Effects of Glyphosate on the Growth and Toxicity of the Harmful Alga Prymnesium parvum Under Phosphorus Sufficiency and Limitation

Nutrient-rich inputs into surface waters can promote harmful blooms of Prymnesium parvum by stimulating growth under favorable environmental conditions. Glyphosate can enter aquatic ecosystems and affect algal growth, potentially further influencing P. parvum blooms. The interaction effects of nutrient conditions and glyphosate on P. parvum toxicity, however, remain unknown. This study determined the effects of glyphosate on the growth and toxicity of P. parvum. Cultures were exposed to glyphosate (0, 0.1, 0.25, and 0.5 mg L−1) under phosphorous (P)-sufficient (f/2 medium) and P-limited conditions (phosphate-free medium) and, for the 0.1 mg L−1 cultures, P. parvum toxicity (10,000 cells mL−1) to Daphnia pulex was assessed. The addition of 0.1 and 0.25 mg glyphosate L−1 significantly stimulated P. parvum growth in P-sufficient conditions, resulting in the highest growth rates and final cell densities. In P-limited conditions, however, glyphosate had no effect. P-sufficient cultures with 0.1 mg glyphosate L−1 exhibited higher toxicity compared to controls. Although toxicity under P limitation was consistently higher than under P sufficiency, glyphosate did not affect this toxicity. These results suggest that glyphosate, at environmentally relevant concentrations, stimulates the growth and toxicity of P. parvum under nutrient-sufficient but not nutrient-limited conditions.