Copper Speciation and Transformation in Soil-Plant System

The widespread use of copper (Cu) resulted in anthropogenic inputs to soils through industrial and agricultural practices. Elevated soil Cu concentrations can cause toxicity effects on soil organisms and affect soil ecosystem functioning. The toxicity of Cu to organisms strongly depends on its bioavailability in soil and the sensitivity of the organisms. Molecular-level knowledge about chemical speciation of Cu in soils will advance our ability to predict its reactivities and environmental fates. The speciation and transformation of Cu in soil-plant system were intensively studied during the past decade by combining traditional chemical analysis methods and advanced synchrotron radiation-based techniques, including X-ray absorption fine structure (XAFS), X-ray fluorescence (XRF), and scanning transmission X-ray microscopy (STXM). Bioavailability of Cu in soil can be mediated by sulfur and some specific rhizobacterium like Cu-tolerant Pseudomonas putida CZ1, which is isolated from rhizosphere of Elsholtzia splendens. Excess Cu will cause phytotoxicity to plants; XAFS analysis demonstrated that Cu bound by N/O/S ligands played a key role in Cu detoxification of E. splendens. Copper stress leads to different functional protein expression in different E. splendens tissues, which includes signal transduction, regulation of transcription and translation, energy metabolism, regulation of redox homeostasis, and cell defense. CuO nanoparticles can be absorbed by the roots and transported to the shoots in E. splendens and rice (Oryza sativa L.) and then transformed into different Cu species, like Cu-citrate and Cu-cysteine. Sulfur- and rhizobacterium (such as Pseudomonas putida CZ1)-enhanced phytoremediation of Cu-contaminated soil is considered to be promising, and further practices should be in field conditions.