Immediate and mid-term effects of pyrimethanil toxicity on microalgae by simulating an episodic contamination

Since pesticides can represent a threat for non-target aquatic communities, including microalgae, we looked at the effects of the fungicide pyrimethanil on the growth of the freshwater green microalgae Selenastrum capricornutum. Additionally, attenuation of the toxicity of pyrimethanil due to its dissipation in the water was assessed. Pyrimethanil-contaminated samples were taken from outdoor mesocosms one (1.4 mg L−1 of pyrimethanil) and ten (0.78 mg L−1 of pyrimethanil) days after pyrimethanil application. Different dilutions were prepared using both nutrient-rich culture medium (LC Oligo) and non-contaminated mesocosm samples, and cell growth inhibition was assessed. Reference mesocosm samples were also diluted with LC Oligo in order to verify how the nutrient concentration in the LC Oligo could improve cell growth. Comparing cell growth of population exposed to pyrimethanil-treated sample taken at day 1 with cells growing in reference sample and LC Oligo, the growth inhibition was 80% (±6.5) and 95% (±2.0), respectively. The toxicity of samples taken from contaminated mesocosms at day 10 was attenuated to 34% (±15) (when compared with reference sample) and 88% (±3.0) (when compared with LC Oligo), as pyrimethanil concentrations in the mesocosms decreased. In conclusion, (i) pyrimethanil can be an environmental disturber for the microalgae; (ii) the toxicity of pyrimethanil in water was reduced almost 2.4 times (when compared with the reference sample) at as short a period as 10 d if assuming that pesticide entrance is not continuous; (iii) toxicity of an environmental sample could be underestimated if the sample/medium used in dilution presents different nutrient levels.