Toxicity of oxadiargyl to marine microalgae on competitive displacement and overlooked risk from transformation products.

Abstract

Protoporphyrinogen oxidase (PPO) inhibitors are widely used in agriculture, yet their risks to marine ecosystems are often underestimated due to assumptions of target-site specificity and rapid environmental dissipation. Here, we demonstrated that oxadiargyl (OXA), a representative PPO herbicide, exhibits pronounced toxicity toward marine microalgae at 1-32 µg/L, highlighting the need for monitoring and risk assessment in coastal waters affected by agricultural runoff. Acute 96h-EC50 values for the keystone species Chlorella sp. and Skeletonema costatum were 6.02 and 9.35 μg/L, respectively, indicating they were highly sensitive taxa. OXA could disrupt community hierarchies through the competitive displacement of sensitive species by the tolerant diatom Phaeodactylum tricornutum. Although parent OXA dissipated rapidly in seawater (half-life 0.95 d), persistent toxicity in the solution was observed as the growth of microalgae remained suppressed even after parent OXA fully degraded. Using solid phase extraction-liquid chromatography-high resolution tandem mass spectrometry (SPE-LCHRMS/MS), nine (seven new) transformation products (TPs) were identified. Computational toxicity screening predicted that four TPs retain algal toxicity comparable to that of OXA. These findings demonstrate that rapid dissipation of parent compound does not equate to risk elimination, emphasizing the necessity of integrating community-level competition endpoints and TP profiling in future pesticide risk assessments.

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