Effects of dimethoate, an organophosphate insecticide, on photosynthesis of five selected phytoplankton species

Several studies in the late eighties have shown that the insecticide dimethoate is not only toxic to insects due to the inhibition of the acetylcholinesterase, but also to phototrophs by the inhibition of photosynthesis. Although the use of dimethoate and also its commercial formulations are not any longer permitted in the EU, it is still under application mostly in private hands or small farms in the Mediterranean area and in legal use in many non-EU countries. The mode of action in the aquatic environment is not clear until now. In the present study we have extended the aquatic toxicity test via photo-synthetic oxygen production by the analysis of Chl a -in vivo fluorescence and xanthophyll cycle pigments, which indicate the capacity to resist against photoinhibition. The study presents a data set including three different green algae and two diatoms to test the variety of responses from the major algal taxa in the aquatic environment. The data show that dimethoate as well as the commercial product Perfekthion inhibits PSII in a similar manner as DCMU (3-(3, 4-Dichlorophenyl)-1, 1-dimethylurea), a PSII inhibitor that decreases the photosynthetic oxygen evolution rate. The formulated product shows a higher toxicity than the pure chemical compound, which indicates that the formulation cocktail increases stability/permeability as key elements controlling toxicity, leading probably to photoinhibition.