Resistance, resilience, and functional redundancy of freshwater bacterioplankton communities facing a gradient of agricultural stressors in a mesocosm experiment

Abstract

Agricultural pollution with fertilizers and pesticides is a common disturbance to freshwater biodiversity. Bacterioplankton communities are at the base of aquatic food webs, but their responses to these potentially interacting stressors are rarely explored. To test the extent of resistance and resilience in bacterioplankton communities faced with agricultural stressors, we exposed freshwater mesocosms to single and combined gradients of two commonly used pesticides: the herbicide glyphosate (0-15 mg/L) and the neonicotinoid insecticide imidacloprid (0-60 μg/L), in high or low nutrient backgrounds. Over the 43-day experiment, we tracked variation in bacterial density with flow cytometry, carbon substrate use with Biolog EcoPlates, and taxonomic diversity and composition with environmental 16S rRNA gene amplicon sequencing. We show that only glyphosate (at the highest dose, 15 mg/L), but not imidacloprid, nutrients, or their interactions measurably changed community structure, favouring members of the Proteobacteria including the genus Agrobacterium.However, no change in carbon substrate use was detected throughout, suggesting functional redundancy despite taxonomic changes. We further show that communities are resilient at broad, but not fine taxonomic levels: 24 days after glyphosate application the precise amplicon sequence variants do not return, and tend to be replaced by phylogenetically close taxa. We conclude that high doses of glyphosate -but still within commonly acceptable regulatory guidelines -alter freshwater bacterioplankton by favouring a subset of higher taxonomic units (i.e., genus to phylum) that transiently thrive in the presence of glyphosate. Longer-term impacts of glyphosate at finer taxonomic resolution merit further investigation.

A Glyphosate-Based Herbicide Cross-Selects for Antibiotic Resistance Genes in Bacterioplankton Communities

“…In phase II, when a single dose of 40 mg/liter glyphosate was applied to all mesocosms except for the phase II controls, a strong effect was observed in all ponds regardless of taxonomic resolution. These results are broadly consistent with our previous 16S rRNA gene amplicon sequencing from the same experiment, which showed community resilience to GBH pulses at higher taxonomic levels only ( 11 ).…”

“…Experiments conducted in diverse environments, such as soil and freshwater ( 9 – 11 ) and the bee gut microbiome ( 12 ), have shown that bacterial taxa from natural ecosystems vary in their sensitivity to glyphosate. Some of this variation is explained by the distribution of different EPSPS classes.…”

“…Moreover, the extent of direct selection on EPSPS mutations compared to indirect selection on ARGs is unclear. In a previous study, we used 16S ribosomal gene amplicon sequencing to assess how the composition of freshwater bacterioplankton communities responds to a GBH applied alone or in combination with a widely used neonicotinoid insecticide ( 11 ). As part of the same experiment, we also showed that phytoplankton and zooplankton communities responded strongly to high doses of GBH ( 25 , 26 ).…”

Widespread agrochemicals differentially affect zooplankton biomass and community structure

“…We recorded physicochemical variables and water level weekly to track homogeneity across ponds. On 10 August 2016, we selected 48 ponds for a collaborative experiment to assess the responses of planktonic communities, including phytoplankton and bacterioplankton; see Fug ere et al (2020) and Barbosa da Costa et al (2021), respectively. Here we only report observations made over the first 43 d, as zooplankton communities were no longer sampled at the same resolution beyond this time point.…”

“…We measured pesticides concentrations via liquid chromatography coupled to mass spectrometry (LC-HRMS) using an Accela 600-Orbitrap LTQ XL (LC-HRMS, Thermo Scientific) in the Department of Chemical Engineering at McGill University. The methods used are more extensively described in Fug ere et al (2020; glyphosate) and Barbosa da Costa et al (2021;imidacloprid). Briefly, we measured glyphosate via heated electrospray ionization in negative mode (mass range = 50-300 m/z [mass m per charge number z]), and imidacloprid in positive mode (mass range = 50-700 m/z).…”