Ecotoxicological and potential endocrine effects of selected aquatic herbicides on life stages of the African clawed frog, Xenopus laevis
Thesis (PhD)--Stellenbosch University, 2016.
ENGLISH ABSTRACT: Many pesticides have shown health impacts in wildlife and humans, including endocrine system modulation. The potential health effects of the pesticides elicit concerns considering high volume being used in South Africa, especially herbicides in agriculture and alien plants eradication at the government “Working for Water” programme. These herbicides included Midstream (diquat dibromide), Basta (glufosinate ammonium), Arsenal (imazapyr), Roundup, Kilo-Max and Enviro-glyphosate (glyphosate) formulation. For most of these herbicides, toxicity and endocrine modulating potential are unknown. The present study therefore assessed the toxicity and endocrine modulating potential through teratogenicity, thyroidal and reproductive disruption, using the African Clawed frog, X. laevis as sentinel species. International validated protocols, including a 96-hour toxicity assay; frog embryo teratogenic assay (FETAX); Xenopus metamorphosis assay (XEMA), and semi-static exposure of adult male frogs, were used to evaluate relevant endpoints. The result showed that in 96-hour comparative toxicity study, the early tadpoles (NF-stage 48) in comparison to the embryos (NF-stage 8-11) and transitional tadpoles (NF-stage 60), were the most sensitive and vulnerable in the developmental stages of X. laevis. This present study confirmed a stage-dependent sensitivity to environmental herbicides during metamorphosis/developmental program. In the FETAX study, the 96-hour lethal concentration (LC50) for Midstream, Arsenal, Basta, Roundup, Kilo-Max and Enviro-glyphosate were 0.833, 35.99, 2.24, 1.052, 207.25 and 465.95 mg/L respectively. The 96-hour effective concentrations (EC50) were 0.241, 28.13, 2.01, 0.76, 150.8 and 287 mg/L, respectively. The LC50/EC50 ratio produced a positive teratogenic index (TI) for Midstream and Enviro-glyphosate with a TI of 3.5, 1.3, 1.1, 1.4, 1.4 and 1.6 for Midstream, Arsenal, Basta, Roundup, Kilo-Max and Enviro-glyphosate formulations, respectively. The minimum concentrations inhibiting growth were computed to be 0.5, 2.0, 2.0, 0.9, 190 and 440 mg/L for Midstream, Basta, Arsenal, Roundup, Kilo Max, Enviro-glyphosate formulations, respectively. Observed malformations from these formulations included edema (cardiac, abdominal and severe), gut, axial, blisters and eye malformations. The XEMA results (thyroidal modulation) revealed that Arsenal, Midstream, and Kilo-Max formulations resulted in thyroidal modulation as these significantly reduced the average developmental stages relative to the control tadpoles after 21-days exposure. Extending XEMA to completion of metamorphosis revealed a significantly skewed sex ratios at 0.14 and 280 mg/L of Midstream and Kilo-Max formulations, respectively. The gonadotoxicity assessment showed that the highest abnormality index ranged from 60%, 43%, 37.5%, 35%, 30% and 27.5% for Midstream, Kilo-Max, Enviro-Glyphosate, Arsenal, Roundup and Basta formulations, respectively. The observed gonadal malformations included folded gonads, segmented hypertrophy, hypoplasia, segmented aplasia and translucence. For adult exposure, no significant difference in the results, showing that adult, relative to other developmental stages, are less sensitive to exposure impacts of pesticides. In summary, different stages were differentially sensitives to the herbicide toxicity. The six formulations showed differential endocrine modulation, including teratogenicity (Midstream and Enviro-glyphosate), thyroid and growth disruption (all the six formulations), gonadotoxicity (all the six formulations), skewed sex ratios (Midstream and Kilo-Max) and estrogenicity/antiandrogen (Midstream). Finally, these herbicides have shown secondary effects on non-targets organisms, therefore their inclusion in aquatic weeds control should be investigated beyond lethal toxicity but also endocrine modulation.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar