Toxicity of ten herbicides to the tropical marine microalgae Rhodomonas salina

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/F_m') and subsequent reductions in specific growth rates (SGR). The concentrations which reduced ΔF/F_m' 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/F_m' indicated reduced photosynthetic capacity, and this correlated linearly with reduced SGR (R² = 0.89), supporting the application of ∆F/F_m' 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.

Figure 1

Concentration-response curves for EC_x derivation. Sigmoidal, 4-parameter curve fit (solid line) and 95% confidence intervals (shaded area) on the relative percent inhibition of 3-day specific growth rate (SGR; full ring, mean ± SE) and 24 h effective quantum yield (ΔF/F_m′; open ring, mean ± SE) following herbicide exposure to (a) diuron; (b) metribuzin; (c) hexazinone; (d) bromacil; (e) tebuthiuron; (f) simazine; (g) propazine; and (h) imazapic at increasing concentrations. All concentrations in µg L⁻¹ (n = 5 for each treatment, bars not visible are smaller than symbol).

Figure 2

Concentration-response curves for NEC derivation. Bayesian non-linear gaussian model fit on the proportional decline in 3-day specific growth rate (SGR) relative to the control treatment (solid black line) and 95% confidence interval (black dashed line) to derive the no effect concentration (NEC) (red line) and 95% confidence interval (red dashed line) of (a) diuron; (b) metribuzin; (c) hexazinone; (d) bromacil; (e) tebuthiuron; (f) simazine; (g) propazine; and h) imazapic. All concentrations in µg L⁻¹.

Figure 3

Response of Rhodomonas salina to (a) haloxyfop and (b) 2,4-D. Boxplots showing percent inhibition relative to control treatments in 3-day specific growth rate (SGR d⁻¹) and 24 h effective quantum yield (ΔF/F_m′) (n = 5 for each treatment).

Figure 4

Linear relationship between effective quantum yield (ΔF/F_m′) and specific growth rate (SGR). Comparison of EC₅₀ values (Slope = 3.48; R² = 0.87) of seven PSII inhibitor herbicides (Diu – diuron, Met - metribuzin, Brom - bromacil, Hex - hexazinone, Teb - tebuthiuron, Pro - propazine, Sim - simazine). Dashed red line indicates 1:1 relationship.