Algal species vary considerably in sensitivity to the commonly used herbicide atrazine, and it is hypothesized that several factors may contribute to species-specific sensitivity. In this study four estuarine microalgal species, a planktonic chlorophyte (Dunaliella tertiolecta), a benthic chlorophyte (Ankistrodesmus sp.), a cryptophyte (Storeatula major) and a dinoflagellate (Amphidinium operculatum), were exposed to atrazine to determine toxicity and identify factors that might influence species sensitivity. Atrazine effects were examined at the population (cell density, primary productivity and biomass), cellular (biovolume), and subcellular (pigment composition, protein concentration and lipid concentration) levels. Atrazine significantly decreased cell density, productivity rate, biomass and biovolume in all the algal populations tested at atrazine concentrations ≥ 12.5 μ g l−1. Toxicity values (96 h EC50 values) for the four species ranged from 11.87 to 146.71 μ g l−1. Species sensitivity rankings varied with endpoint measured. Overall, Ankistrodesmus sp. was the most sensitive species followed by Storeatula, Dunaliella and Amphidinium. Cellular biovolume was a significantly more sensitive test endpoint for Amphidinium, chlorophyll a was a significantly more sensitive test endpoint for Ankistrodesmus sp., and phototrophic carbon assimilation was a significantly more sensitive test endpoint for both Storeatula and Amphidinium. Algal subcellular responses to atrazine were also species dependent. Chlorophyll a concentration per cell decreased in the green algae, increased in Amphidinium and did not change in Storeatula. Total lipids per cell increased in Storeatula, decreased in Amphidinium and did not change in the green algae. Ankistrodesmus sp. pigments were not significantly altered after atrazine exposure; however selected Amphidinium pigments increased per cell, and selected Dunaliella and Storeatula pigments decreased per cell in the atrazine treatments. Atrazine significantly reduced cellular biovolume in all test species. Species with smaller biovolumes and less chlorophyll a per cell tended to be more sensitive to atrazine exposure based on population growth rate.

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