Toxicity of the herbicides diuron, propazine, tebuthiuron, and haloxyfop to the diatom Chaetoceros muelleri

Marie C Thomas,Timothy A Reeks,Sarit Kaserzon,Andrew P Negri,Florita Flores

doi:10.1038/s41598-020-76363-0

Marie C Thomas, Timothy A Reeks + Show 3 more

Open Access

https://doi.org/10.1038/s41598-020-76363-0

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Abstract

Conventional photosystem II (PSII) herbicides applied in agriculture can pose significant environmental risks to aquatic environments. In response to the frequent detection of these herbicides in the Great Barrier Reef (GBR) catchment area, transitions towards ‘alternative’ herbicides are now widely supported. However, water quality guideline values (WQGVs) for alternative herbicides are lacking and their potential ecological impacts on tropical marine species are generally unknown. To improve our understanding of the risks posed by some of these alternative herbicides on marine species under tropical conditions, we tested the effects of four herbicides on the widely distributed diatom Chaetoceros muelleri. The PSII herbicides diuron, propazine, and tebuthiuron induced substantial reductions in both 24 h effective quantum yields (ΔF/Fm′) and 3-day specific growth rates (SGR). The effect concentrations, which reduced ΔF/Fm′ by 50% (EC50), ranged from 4.25 µg L−1 diuron to 48.6 µg L−1 propazine, while the EC50s for SGR were on average threefold higher, ranging from 12.4 µg L−1 diuron to 187 µg L−1 tebuthiuron. Our results clearly demonstrated that inhibition of ΔF/Fm′ in PSII is directly linked to reduced growth (R2 = 0.95) in this species, further supporting application of ΔF/Fm′ inhibition as a valid bioindicator of ecological relevance for PSII herbicides that could contribute to deriving future WQGVs. In contrast, SGR and ΔF/Fm′ of C. muelleri were nonresponsive to the non-PSII herbicide haloxyfop at the highest concentration tested (4570 µg L−1), suggesting haloxyfop does not pose a risk to C. muelleri. The toxicity thresholds (e.g. no effect concentrations; NECs) identified in this study will contribute to the derivation of high-reliability marine WQGVs for some alternative herbicides detected in GBR waters and support future assessments of the cumulative risks of complex herbicide mixtures commonly detected in coastal waters.

Highlights

Herbicide contamination of nearshore waters is common across tropical regions, including the Caribbean1, Mexico[2], Central A merica[3] and the Asia-Pacific[4,5,6,7,8]; the Great Barrier Reef (GBR) World Heritage Area located on the east coast of northern Queensland, Australia represents the most studied location for herbicide contamination in tropical waters[9]
The PC99 proposed guideline values (PGVs) of 590 μg L−1 is an order of magnitude lower than the no effect concentrations (NECs) value we report for C. muelleri of > 4570 μg L−1, indicating C. muelleri is well protected by this PGV and that environmental concentrations currently recorded do not pose a risk to this species in comparison to photosystem II (PSII) herbicides
A revision of the current water quality guideline values (WQGVs) has recently been proposed, most of the PGVs were derived from freshwater toxicity thresholds and are of low reliability, signifying data gaps for tropical marine species, especially for marine phototrophs

Summary

Introduction

Herbicide contamination of nearshore waters is common across tropical regions, including the Caribbean1, Mexico[2], Central A merica[3] and the Asia-Pacific[4,5,6,7,8]; the Great Barrier Reef (GBR) World Heritage Area located on the east coast of northern Queensland, Australia represents the most studied location for herbicide contamination in tropical waters[9]. Non-PSII herbicides can affect microalga, but their vulnerability depends on whether the mode of action of the herbicide is relevant to each specific type of algae Their ecological importance, potential vulnerability to herbicides, along with rapid growth rates that allow for chronic exposure testing in a short period, mean that marine microalgae represent a suitable taxon to contribute to improving WQGVs. Currently, SSDs used to derive high-quality WQGVs require ecologically relevant toxicity data, and for microalgae, the inhibition of growth is the most common e ndpoint[27,28]. Further assessment of ΔF/Fm′ inhibition as an effective endpoint for herbicides is warranted to investigate its suitability as an ecologically relevant endpoint to support herbicide risk assessments

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