In the present study, two strains of Ectocarpus siliculosus with different pollution histories, LIA4 and Es524, were exposed to three PSII inhibitor herbicides, diuron (DIU), terbuthylazine (TBA) and isoproturon (IPU). Evaluation of their effects on growth and photosynthetic efficiency (Fv/Fm, ΦPSII, qP) have demonstrated negative impacts of all herbicides on both strains. With reference to the 7 d RGR EC50 values; DIU (9.9 - 25 μg L-1); TBA (18 – 28 μg L-1); IPU (257 – 315 μg L-1), the reported environmental concentrations for DIU were higher than the values that exert negative effects to E. siliculosus. However, as for TBA and IPU, the environmental concentrations were below the values which caused detrimental effects to E. siliculosus. Strain Es524, which originated from a Cu-polluted site in Chile, was found to exhibit greater resistance to the herbicides, with higher EC50 values recorded, compared to LIA4. To further elucidate the factors contributing to the relative tolerances of the two strains, responses associated with reactive oxygen species (ROS) were investigated. Measurements of H2O2 concentrations and lipid peroxidation showed significant differences between the strains, with increases in both parameters recorded at lower concentrations in LIA4 than Es524. Activities of antioxidant enzymes (CAT, APX and GR) were significantly (P < 0.05) greater in Es524 than LIA4, and total phenolic content and DPPH scavenging activity were also greater in the more tolerant strain. The rank order of toxicity of the three herbicides was diuron > terbuthylazine > isoproturon in both strains of E. siliculosus. In addition to exposure to individual herbicides, the interactions between binary mixtures were also investigated in both strains using physiological and biochemical biomarkers. All three ways of action (synergistic,additive and antagonistic) were exhibited through different endpoints applied in the present study. Significant differences (P < 0.05) between LIA4 and Es524 were observed for the DIU+TBA and TBA+IPU mixtures, while higher impacts were recorded in LIA4 strain compared to Es524. Further studies showed synergistic interactions were observed in Es524 for the aforementioned mixtures (DIU+TBA, TBA+IPU) on the antioxidative enzyme activities while in LIA4 different interactions were exhibited, which probably contribute to the higher tolerance of Es524 to the mixtures. The presence of TBA together with the phenylureas DIU/IPU was also observed to increase the stimulation of antioxidative enzymes (CAT, APX, GR) in both strains of E. siliculosus. This investigation provides new information on the abiotic stress metabolism in brown algae, and HPLC analysis demonstrates the important role of polyphenols in overcoming the impact of oxidative stress. In conclusion, exposure to the herbicides, singly and in mixtures, caused significant (P < 0.05) changes in the growth, photosynthetic efficiency, and ROM of both strains of E. siliculosus. Strain Es524 was found to be more tolerant than LIA4.

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