Glyphosate is a commonly-used agricultural herbicide which enters freshwater sources and risks affecting non-target aquatic organisms, including algae. In this study, lab cultures of Scenedesmus quadricauda and Selenastrum capricornutum were inoculated with glyphosate (Roundup® formulation) to determine its impact on cellular growth and chlorophyll-a (Ch-a) synthesis. A concentration of 10 mg/L of glyphosate significantly inhibited growth and Ch-a synthesis in S. quadricauda and S.

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... . Concentrations ranging from 0 to 3 mg/L of glyphosate did not affect cellular growth or Ch-a synthesis in either species. A concentration of 6 mg/L of glyphosate did initially reduce the growth of S. quadricauda, but growth recovered and Ch-a concentrations were high. For S. capricornutum, growth and Ch-a synthesis were low, and pheophytin concentrations were significantly elevated relative to the control at 6 mg/L of glyphosate. Based these results, S. capricornutum was more sensitive to glyphosate than S. quadricauda, which was likely due to differences in the surface area to volume ratios between the species. In the future, algal toxicity should be studied in greater detail by conducting mesocosm studies within the natural aquatic environment.