Herbicide Butafenacil and Its Transformation Products Significantly Influence Ecological Dynamics in Marine Environments

Abstract Protoporphyrinogen oxidase (PPO) inhibitors have long been considered environmentally friendly herbicides due to their high efficacy and low toxicity. However, we found that a widely used PPO inhibitor butafenacil (BFA) and its transformation products (TPs) possessed exceptionally high toxicity to some marine phytoplankton (96-h EC50 of BFA were 0.71 ± 0.08 μg/L and 1.3 ± 0.06 μg/L for Skeletonema costatum and Chlorella sp., respectively). Bialgal culture experiments revealed that BFA exposure enhanced the competitive advantage of Chlorella sp., triggering a shift in dominance from other microalgae to Chlorella sp., which could alter the community structure and potentially destabilize ecological imbalances in marine environments. The degradation process of BFA in seawater could be accelerated by Phaeodactylum tricornutum, as its half-life decreased from 7.5 to 3.3 days, and 16 (13 new) TPs were identified by solid-phase extraction-liquid chromatography–high-resolution tandem mass spectrometry. Using computational toxicology, TPs with predicted toxicity comparable to parent BFA were isolated by preparative liquid chromatography and subjected to algal toxicity tests. The semiquantitative estimates indicated that the persistent high toxicity to microalgae after BFA degradation was attributable to these toxic TPs on a BFA-equivalent basis, underscoring the necessity to consider the potential environmental risks posed by TPs.