Uptake and toxic effects of triphenyl phosphate on freshwater microalgae Chlorella vulgaris and Scenedesmus obliquus: Insights from untargeted metabolomics

Abstract

The flame retardant triphenyl phosphate (TPhP) has been widely detected in surface waters. Yet, little information is known regarding its impact on microalgae. We investigated the uptake and toxicity of TPhP on two freshwater microalgae Chlorella vulgaris (CV) and Scenedesmus obliquus (SO) after exposure to 10 μg/l-10 mg/l for 5 days. The presence of microalgae significantly enhanced TPhP degradation, with the final concentrations dropped to 5.5-35.1% of the original concentrations. Most of the medium TPhP were sorbed and transformed by microalgae in just one day. Growth of CV was inhibited in a concentration-dependent manner, whereas growth of SO were only inhibited significantly at 10 mg/l TPhP exposure. Mass spectrometry-based untargeted metabolomics revealed concentration- and species-dependent metabolic responses. Exposure to TPhP in CV resulted in enhanced respiration (increase of fumarate and malate) and osmoregulation (increase of sucrose and myo-inositol), synthesis of membrane lipids (accumulation of monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), decrease of lysoglycerolipids, fatty acids, and glyceryl-glucoside). Exposure to TPhP in SO resulted in enhanced osmoregulation (increase of valine, proline, and raffinose) and lipolysis (decrease of MGDG, accumulation of fatty acids, lysophospholipids, and glycerol phosphate). Although chlorophyll a and b contents did not change significantly, decrease of chlorophyll derivatives was observed in both CV and SO at high exposure concentrations. Further bioassays confirmed that CV exhibited enhanced membrane integrity and decreased cellular reactive oxygen species (ROS) possibly as a defense strategy, whereas SO showed disruption of membrane integrity and induction of ROS at 10 mg/l exposure. This study demonstrated the potential of microalgae to remove TPhP in water, and offered new insights for the risk assessment of TPhP on freshwater microalgae using metabolomics.

Keywords: GC–MS; LC-QTOF; Membrane integrity; Metabolic pathway; Reactive oxygen species; Toxicity.