Diuron and Irgarol 1051 are widely used antifouling booster herbicides to control the growth of redundant algae on submerged structures. They pose serious threats to the marine ecosystem especially on the non-target algal species which is of serious environmental concern. In the present study, the acute (1 h) individual and joint toxicities of the foresaid herbicides were assessed in the economically important cyanobacterium Arthrospira maxima, using robotic chlorophyll a (Chl a) fluorescence imaging method. When tested individually, Diuron was found to be more toxic to photosynthesis than Irgarol 1051, with median effective concentration (EC50) values of 4.96-9.51 μg L-1 and 7.15-14.80 μg L-1, respectively. The most sensitive endpoint was the effective quantum yield (Y (II)) of photosystem II (PSII) for Diuron and the relative maximum electron transport rate (rETRmax) for Irgarol 1051. Mixture toxicity of Diuron and Irgarol 1051 was evaluated based on a factorial design with two factors at two levels, considering Y (II) as the sensitive biomarker. There were additive effects when low concentration of Irgarol 1051 (6.5 μg L-1) was mixed with high and low concentrations of Diuron (5 μg L-1 and 2.5 μg L-1, respectively), with ratio of inhibition (RI) being 1.07±0.17 and 0.92±0.13, respectively. In contrast, high concentration of Irgarol 1051 (13 μg L-1) resulted in antagonistic effects when added to low and high concentrations of Diuron, showing RI values of 0.89±0.03 and 0.81±0.06, respectively. The observations presented here indicate the need of consideration of interactive mode of action in evaluating herbicide toxicities in natural waters. © 2013 Korean Society of Environmental Risk Assessment and Health Science and Springer Science+Business Media Dordrecht.
