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

## TL;DR

This study shows how heavy metals affect antioxidant defenses in a soil ciliate, suggesting it can be used as a bioindicator for soil pollution.

## Contribution

The study introduces Rigidohymena tetracirrata as a potential model organism for ecotoxicological assessments of heavy metal-polluted soils.

## Key findings

- Copper, Zinc, and Cadmium showed varying levels of cytotoxicity in R. tetracirrata.
- Cd and Zn mixtures exhibited antagonistic effects compared to other metal combinations.
- LC20 and LC50 values of metals increased antioxidant activity in the ciliate.

## Abstract

In this study, we evaluated the cytotoxicity and antioxidant activity of the soil ciliate Rigidohymena tetracirrata (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 (p ≤ 0.01) and the LC20 of intracellular Cd (p ≤ 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 p ≤ 0.05, respectively. Regarding hydroxyl scavenging activity (HRSA), the LC50 of extracellular Cd (p ≤ 0.001) and LC50 of intracellular Cu (p ≤ 0.001) exhibited higher antioxidant activity. Therefore, the present study suggests that R. tetracirrata holds considerable potential as bioindicators and could be used as a model organism in ecotoxicological studies of soil polluted by HMs.