Microalgal sensitivity varies between a diuron-resistant strain and two wild strains when exposed to diuron and irgarol, alone and in mixtures

•Two microalgal species were exposed to irgarol, diuron, and mixtures of both.•At 0.5 μg L−1, irgarol was more toxic than diuron, for both species.•A mutation was found in the psbA gene coding sequence of the diuron-resistant strain.•The mutation induced no resistance to irgarol in the diuron-resistant strain.•Mixture (D5+I0.5) induced stronger effects than I0.5 in the diuron-resistant strain.

A wild strain of Chaetoceros calcitrans and wild and diuron-resistant strains of Tetraselmis suecica, were exposed to the PSII inhibitor herbicides diuron and irgarol, individually and in mixtures. The effects of three concentrations of diuron and irgarol and four binary mixtures were evaluated on doubling time, relative reactive oxygen species and lipid content by flow cytometry, and on photosynthetic efficiency by pulse amplitude modulated fluorescence.In both wild strains, significant effects were observed for each molecule at the highest concentration tested: at irgarol 0.5 μg L−1, C. calcitrans was shown to be more sensitive than T. suecica (+52% and +19% in doubling time, respectively), whereas at diuron 5 μg L−1, T. suecica was more affected (+125% in doubling time) than C. calcitrans (+21%). Overall, irgarol had a higher toxicity at a lower concentration than diuron (no effect at diuron 0.5 μg L−1) for both wild strains. The strongest mixture (irgarol 0.5 μg L−1 + diuron 5 μg L−1) increased doubling time by 356% for T. suecica, thus showing amplified effects when the two compounds were mixed.Sequencing of the diuron-resistant strain demonstrated a single mutation in the psbA gene coding sequence. Although resistance of this strain to diuron was confirmed with no effect at the highest diuron concentration, no resistance to irgarol was shown. In addition, the mutant strain exposed to the strongest mixture showed a 3.5-fold increase in doubling time compared with irgarol alone, thereby supporting the hypothesis of a biochemical interaction between these two compounds.