Abstract
One consequence of the intensive use of glyphosate is the contamination of rivers by the active substance and its metabolites aminomethyl phosphonic acid (AMPA) and sarcosine, inducing river eutrophication. Biofilms are the predominant lifestyle for microorganisms in rivers, providing pivotal roles in ecosystem functioning and pollutant removal. The persistence of glyphosate in these ecosystems is suspected to be mostly influenced by microbial biodegradation processes. The present study aimed to investigate the tripartite relationship among biofilms, phosphorus and glyphosate in rivers. The first part consists of a co-occurrence analysis among glyphosate, AMPA and phosphorus using an extensive dataset of measurements (n = 56,198) from French surface waters between 2013 and 2017. The second part investigated the capacity of natural river biofilms to dissipate glyphosate, depending on phosphorus availability and the exposure history of the biofilm, in a microcosm study. A strong co-occurrence among glyphosate, AMPA and phosphorus was found in surface waters. More than two-thirds of samples contained phosphorous with glyphosate, AMPA or both compounds. Seasonal fluctuations in glyphosate, AMPA and phosphorus concentrations were correlated, peaking in spring/summer shortly after pesticide spreading. Laboratory experiments revealed that natural river biofilms can degrade glyphosate. However, phosphorus availability negatively influenced the biodegradation of glyphosate and induced the accumulation of AMPA in water. An increase in alkaline phosphatase activity and phosphorus uptake was observed in glyphosate-degrading biofilms, evidencing the tight link between phosphorus limitation and glyphosate degradation by biofilms. The results of the present study show that phosphorus not only is a key driver of river eutrophication but also can reduce complete glyphosate degradation by biofilms and favour the accumulation of AMPA in river water. The predominant role of biofilms and the trophic status of rivers must therefore be considered in order to better assess the fate and persistence of glyphosate.
