Acute effects of Anabaena spiroides extract and paraoxon-methyl on freshwater cladocerans from tropical and temperate regions: Links between the ChE activity and survival and its implications for tropical ecotoxicological studies

Abstract

Cholinesterase (ChE) activity was measured in Pseudosida ramosa and Daphnia magna, which had previously been exposed to Anabaena spiroides extract or to paraoxon-methyl for 48h. These activities were then related to survival at 48h. For A. spiroides extract, the observed 48-h LC50 was 2.27 and 2.70×106cellsmL−1, while for paraoxon-methyl it was 0.60 and 2.17μgL−1, respectively, for P. ramosa and D. magna. Dose–response relationships were obtained for both P. ramosa and D. magna, when exposed to A. spiroides extract or paraoxon-methyl. Thus, when the tested concentrations of the toxicants increased, ChE activity and survival decreased. The ratio between 48-h IC50 for ChE and 48-h LC50 ranged from 75% to 81% for P. ramosa and from 77% to 81% for D. magna. This indicated that the concentrations of both A. spiroides extract and paraoxon-methyl that cause 50% mortality also inhibit ChE activity by 50%. Also, it was found that, for P. ramosa, a 50% inhibition of ChE activity was associated with a survival of 59.5% and 60.9%, respectively, for A. spiroides extract and paraoxon-methyl. However, for D. magna, at high levels of inhibition of ChE activity, almost no mortality was detected. In this specific case, 50% inhibition of the ChE activity was associated with 90.4 and 95.4% survival for A. spiroides extract and paraoxon-methyl, respectively. In contrast, enzyme inhibition slightly above 60% had a strong detrimental effect on survival in D. magna. These different patterns found in the relationship between ChE inhibition and survival may be due to species-specific differences in the affinities of both acetylcholinesterase and pseudocholinesterases, since the cladoceran ChE assays were performed with whole-body homogenates. In conclusion, when using ChE as a biochemical biomarker in risk assessment of cyanobacterial neurotoxic blooms in tropical regions, it is strongly recommended that native species are used, since our results revealed that P. ramosa was more sensitive than D. magna for both assay endpoints and both toxicants. Furthermore, the relationship between ChE activity and survival had a species-specific response. Therefore, the use of the model species D. magna in acute toxicity tests and ChE assays in tropical regions may lead to errors in the estimation of risks to the local species.