Toxicity of ten herbicides to the tropical marine microalgae Rhodomonas salina

Marie C Thomas,Sarit Kaserzon,Rebecca Fisher,Andrew P Negri,Florita Flores

doi:10.1038/s41598-020-64116-y

Abstract

Herbicide contamination of nearshore tropical marine ecosystems is widespread and persistent; however, risks posed by most ‘alternative’ herbicides to tropical marine microalgae remain poorly understood. Experimental exposures of the important but understudied microalgae Rhodomonas salina to seven individual Photosystem II (PSII) inhibitor herbicides (diuron, metribuzin, hexazinone, tebuthiuron, bromacil, simazine, propazine) led to inhibition of effective quantum yield (ΔF/Fm′) and subsequent reductions in specific growth rates (SGR). The concentrations which reduced ΔF/Fm′ by 50% (EC50) ranged from 1.71-59.2 µg L−1, while the EC50s for SGR were 4-times higher, ranging from 6.27-188 µg L−1. Inhibition of ΔF/Fm′ indicated reduced photosynthetic capacity, and this correlated linearly with reduced SGR (R2 = 0.89), supporting the application of ∆F/Fm’ inhibition as a robust and sensitive indicator of sub-lethal toxicity of PSII inhibitors for this microalga. The three non-PSII inhibitor herbicides (imazapic, haloxyfop and 2,4-Dichlorophenoxyacetic acid (2,4-D)) caused low or no toxic responses to the function of the PSII or growth at the highest concentrations tested suggesting these herbicides pose little risk to R. salina. This study highlights the suitability of including R. salina in future species sensitivity distributions (SSDs) to support water quality guideline development for the management of herbicide contamination in tropical marine ecosystems.

Highlights

Poor water quality, including pesticide contamination, has long been recognized as a threat to the health and resilience of tropical and subtropical marine ecosystems (Asia Pacific[1,2,3,4,5], Central America[6], Mexico[7], Caribbean[8])
Compared to Photosystem II (PSII) inhibitor herbicides, non-PSII inhibitor herbicides exhibit a range of different modes of action, such as inhibiting acetohydroxyacid synthase (AHAS)[34,35] or acetyl-CoA carboxylase (ACCase)[36] that result in reduced cell growth in plants
Exposures of R. salina to increasing herbicide concentrations resulted in inhibition of ΔF/Fm′ within 24 h, indicating reduced photosynthetic efficiency which led to reduced growth rates over 72 h chronic exposures

Summary

Introduction

Poor water quality, including pesticide contamination, has long been recognized as a threat to the health and resilience of tropical and subtropical marine ecosystems (Asia Pacific[1,2,3,4,5], Central America[6], Mexico[7], Caribbean[8]). According to the Australian and New Zealand Guidelines for Fresh and Marine Water Quality (ANZG)[39], there are currently only four WQGVs for alternative PSII and non-PSII inhibitor herbicides (bromacil, simazine, 2,4-D, MCPA) and five guidelines for the priority herbicides (diuron, atrazine, ametryn, tebuthiuron, hexazinone) detected in GBR waters; all were derived from freshwater toxicity thresholds and are of low reliability (Table 1). The inhibition of effective quantum yield (ΔF/Fm′) by PSII inhibitor herbicides is proportional to the inhibition of photosynthetic efficiency at a given irradiance[46] and could be considered as a rapid, sensitive and non-invasive alternative for growth measurements in microalgae toxicity tests involving PSII inhibitor herbicides[23]. Further comparisons between the inhibition of growth and ΔF/Fm′ as endpoints for herbicide toxicity in marine microalgae are warranted to demonstrate the relevance of using ΔF/Fm′ as an ecological relevant endpoint in future SSDs

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