Synthesizing Laboratory and Field Experiments to Quantify Dominant Transformation Mechanisms of 2,4-Dichlorophenoxyacetic Acid (2,4-D) in Aquatic Environments

Abstract

Laboratory studies used to assess the environmental fate of organic chemicals such as pesticides fail to replicate environmental conditions, resulting in large errors in predicted transformation rates. We combine laboratory and field data to identify the dominant loss processes of the herbicide 2,4dichlorophenoxyacetic acid (2,4-D) in lakes for the first time. Microbial and photochemical degradation are individually assessed using laboratory-based microcosms and irradiation studies, respectively. Field campaigns are conducted in six lakes to quantify 2,4-D loss following large-scale herbicide treatments. Irradiation studies show that 2,4-D undergoes direct photodegradation, but modeling efforts demonstrated that this process is negligible under environmental conditions. Microcosms constructed using field inocula show that sediment microbial communities are responsible for degradation of 2,4-D in lakes. Attempts to quantify transformation products are unsuccessful in both laboratory and field studies, suggesting that their persistence is not a major concern. The synthesis of laboratory and field experiments is used to demonstrate best practices in designing laboratory persistence studies and in using those results to mechanistically predict contaminant fate in complex aquatic environments.

Expanded diversity of tfdA harboring bacteria across the natural and built environment

“…We conducted additional recovery experiments with sediment from four additional lakes recently unexposed to 2,4-D (4). We added 10 3 copies of the tfdA gene and amplified with the 215 bp primer ( Figure 2b ).…”

“…Environmental sample collection. Lake water and sediment were collected from several Wisconsin Lakes undergoing large-scale 2,4-D treatments (i.e., lake wide concentrations >0.45 M), described in detail in (4). Water and sediment used in TOPO cloning were from Random Lake, which had whole-lake 2,4-D treatments in 2018 and 2019.…”

“…Sediment, water, and soil samples exposed to 2,4-D were collected and amplified as described previously in (4) and (77). DNA was extracted using an MP Bio Fast DNA Spin Kit and quantified for DNA concentration using an Invitrogen Qubit 3.0 fluorimeter.…”

“…Two primer sets that amplify an 81 or 215 bp fragment are expected to amplify all three gene classes. These primers have been applied with variable success in soil (20, 30), sediments (31), aquifer solids (32), and glacial ice (33), but have been infrequently applied in aquatic environments (4, 34, 35). Despite a wide range of environmental applications, the qPCR primers were developed from cultured representatives of soil environments that may miss other environmental representatives.…”

“…The herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) is used extensively to control invasive and nuisance plants in terrestrial and aquatic environments (1)(2)(3)(4) but is a chemical of concern due to its toxicity and potential non-target effects (5,6). 2,4-D is degraded by bacteria harboring the tfd gene cassette, named after the 2,4-D compound (TFD) (7)(8)(9)(10)(11).…”