pasture-sugarcane conversion (SC) (fipronil + 2,4-D + fertilizers). After 7 days of exposure, IP reduced catalase (CAT) and increased malondialdehyde (MDA) levels in P. nattereri, while this treatment decreased glucose-6-phosphate dehydrogenase (G6PDH) and CAT activities in S. fuscovarious. SC decreased CAT, G6PDH, and glutathione S-transferase (GST) activities in P. nattereri. In S. fuscovarius, SC reduced G6PDH, acetylcholinesterase (AChE), and carboxylesterase (CbE) activities. MDA was raised in both tadpole species exposed to SC, evidencing oxidative stress. Integrated biomarker responses showed higher scores in both species exposed to SC. Our results warn that management practices currently applied to sugarcane cultivation in Brazil can negatively impact the functional responses of amphibians at natural systems.

DNA damage exerted by mixtures of commercial formulations of glyphosate and imazethapyr herbicides in Rhinella arenarum (Anura, Bufonidae) tadpoles

Glyphosate (GLY) and imazethapyr (IMZT) are two herbicides commonly used worldwide, either alone or in mixtures. They represent key pesticides in modern agricultural management. The toxicity that results when employed as mixtures has not been characterized so far. Acute toxicity of the 48% GLY-based herbicide (GBH) Credit® and the 10.59% IMZT-based herbicide (IBH) Pivot® H alone and their binary combinations was analyzed in Rhinella arenarum tadpoles exposed in a semi-static renewal test. Lethal effects were determined using mortality as the end-point, whereas sublethal effects were determined employing the single-cell gel electrophoresis (SCGE) bioassay. Based on mortality experiments, results revealed LC5096 h values of 78.18 mg/L GBH and 0.99 mg/L IBH for Credit® and Pivot® H, respectively. An increase in the genetic damage index (GDI) was found after exposure to Credit® or Pivot® H at 5 and 10% of LC5096 h values. The combinations of 5% Credit®-5% Pivot® H LC5096 h and 10% Credit®-10% Pivot® H LC5096 h concentrations significantly enhanced the GDI in comparison with tadpoles exposed only to Credit® or Pivot® H. Thus, the effect of interaction between GBH and IBH inducing DNA damage in R. arenarum blood cells can be considered to be synergistic.

Population and life-stage-specific effects of two herbicide formulations on the aquatic development of European common frogs (Rana temporaria)

Environmental contamination is suggested to contribute to amphibian population declines. However, the effects of a contaminant on a particular amphibian species can differ among populations. The authors investigated the toxic effects of 2 herbicide formulations on different populations and on representative developmental stages of the European common frog (Rana temporaria). Larvae from forest populations were more sensitive to a commonly used glyphosate-based herbicide compared with individuals from agrarian land. Median lethal concentrations correlated with measured glyphosate levels in the breeding ponds, which may be a sign of evolved tolerances. The reverse result was observed for a less commonly used cycloxydim-based herbicide. Effects of the glyphosate-based herbicide were stronger for earlier larval stages compared with later larval stages. Hence, applications in early spring (when early larvae are present in breeding ponds) pose greater risk concerning acute toxic effects on R. temporaria. With regard to late larval stages, short exposure (96 h) of prometamorphic larvae prolonged time to metamorphosis, but only at the highest test concentration that did not significantly induce mortality. This could be due to impairment of the thyroid axis. Notably, nearly all test concentrations of the 2 herbicides provoked growth retardation. Further research on how evolved or induced tolerances are acquired, actual contamination levels of amphibian habitats, and potential endocrine effects of glyphosate-based herbicides is necessary. Environ Toxicol Chem 2017;36:190-200. © 2016 SETAC.

Effects of a commonly used glyphosate-based herbicide formulation on early developmental stages of two anuran species

Environmental contamination, especially due to the increasing use of pesticides, is suggested to be one out of six main reasons for the global amphibian decline. Adverse effects of glyphosate-based herbicides on amphibians have been already discussed in several studies with different conclusions, especially regarding sublethal effects at environmentally relevant concentrations. Therefore, we studied the acute toxic effects (mortality, growth, and morphological changes) of the commonly used glyphosate-based herbicide formulation Roundup® UltraMax on early aquatic developmental stages of two anuran species with different larval types (obligate vs. facultative filtrating suspension feeders), the African clawed frog (Xenopus laevis) and the Mediterranean painted frog (Discoglossus pictus). While X. laevis is an established anuran model organism in amphibian toxicological studies, we aim to establish D. pictus as another model for species with facultative filtrating larvae. A special focus of the present study lies on malformations in X. laevis embryos, which were investigated using histological preparations. In general, embryos and larvae of X. laevis reacted more sensitive concerning lethal effects compared to early developmental stages of D. pictus. It was suggested, that especially the different morphology of their filter apparatus and the higher volume of water pumped through the buccopharynx of X. laevis larvae lead to higher exposure to the formulation. The test substance induced similar lethal effects in D. pictus larvae as it does in the teleost standard test organism used in pesticide approval, the rainbow trout (Oncorhynchus mykiss), whereas embryos of both species are apparently more tolerant and, conversely, X. laevis larvae about two times more sensitive. In both species, early larvae always reacted significantly more sensitive than embryos. Exposure to the test substance increased malformation rates in embryos of both species in a concentration-dependent manner, but not at environmentally relevant concentrations. However, the assumed field safety, based on calculated surface water concentrations of the active ingredient (glyphosate), should be validated with realistic field data and buffer strips have to be urgently regarded to any aquatic amphibian habitat.

Genotoxic effect of a binary mixture of dicamba- and glyphosate-based commercial herbicide formulations on Rhinella arenarum (Hensel, 1867) (Anura, Bufonidae) late-stage larvae

The acute toxicity of two herbicide formulations, namely, the 57.71 % dicamba (DIC)-based Banvel(®) and the 48 % glyphosate (GLY)-based Credit(®), alone as well as the binary mixture of these herbicides was evaluated on late-stage Rhinella arenarum larvae (stage 36) exposed under laboratory conditions. Mortality was used as an endpoint for determining acute lethal effects, whereas the single-cell gel electrophoresis (SCGE) assay was employed as genotoxic endpoint to study sublethal effects. Lethality studies revealed LC5096 h values of 358.44 and 78.18 mg L(-1) DIC and GLY for Banvel(®) and Credit(®), respectively. SCGE assay revealed, after exposure for 96 h to either 5 and 10 % of the Banvel(®) LC5096 h concentration or 5 and 10 % of the Credit(®) LC5096 h concentration, an equal significant increase of the genetic damage index (GDI) regardless of the concentration of the herbicide assayed. The binary mixtures of 5 % Banvel(®) plus 5 % Credit(®) LC5096 h concentrations and 10 % Banvel(®) plus 10 % Credit(®) LC5096 h concentrations induced equivalent significant increases in the GDI in regard to GDI values from late-stage larvae exposed only to Banvel(®) or Credit(®). This study represents the first experimental evidence of acute lethal and sublethal effects exerted by DIC on the species, as well as the induction of primary DNA breaks by this herbicide in amphibians. Finally, a synergistic effect of the mixture of GLY and DIC on the induction of primary DNA breaks on circulating blood cells of R. arenarum late-stage larvae could be demonstrated.

Thyroid endocrine disruption of azocyclotin to Xenopus laevis during metamorphosis

Organotin compounds are ubiquitous contaminants that are frequently detected in the environment and in biota, which raises concern about their risk to wildlife and human health. In the present study, Nieuwkoop & Faber stage 51 Xenopus laevis tadpoles were exposed to different concentrations of azocyclotin (0, 0.02, 0.1 and 0.5μg/L) for 21 days, during which time the tadpoles underwent morphological development. Exposure to azocyclotin caused an inhibitory effect on the pre-metamorphic development of X. laevis (e.g., a shortened hind limb length). Azocyclotin induced an alteration of the triiodothyronine (T3) content, which indicated thyroid endocrine disruption. Real-time PCR was performed to examine the expression levels of the genes involved in the thyroid hormone (TH) signaling pathway. Significant down-regulation of the type 2 deiodinase gene was observed, which may be partially responsible for the decreased T3 concentrations. Furthermore, the expression of T3 responsive genes, including thyroid hormone receptor, basic transcription element binding protein, 2tromelysins-3 and matrix metalloproteinase 2, were down-regulated in tadpoles, suggesting that azocyclotin induced a decrease in the T3 contents and, in turn, affected the mRNA expression of downstream genes involved in multiple physiological responses. Chemical analysis showed that azocyclotin could accumulate in X. laevis after 21 days of exposure. In conclusion, the results of the present study showed that azocyclotin could alter the mRNA expression of genes involved in TH signaling as well as the thyroid hormone concentrations in X. laevis tadpoles, leading to endocrine disruption of thyroid system, and that azocyclotin had obvious inhibitory effects on X. laevis metamorphosis.

Integrated assessment of biochemical markers in premetamorphic tadpoles of three amphibian species exposed to glyphosate- and methidathion-based pesticides in single and combination forms

In this study, we evaluated the toxic effects of a glyphosate-based herbicide (GBH) and a methidathion-based insecticide (MBI), individually and in combination, on premetamorphic tadpoles of three anuran species: Pelophylax ridibundus, Xenopus laevis, and Bufotes viridis. Based on the determined 96-h LC50 values of each species, the effects of a series of sublethal concentrations of single pesticides and their mixtures after 96-h exposure and also the time-related effects of a high sublethal concentration of each pesticide were evaluated, with determination of changes in selected biomarkers: glutathione S-transferase (GST), glutathione reductase (GR), acetylcholinesterase (AChE), carboxylesterase (CaE), aspartate aminotransferase (AST), and lactate dehydrogenase (LDH). Also, the integrated biomarker response (IBR) was used to assess biomarker responses and quantitatively evaluate toxicological effects. Isozyme differences in CaE inhibition were assessed using native page electrophoresis; results showed that GBH to cause structural changes in the enzyme but not CaE inhibition in P. ridibundus. In general, single MBI and pesticide mixture exposures increased GST activity, while single GBH exposures decreased GST activity in exposed tadpoles. The AChE and CaE activities were inhibited after exposure to all single MBI and pesticide mixtures. Also, higher IBR values and GST, GR, AST, and LDH activities were determined for pesticide mixtures compared with single-pesticide exposure. This situation may be indicative of a synergistic interaction between pesticides and a sign of a more stressful condition.