Effects of glyphosate on cyanobacteria and phytoplankton communities

Glyphosate, the most widely used herbicide in agriculture, is frequently detected in rivers and coastal waters influenced by agricultural runoff, raising concerns about its potential effects on non-target aquatic primary producers. Here, we combined mono-culture and community-level experiments to examine how glyphosate influences cyanobacteria and phytoplankton. We conducted a 20-day mono-culture experiment using the toxin-producing cyanobacterium Nodularia spumigena AV1 grown under phosphate-replete (P+), phosphate-free (P-) and glyphosate-amended conditions, in which glyphosate was the only added phosphorus-containing compound (GLY). In parallel, a 7-day microcosm experiment exposed a natural Baltic Sea phytoplankton assemblage to a glyphosate concentration gradient (10–1000 μg l‒1). The lower concentrations (10–100 μg l‒1) reflected upper-end levels reported for agriculturally influenced and freshwater-influenced waters, which may occur as short-term pulses in nearshore mixing zones, whereas higher concentrations (500–1000 μg l‒1) represented extreme or accidental exposure scenarios. In the Nodularia mono-culture, TP and PO4-P concentrations remained low and similar in the GLY and P- treatments. The GLY treatment showed overall lower Chl a concentrations than the P- and P+ treatments, indicating that glyphosate did not stimulate Chl a accumulation or measurable inorganic phosphorus availability under the tested conditions. By contrast, glyphosate exposure was associated with modest, taxon-specific differences in phytoplankton community composition. Several cyanobacterial taxa increased in relative abundance across the concentration gradient, with Chroococcus minutus occurring in all glyphosate treatments and Aphanothece sp. becoming more prominent at the highest concentration. Cyanobacterial relative abundance was positively correlated with glyphosate concentration, whereas overall Chl a remained unchanged. Together, our results indicate an association between glyphosate exposure and taxon-specific compositional differences but do not resolve the mechanisms underlying these patterns. These differential patterns highlight the need to consider taxon-specific associations when evaluating the potential ecological effects of glyphosate in eutrophic aquatic systems.