The toxicity of Roundup ® 360 SL formulation and its main constituents: Glyphosate and isopropylamine towards non-target water photoautotrophs

Abstract

The toxicity of commercial formulation of Roundup ® 360 SL, widely used, nonselective herbicide and its main constituents, glyphosate (PMG), equimolar (1:1) isopropylamine salt of glyphosate (GIPA) and isopropylamine (IPA) was examined towards eight aquatic microphotoautotrophs; seven cyanobacterial strains representing either saline or freshwater communities, and common eukaryotic algae Chlorella vulgaris Beijerinck. Autotrophs were cultured 21 days in their appropriate standard media supplemented with various amounts of Roundup ®, glyphosate, GIPA and IPA. The determination of the growth of examined photoautotrophs was performed by time-course measurements of total chlorophyll content in experimental cultures. The growth rates related to corresponding concentrations of chemicals, the EC 50 values and generation doubling time were determined in order to present the toxicity Roundup ® 360 SL formulation and its main constituents. Market available formulation of Roundup ® was found to possess toxicity significantly higher than this, attributed to its main constituents; however both these compounds, isopropylamine and glyphosate, also inhibited the growth of examined strains in a dose-dependent manner. Notably, the interpretation of toxicity of the examined substances was found to be significantly dependent on the method of EC 50 calculation. The choice of molar or weight concentration of substances tested separately and in specific formulation was found to be essential in this matter. Due to these findings the EC 50 values were calculated based either on molar or on weight concentrations. Considering Roundup ® 360 SL formulation, these values ranged from 10 −3 up to 10 −1 mM and they were one order of magnitude lower than those found for isopropylamine. Quite surprisingly the minimum EC 50 values found for glyphosate did not reach micromolar concentrations, whereas most of the EC 50 values revealed to IPA did not exceed this range. Notably, in all the cases except for Synechocystis aquatilis Sauvageau, isopropylamine alone was indicated as more toxic than glyphosate.