Resistance to degradation and effect of the herbicide glyphosate on the bacterioplankton community of a large river system dominated by agricultural activities

Glyphosate-based herbicides are widely used for several crops, such as transgenic soybean and forestry. The aim of this study was to determine the effect of glyphosate on the community structure of riverine bacterioplankton and to evaluate the potential of bacterioplankton to degrade the herbicide. River water to which C-labelled glyphosate (10, 100 µg L) was added or not (control) was incubated for 6 days at the temperature measured in situ (20°C). Significant differences in bacterioplankton community composition, as assessed by microfluidics-based automated ribosomal intergenic spacer analysis, were found among treatments, with differences in the presence of 100 µg L of glyphosate being more pronounced, namely significant decreases in bacterial richness and diversity. The glyphosate degradation product aminomethylphosphonic acid (AMPA) was detected, accounting for 1.2% of glyphosate conversion in water with 100 µg L of C-labelled glyphosate, together with a significant enrichment of C in the bacterial biomass. These findings suggest that glyphosate had a direct detrimental effect on most bacterioplankton taxa, but enriched those that were able to degrade the herbicide. Together, the results indicate that glyphosate degradation in the river assessed would be a slow process (months-years), taking place through the AMPA degradation pathway and meaning glyphosate accumulate in the ecosystem.