The soil ciliated protozoa Cyrtoymena tetracirrata as bioindicators of soil health: cytotoxicity of single and bimetallic mixtures of heavy metals and antioxidant defenses

As far as we known, the releases of heavy metals (HMs) to the environments and particularly in agricultural soils, has increased globally in the last few years. Although, a number of reviews have analysed the effects of HM contaminated soils using various microbial test organisms, few studies have focused on soil ciliated protozoa as potential bioindicators of soil health. Thus, in this study we have evaluated the cytotoxic effects and the presence of antioxidant activities in the soil ciliate Cyrtoymena tetracirrata exposed for 24hrs to both single (Cd, Cu, and Zn) and bimetallic mixtures (Cd + Zn, Cd + Cu, and Cu + Zn) of HMs. Ecotoxicological tests showed, LC20 values to Cd, Cu, and Zn of 0.53, 0.22 and 23.0 mg L-1, respectively and, LC50 values of 1.16, 0.37 and 32.7 mg L-1, respectively. The order of toxicity was Cu > Cd >> Zn. Using the Concentration Addition model approach we found that the Cd + Zn bimetallic mixtures treatments were able to prevalently produce antagonistic effects in comparison to the other mixtures (Cd + Cu, and Cu + Zn). With regards to the analysis of the antioxidant activities, the total phenolic contents (TPC) were significantly higher in the ciliates exposed to Cu at the LC20 value previously identified (extracellular: P ≤ 0.01) and to Cd at the LC20 value (intracellular: P ≤ 0.001). Higher α,α-diphenyl-β-picrylhydrazyl (DPPH) scavenging activities were found at the Cd and Zn LC50 values for both the extracellular and intracellular contents (P ≤ 0.05). Hydroxyl scavenging activity (HRSA), unveiled that higher levels of antioxidants were produced following exposures to Cd LC50 (extracellular: P ≤ 0.001) and Cu LC50 values (intracellular: P ≤ 0.001). Overall the present study reveals, that C. tetracirrata has the potential to be used as bio-indicator of HM polluted soils.