Phytoplankton growth and PSII efficiency sensitivity to a glyphosate-based herbicide (Factor 540^®)

The use of glyphosate-based herbicides in agriculture has increased steadily since the mid 90's and there is now evidence of glyphosate leaching and contamination of aquatic ecosystems. The aim of this study was to evaluate the effects of a glyphosate-based herbicide (Factor 540^®) on growth and photosynthetic capacity of algae and cyanobacteria. Six algal and three cyanobacterial species/strains, of three different taxonomic groups, were exposed to five glyphosate concentrations (10, 50, 100, 500 and 1000μgl^-1) during 48h. All species have significant growth inhibition at concentrations varying between 50 and 500μgl^-1. The photosynthetic response, after glyphosate exposure, varied among species, but a general pattern has emerged. There was an increase in the amount of photons absorbed (ABS/RC), in dissipated (DI_O/RC) and trapped (TR_O/RC) energy in the photosystem II reaction centers, along with a decreased of the maximum photosystem II quantum yield (F_V/F_M) and electron transport per reaction center (ET_O/RC). The EC₅₀ and LOEC values for growth and photosynthesis were calculated and established that growth was the most affected parameter by glyphosate-based herbicide, while parameter TR_O/RC was the least affected. All species showed reduced growth at glyphosate concentrations lower than the Canadian standard for the protection of aquatic life, set at 800μgl^-1 or the American aquatic life benchmark for acute toxicity in non vascular plants of 12 100μgl^-1 questioning the validity of these thresholds in assessing the risks related to the presence of glyphosate and glyphosate-based herbicides in aquatic systems.