First evaluation of novel potential synergistic effects of glyphosate and arsenic mixture on Rhinella arenarum (Anura: Bufonidae) tadpoles

Abstract

The toxicity of glyphosate-based herbicide (GBH) and arsenite (As(III)) as individual toxicants and in mixture (50:50 v/v, GBH-As(III)) was determined in <i>Rhinella arenarum</i> tadpoles during acute (48 h) and chronic assays (22 days). In both types of assays, the levels of enzymatic activity [Acetylcholinesterase (AChE), Carboxylesterase (CbE), and Glutathione S-transferase (GST)] and the levels of thyroid hormones (triiodothyronine; T3 and thyroxine; T4) were examined. Additionally, the mitotic index (MI) of red blood cells (RBCs) and DNA damage index were calculated for the chronic assay. The results showed that the LC₅₀ values at 48 h were 45.95 mg/L for GBH, 37.32 mg/L for As(III), and 30.31 mg/L for GBH-As(III) (with similar NOEC = 10 mg/L and LOEC = 20 mg/L between the three treatments). In the acute assay, Marking's additive index (<i>S</i> = 2.72) indicated synergistic toxicity for GBH-As(III). In larvae treated with GBH and As(III) at the NOEC-48h (10 mg/L), AChE activity increased by 36.25% and 33.05% respectively, CbE activity increased by 22.25% and 39.05 % respectively, and GST activity increased by 46.75% with the individual treatment with GBH and by 131.65 % with the GBH-As(III) mixture. Larvae exposed to the GBH-As(III) mixture also showed increased levels of T4 (25.67 %). In the chronic assay at NOEC-48h/8 (1.25 mg/L), As(III) and GBH-As(III) inhibited AChE activity (by 39.46 % and 35.65%, respectively), but did not alter CbE activity. In addition, As(III) highly increased (93.7 %) GST activity. GBH-As(III) increased T3 (97.34%) and T4 (540.93%) levels. Finally, GBH-As(III) increased the MI of RBCs and DNA damage. This study demonstrated strong synergistic toxicity of the GBH-As(III) mixture, negatively altering antioxidant systems and thyroid hormone levels, with consequences on RBC proliferation and DNA damage in treated <i>R. arenarum</i> tadpoles.