Heavy Metals Affect the Antioxidant Defences in the Soil Ciliate Rigidohymena tetracirrata

In this study, we evaluated the cytotoxicity and antioxidant activity of the soil ciliate <i>Rigidohymena tetracirrata</i> (Gellért, 1942) Berger 2011, exposed to single and bimetallic mixtures of heavy metals (HMs) for 24 h. Ecotoxicological tests showed LC₂₀ values of 0.16, 19.86 and 0.68 mg L^-1 to Copper (Cu), Zinc (Zn), and Cadmium (Cd), respectively, and LC₅₀ values of 0.25, 44.12 and 1.12 mg L^-1, respectively. Furthermore, it was observed that the mixture of Cd and Zn exhibited antagonism in comparison to other mixtures, (Cd + Cu and Cu + Zn). In the total phenolic content (TPC) assay, a higher phenolic content was observed for the LC₂₀ of extracellular Cu (<i>p</i> ≤ 0.01) and the LC₂₀ of intracellular Cd (<i>p</i> ≤ 0.001). The LC₅₀ values for Cd and Zn in both extracellular and intracellular contents demonstrated increased α,α-diphenyl-β-picrylhydrazyl (DPPH) scavenging activity with significant values of <i>p</i> ≤ 0.05, respectively. Regarding hydroxyl scavenging activity (HRSA), the LC₅₀ of extracellular Cd (<i>p</i> ≤ 0.001) and LC₅₀ of intracellular Cu (<i>p</i> ≤ 0.001) exhibited higher antioxidant activity. Therefore, the present study suggests that <i>R. tetracirrata</i> holds considerable potential as bioindicators and could be used as a model organism in ecotoxicological studies of soil polluted by HMs.