Toward Integrative Bacterial Monitoring of Metolachlor Toxicity in Groundwater

Abstract

Common herbicides such as metolachlor (MET), and their transformation products, are frequently detected in groundwater worldwide. Little is known about the response of groundwater bacterial communities to herbicide exposure, and its potential use for ecotoxicological assessment. The response of bacterial communities exposed to different levels of MET from the Ariège alluvial aquifer (Southwest of France) was investigated in situ and in laboratory experiments. Variations in both chemistry and bacterial communities were observed in groundwater, but T-RFLP analysis did not allow to uncover a pesticide-specific effect on endogenous bacterial communities. To circumvent issues of hydrogeochemical and seasonal variations in situ, groundwater samples from two monitoring wells of the Ariège aquifer with contrasting records of pesticide contamination were exposed to different levels of MET in laboratory experiments. The standard Microtox® acute toxicity assay did not indicate toxic effects of MET, even at 5 mg L-1 (i.e., 1000-fold higher than in contaminated groundwater). Analysis of MET transformation products and compound-specific isotope analysis (CSIA) in laboratory experiments demonstrated MET biodegradation but did not correlate with MET exposure. High-throughput sequencing analysis (Illumina MiSeq) of bacterial communities based on amplicons of the 16S rRNA gene revealed that bacterial community differed mainly by groundwater origin rather than by its response to MET exposure. OTUs correlating with MET addition ranged between 0.4 to 3.6% of the total. Predictive analysis of bacterial functions impacted by pesticides using PICRUSt suggested only minor changes in bacterial functions with increasing MET exposure. Taken together, results highlight MET biodegradation in groundwater, and the potential use of bacterial communities as sensitive indicators of herbicide contamination in aquifers. Although detected effects of MET on groundwater bacterial communities were modest, this study illustrates the potential of integrating DNA- and isotopic analysis-based approaches to improve ecotoxicological assessment of pesticide-contaminated aquifers. GRAPHICAL ABSTRACTAn integrative approach was develop to investigate in situ and in laboratory experiments the response of bacterial communities exposed to different levels of MET from the Ariége alluvial aquifer (Southwest of France).

Side Effects of Pesticides and Metabolites in Groundwater: Impact on Denitrification

“…Moreover, as mentioned above, microbial community diversity as well as the history of pesticide contamination (i.e., communities previously in contact or not with pesticides) can influence the impact of pesticides. Finally, the work of Imfeld et al (2018) also suggested a little effect of metolachlor on in situ bacterial groundwater community (Microtox test and T-RFLP fingerprints). In this same work, the transformation products of S-metolachlor (as ESA and OXA-metolachor) were shown to have no effect on microbial metabolism.…”

“…This last point is not in agreement with our results that showed no impact of the tested pesticides on the relative abundance of the nitrate reductase narG and napA genes involved in the first step of denitrification. One explanation is probably that the data of Mauffret et al (2017) were obtained from in situ groundwater, whereas our results concern batch experiments, and it has been demonstrated that for in situ studies particularly, there are difficulties to tease apart the effect of pesticides from the effects of variations of hydrogeological conditions ( Imfeld et al, 2018 ). Moreover, as mentioned above, microbial community diversity as well as the history of pesticide contamination (i.e., communities previously in contact or not with pesticides) can influence the impact of pesticides.…”

“…The strategy used in the study was to perform all the tests (batch experiments at the lab scale) with the same groundwater sample, which allowed comparing the results. Indeed, such a strategy enables to circumvent the potential effects due to hydrogeochemical and seasonal variations in groundwater and to strong spatial and temporal variations in groundwater bacterial communities ( de Lipthay et al, 2004 ; Imfeld et al, 2018 ). In our study, nitrate reduction rate measured in the absence of pesticides/metabolites (control) was about 5.6 × 10 –5 g N/L/h.…”

“…Results first suggested that the presence of pesticides could increase microbial biodiversity. For instance, Imfeld et al (2018) obtained higher Shannon and Simpson diversity indices (from T-RFLP fingerprints) for groundwater communities in batch experiments amended with pesticides (metolachlor) compared with experiments without the addition of pesticides. de Lipthay et al (2004) also demonstrated that the diversity of colony morphotypes (culturable) in sediments from a subsurface aquifer exposed to herbicides (as mixture) was higher compared with the non-exposed samples.…”

“… Janniche et al (2012) also noticed high diversity indices from microbial community-level physiological profiles (CLPP) using EcoPlates TM in herbicide (isoproturon, atrazine, and acetochlor)-exposed groundwater. Studies, however, underlined the difficulties to determine in situ the impact of pesticides on groundwater biodiversity due to natural spatial and temporal variations of groundwater bacterial communities ( de Lipthay et al, 2004 ; Imfeld et al, 2018 ). Moreover, they suggested that biodiversity modifications induced by pesticides were not associated to changes in general catabolic properties.…”