Rethinking the term “glyphosate effect” through the evaluation of different glyphosate-based herbicide effects over aquatic microbial communities

Abstract

Glyphosate-based herbicides (GBH) -the most widely used herbicides in agriculture worldwide-are frequently generalized by the name of “glyphosate”. However, GBH encompass a variety of glyphosate salts as active ingredient and different adjuvants, which differ between products. These herbicides reach water bodies and produce diverse impacts over aquatic communities. Yet, the risk assessment assays required for the approval focus mostly on active ingredients. Herein, we compared the effect of five different GBH as well as of monoisopropylamine salt of glyphosate (GIPA) on aquatic microbial communities from natural shallow lakes that were mixed and allowed to evolve in an outdoor pond. We performed an 8-day long assay under indoor control conditions to evaluate the effects of exposure on the structure of nano-plus microphytoplankton (net phytoplankton, with sizes between 2 and 20 μm and >20 μm, respectively) and picoplankton (size ranging between 0.2 and 2 μm) communities through microscopy and flow cytometry, respectively. Significantly different effects were evident on the structure of microbial communities dependent on the GBH, even with herbicides sharing similar active ingredients. Each GBH evoked increases of different magnitude in bacterioplankton abundance. Furthermore, GIPA and a formulation decreased the abundance of a phycocyanin-rich (PC-rich) picocyanobacteria (Pcy) cytometric population and GIPA further altered Pcy composition. Also, two GBH increased net phytoplankton total abundance and, unlike the tested GBH, no apparent effect of GIPA was detected on this community structure. These results demonstrate that GBH effects on aquatic microbial communities should not be summarized as “glyphosate” effects considering that the formulations have effects beyond those exerted by the active ingredients alone. This work intends to alert on the lack of real knowledge regarding the consequences of the variety of GBH on natural aquatic ecosystems. Indeed, the wide use of the term “glyphosate effect” should be thoroughly rethought.