Past exposure determines response of freshwater microalgae to glyphosate-based contamination

Glyphosate-based herbicides are the most widely agrochemicals used in the world. This toxic compound reaches aquatic ecosystems and can affect microalgae, which are the basis of the trophic web and the main primary producers in these environments. We tested the sensitivity of two species of green algae (Chlorophyta) to a glyphosate-based herbicide (Roundup Transorb) and the effect of contamination history in population rescue. Strains of Desmodesmus communis and Pseudopediastrum boryanum were tested in isolation and in co-occurrence in a two-step experiment, with three and 10 days each, respectively. In the first step, we exposed populations of the two species to Roundup Transorb at 0, 350, 500, and 1000 μg‧L-1 glyphosate. Next, we transferred aliquots of each treatment to a new media where 20,000 μg‧L-1 of the glyphosate-based herbicide was added. Growth inhibition at the end of Step 1 was dose-dependent regardless of species and whether they were isolated or combined. Rescue after exposure to 20,000 μg‧L-1 of the glyphosate-based herbicide for three and 10 days was more successful for populations previously exposed to intermediate concentrations of 350 and 500 μg‧L-1. We suggest that these concentrations triggered a process that increased algal tolerance to the glyphosate-based herbicide. In addition, neither the species nor the fact that they were isolated or combined influenced glyphosate-based herbicide toxicity, at least for the concentrations tested in this study.