Multistressor negative effects on an experimental phytoplankton community. The case of glyphosate and one toxigenic cyanobacterium on Chlorophycean microalgae

Aquatic ecosystems face serious pollution issues. Discharges of toxic substances and eutrophication may lead to changes in the phytoplankton community and foster cyanobacterial blooms. Glyphosate-based herbicides are chemical stressors of microalgae that may affect the structure of phytoplankton communities, and also stimulate the synthesis of cyanotoxins by cyanobacteria. The simultaneous presence of glyphosate and toxigenic cyanobacteria increases the stress on microalgae, jointly affecting their growth and development. This study evaluated the combined effect of a toxigenic cyanobacterium and glyphosate in the development of an experimental microalgal community. We studied the effect of Microcystis aeruginosa on the population growth of the microalgae Ankistrodesmus falcatus, Chlorella vulgaris, Pseudokirchneriella subcapitata, and Scenedesmus incrassatulus. We also evaluated the combined effect of sub-inhibitory glyphosate (Faena®) concentrations on the content of macromolecules and the enzymes superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx), as well as on the concentration of TBARS. These effects were evaluated through the integrated biomarker response (IBR). In individual experiments, microalgae showed lower growth rates versus M. aeruginosa. In the mixed bioassays, both M. aeruginosa and microalgae showed reduced growth. IC₅₀ values for Faena® ranged from 1.022 to 2.702 mg L⁻¹. In the microalgae + cyanobacteria bioassays, the herbicide lowered the growth rates of microalgae but stimulated the proliferation of M. aeruginosa. The joint action of both stressors affected growth rate and population dynamics, macromolecule content, and led to increased CAT and GPx levels. Faena® influenced growth rate and caused oxidative stress. On the other hand, the herbicide stimulated the synthesis of cyanotoxins, which further affected microalgal development. The experimental community was not only affected by the herbicide, but the mixed culture with cyanobacteria magnified the effects of chemical stress. These results illustrate the potential damage to phytoplankton expected in anthropically eutrophic water bodies that are also polluted by glyphosate.