N2 - Multiple anthropogenic agents have turned Lake Maracaibo into a hypereutrophic environment. Heavy metals resulting from the steel and oil industry augment pollution in the lake. There is a lack of research on the ecotoxicological effect of heavy metals in protozoa. To evaluate the ecotoxicological effect of Cr3+, Cr6+, Cd2+, Pb2+ and Ni2+ on free-living ciliated protozoa and to identify suitable ciliated protozoa candidates for bioindicators of water quality; we estimated the lethal concentration for 50% of the protozoa population (LC50) in samples from two stations (S1: narrow of Maracaibo and S2: South of the lake) using ecotoxicological tests in the Sedgewick–Rafter chamber and Probit analysis. The general toxicity patterns obtained for S1 protozoa (Euplotes sp. and Oxytricha sp.) were Cr3+ > Cd2+ > Pb2+ > Cr6+ > Ni2+; and those corresponding to S2 (Coleps sp. and Chilodonella sp.) were Cr6+ > Cr3+ > Cd2+ > Pb2+ > Ni2+. We found statistically significant difference (p < 0.05) in the LC50 of protozoa exposed to Cr3+, Cr6+, Ni2+ and Pb2+ when comparing the two sampling stations. The differences observed in toxicity patterns are probably the result of various kinds of protozoa adaptation, possibly induced by various sources, levels and incidents of exposure to heavy metals contamination of the protozoa studied and to the physicochemical conditions prevailing in the two selected stations. The levels of tolerance observed in the present study, allow us to infer that S2 ciliates are the most susceptible to the contaminants studied and can be used as possible microbiological indicators that provide early warning in studies of contamination by heavy metals in Lake Maracaibo. L1 - http://journals.pan.pl/Content/122152/PDF/2021-04-JWLD-13-Echavez.pdf L2 - http://journals.pan.pl/Content/122152 PY - 2021 IS - No 51 EP - 116 DO - 10.24425/jwld.2021.139020 KW - contamination KW - ecotoxicological tests KW - lethal concentration LC50 KW - microbiological indicators of early warning KW - toxicity A1 - Echavez, Fernando Luis Castro A1 - Leal, Julio César Marín PB - Polish Academy of Sciences; Institute of Technology and Life Sciences - National Research Institute DA - 2022.01.10 T1 - Ecotoxicological effect of heavy metals in free-living ciliate protozoa of Lake Maracaibo, Venezuela SP - 102 UR - http://journals.pan.pl/dlibra/publication/edition/122152 T2 - Journal of Water and Land Development