A Glyphosate Pulse to Brackish Long-Term Microcosms Has a Greater Impact on the Microbial Diversity and Abundance of Planktonic Than of Biofilm Assemblages

René Janßen,Johannes Werner,Wael Skeff,Detlef Schulz-Bull,Bernd Kreikemeyer,Marisa A Wirth,Matthias Labrenz

doi:10.3389/fmars.2019.00758

Abstract

The widespread herbicide glyphosate has been detected in aquatic coastal zones of the southern Baltic Sea. We monitored community dynamics in glyphosate-impacted chemostats for 20 weeks to evaluate the potential impact of the herbicide on free-living and biofilm-associated bacterial community assemblages in a brackish ecosystem. A HPLC-MS/MS method was developed to measure glyphosate, aminomethylphosphonic acid and sarcosine concentrations within a brackish matrix. These concentrations were analyzed weekly, together with prokaryotic succession, determined by total cell counts and next generation 16S rRNA (gene) amplicon sequencing. Shotgun metagenomics provided insights into the glyphosate degradation potential of the microbial communities. Temporal increases in total cell counts, bacterial diversity and the abundances of distinct bacterial operational taxonomic units were identified in the water column. Biofilm communities proved to be less affected than pelagic ones, but their responses were of longer duration. The increase of gox and thiO gene and the phn operon gene abundance indicated glyphosate degradation by first the aminomethylphosphonic acid pathway and possible subsequently by cleaving the C-P bond. However, although glyphosate concentrations were reduced by 99 %, 1 µM of the herbicide remained until the end of the experiment. Thus, when present at low concentrations, glyphosate may evade bacterial degradation and persist in Baltic Sea waters.

Highlights

Microorganisms are ubiquitous on Earth and respond rapidly to environmental changes
Two major routes of glyphosate biodegradation have been described according to their first respective intermediate: the sarcosine pathway and the aminomethylphosphonic acid (AMPA) pathway, encoded mainly by the phn operon and the glyphosate oxidoreductase gene, respectively
Total cell counts in the water column were in the range of 2–4 × 107 cells·mL−1 both in the treatment and control water samples (Figure 1A)

Summary

Introduction

Microorganisms are ubiquitous on Earth and respond rapidly to environmental changes. The majority of microorganisms live within biofilms, which promote high cell abundances and activities (Costerton et al, 1995). Following assessments demonstrating its relatively low environmental toxicity, it has become the most widely produced and sold herbicide worldwide. Two major routes of glyphosate biodegradation have been described according to their first respective intermediate: the sarcosine pathway and the aminomethylphosphonic acid (AMPA) pathway, encoded mainly by the phn operon and the glyphosate oxidoreductase (gox) gene, respectively. An alternative pathway to yield AMPA from glyphosate was discovered with the enzyme glycine oxidase encoded by thiO. This enzyme possesses an unspecific Km of 87 mM for glyphosate, compared to 0.6 mM for glycine (Pedotti et al, 2009)

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