New insights into the effects of the herbicide imazethapyr on Cu(II) ecotoxicity to the aquatic unicellular alga Scenedesmus obliquus

Abstract

Interactions between heavy metals and organic contaminants can result in speciation changes of heavy metals. Therefore, it is unknown whether organic contaminants in aquatic environments act as selective agents to mediate the toxicity of heavy metals. To elucidate these interactions, the effects of the herbicide imazethapyr (IM) on Cu(II) ecotoxicity to the aquatic unicellular alga Scenedesmus obliquus were investigated. It was found that the toxicity of Cu could be mediated by IM. To explore the mechanisms involved, complex formation, the catalytic activity of the complex, the Cu species and the distribution of Cu and Fe in the algal cell were characterized. The results showed that Cu(II) and IM formed an octahedral complex with an IM:Cu molar ratio of 2:1. These complexes also catalyzed the disproportionation of hydrogen peroxide. Analysis of the K-edge of Cu using XAFS spectroscopy indicated that when treated with Cu, the Cu was bound to polygalacturonic acid (on the cell wall), and once inside the cell, Cu may complex with the reduced glutathione (GSH) (in the cell). When the cell is treated with IM and Cu simultaneously, IM-Cu may be the primary complex formed. Once Cu combines with IM, it is difficult for it to interact with the cell wall. In addition, using scanning transmission soft X-ray microscopy, it was found that Cu could induce changes in the distribution of the essential trace element Fe, whereas IM-Cu cannot. This finding demonstrates the importance of interactions between heavy metals and organic contaminants, which are able to mediate the toxicity of heavy metals and should be considered in future risk assessments.

Keywords: Copper toxicity; Green algae; Herbicides; Speciation.