Effects of xenobiotic compounds on the cell activities of Euplotes crassus, a single-cell eukaryotic test organism for the study of the pollution of marine sediments

It is now widely accepted that assays with protists are relevant to be exploited for the study of environmental modifications due to the presence of xenobiotic compounds. In this work, the possibility of utilizing Euplotes crassus, an interstitial marine ciliate, for the pre-chemical screening of estuarine and coastal sediments was evaluated. For this purpose, the effects of exposure to pollutants were tested on the cell viability, fission rate and lysosomal membrane stability of E. crassus. The following toxicants were used: an organophosphate (OP) pesticide, basudin, an organochlorine hydrocarbon, AFD25, both employed especially for pest control in agricultural sites, a toxic heavy metal, mercury (HgCl2) and different mixtures of the above-mentioned compounds, as they might occur in polluted sites. Exposure to these toxicants affected cell viability at concentrations ranging from 96.6 to 966 x 10(3)mg/l for basudin, from 3.3 to 33 x 10(3)mg/l for AFD25 and from 0.1 to 1mg/l for HgCl2. A significant decrease in the mean fission rate (P<0.001) was found after 24- or 48-h exposures to 9.66 mg/l basudin, 3.3 mg/l AFD25 and 7 x 10(-2)mg/l HgCl2. Furthermore, the Neutral Red Retention Assay showed a significant decrease in lysosomal membrane stability after 60- and 120-min exposures to AFD25 (33 mg/l) and HgCl2 (0.33 mg/l). In addition, as it is well-known that the inhibition of acetylcholinesterase activity represents a specific biomarker of exposure to OP and carbamate pesticides in higher organisms, initially the presence of cholinesterase (ChE) activity was detected in E. crassus, using cytochemical, spectrophotometric and electrophoretic methods. Afterwards, this enzyme activity was characterized spectrophotometrically by its sensitivity to specific ChE inhibitors and to variations in pH and temperature. The ChE activity was inhibited significantly by basudin- (9.66 and 96.6 mg/l) or AFD25-exposure (3.3 mg/l). Conversely, exposure to AFD25 (33 mg/l) or HgCl2 (0.1 and 0.3mg/l) caused a significant increase in this enzyme activity. Moreover, exposure to mixtures containing basudin, AFD25 and HgCl2 was found to affect the cell viability, the mean fission rate and the ChE activity differently, in an unpredictable manner. Our results indicate that E. crassus seems to be a suitable test organism to evaluate the toxicity of marine sediments.