**Author details**

**3.4. Toxicity of the metabolites of fenthion on growth of marine alga** *T. suecica*

The abiotic and biotic degradation of organophosphorus pesticides has been extensively studied in a large number of studies. Various data concerning the metabolism of several organophosphates in terrestrial and aquatic species, either in vivo or in vitro, are available [4–6]. After the application of Eqs. (3) and (4) for the prediction of the toxicity of 13 principal metabolites and degradation products of fenthion that have been identified in environmental samples, the predicted EC50 values for *T. suecica* are listed in **Table 5**. Additionally, the chemical structures of those compounds, called as metabolites and degradation products, are presented in **Figure 5**.

According to predicted EC50 values of Eq. (3), the parent chemical was more toxic than all of its metabolites, while on the contrary, according to Eq. (4), all of the 13 metabolites and degradation products of fenthion were expected to be more toxic than the parent compound. The acquired toxicity based on QSAR containing log *POW* data [Eq. (3)] followed the order: fenthion > demethyl fenthion > fenthion phenol > fenthion sulfone > fenthion oxon > fenthion sulfoxide > demethyl fenthion oxon > demethyl fenthion sulfone > demethyl fenthion sulfoxide > fenthion phenol sulfoxide > fenthion phenol sulfone > fenthion oxon sulfone > fenthion oxon sulfoxide > demethyl fenthion oxon sulfoxide. Interestingly, fenthion oxon that is the transformation product of fenthion by oxidative desulfuration was not predicted to be as toxic to *Tetraselmis suecica* up as the parent compound fenthion. EC50 value of fenthion oxon was estimated to be 25.38 mg L−1, approximately six times higher than EC50 of fenthion, which was 4.27 mg L−1. That fact could be attributed either to physicochemical properties of the compound (such as the highest water solubility and lowest octanol water partition coefficient) or to low persistence of the molecule into marine ecosystems as it undergoes under rapid hydrolysis. On the contrary, the calculated toxicity based on QSAR containing log *S* data [Eq. (4)] followed the order: demethyl fenthion oxon sulfoxide > fenthion phenol sulfoxide > fenthion phenol sulfone > fenthion oxon sulfoxide = demethyl fenthion oxon > fenthion oxon sulfone = demethyl fenthion sulfoxide > fenthion phenol > fenthion oxon > demethyl fenthion sulfone > fenthion sulfoxide > demethyl fenthion > fenthion sulfone > fenthion. This observation is in accordance with EC50 values found for the organophosphorus pesticide fenamiphos and its oxidation products fenamiphos sulfoxide (FSO) and fenamiphos sulfone (FSO2

toward the aquatic alga *Pseudokirchneriella subcapitata* and the terrestrial alga *Chlorococcum* sp.,

Based on the results of the current study, it appeared that fenthion can be highly toxic to the marine microalgal strain *T. suecica*. Experimental data revealed that the examined organophosphorus pesticide had marked effects on the growth of the tested algae since treatment concentrations above 1.00 mg L−1 affected algal densities and significantly decreased specific growth rate values. The finding that reduction of chlorophyll pigment production was observed due to exposure to fenthion indicated that this parameter could be used as a pollution biomarker. Moreover, two quantitative structure activity relationships, QSARs, based on physicochemical properties of the toxicants were applied for the prediction of toxicity values EC50 of the

which proved that parent compound was less toxic than its metabolites [33].

**4. Conclusions**

132 Insecticides - Agriculture and Toxicology

Maria C. Vagi<sup>1</sup> \*, Andreas S. Petsas1,2, Maria D. Pavlaki3 , Niki M. Smaragdaki1 and Maria N. Kostopoulou1

\*Address all correspondence to: mbag@env.aegean.gr

1 Department of Marine Sciences, Laboratory of Environmental Quality and Geo-spatial Applications, School of the Environment, University of the Aegean, Mytilene, Lesvos, Greece

2 Department of Food Sciences and Nutrition, School of the Environment, University of the Aegean, Myrina, Lemnos, Greece

3 Department of Biology and Center for Environmental and Marine Studies (CESAM), University of Aveiro, Aveiro, Portugal
