Ciliates as model organisms for the ecotoxicological risk assessment of heavy metals: A meta-analysis

Ciliates are key components of aquatic ecosystems, significantly contributing to the decomposition of organic matter and energy transfer to higher trophic levels. They are considered good biological indicators of chemical pollution and relatively sensitive to heavy metal contamination. In this study, we performed a meta-analysis of the available toxicity data of heavy metals and ciliates to assess: (1) the sensitivity of freshwater ciliates to different heavy metals, (2) the relative sensitivity of ciliates in comparison to the standard test species used in ecotoxicological risk assessment, and (3) the difference in sensitivity across ciliate taxa. Our study shows that the tolerance of ciliates to heavy metals varies notably, which is partly influenced by differences in methodological conditions across studies. Ciliates are, in general, sensitive to Mercury > Cadmium > Copper > Zinc > Lead > Chromium. Also, this study shows that most ciliates are more tolerant to heavy metal pollution than the standard test species used in ecotoxicological risk assessments, i.e., Raphidocelis subcapitata, Daphnia magna, and Onchornyncus mykiss. Threshold concentrations derived from toxicity data for these species is expected to confer sufficient protection for the vast majority of ciliate species. Our data analysis also shows that the most commonly tested ciliate species, Paramecium caudatum and Tetrahymena thermophila, are not necessarily the most sensitive ones to heavy metal pollution. Finally, this study stresses the importance of developing standard toxicity test protocols for ciliates, which could lead to a better comprehension of the toxicological impact of heavy metals and other contaminants to ciliate species.