| Biotic Ligand Model (Biotic Ligand Model, BLM) is a tool based on direct quantification of the effectiveness of biological susceptibility and water bodies of water chemistry to evaluate the metals’bioavailability. BLM combines the physiology, chemistry, computer science, and toxicology together, whic fully considers the impact of metals on biological toxicity of3factors:concentration, complexation, competition. That is to say, firstlt metal toxicology depends on the activity of the free ions in the water. At the same time, the free ion concentration of metal is related to the total metal concentration, while also influenced by the inorganic and organic ligands in the water.Lanthanides are a group of15elements of similar physicochemical properties, and are moderately abundant in the earth crust despite that they are also called rare earth elements. They have wide applications in high-technology and clean energy products because of their special magnetic, luminescent and catalytic properties.With rare earth elements used in a large number of applications in various fields, numbers of rare earth metals and their compounds came and are coming into the environment. Currently increasing amount of REEs are going into the natural water bodies through farmland runoff and industrial wastewater discharge, which would cause of the potential negative impacts to the natural aquatic ecological environment. Furthermore, it could also be possible to impact on human health through the food chain. As a result, it’s really important to do some related research about REEs’ environmental chemical behavior and its bio-ecological effects in the water bodies. In this study, BLM is used to begin the research of rare earth metals (samarium and europium) to get the final conclusion about the applicability of BLM for rare earth metals and their mixtures.Firstly, the applicability of the biotic ligand model (BLM) was tested for the lanthanide Sm in a freshwater green alga Chlamydomonas reinhardtii. The uptake of Sm3+in the absence of organic ligands was well described by a Michaelis-Menten equation, consistent with the BLM assumption of single transporter, with themaximum influx rate (Jmax) of1.43×10-14mol cm-2s-1and the half saturation concentration of10-7.02M. The addition of organic ligands (i.e., malic acid, diglycolic acid and citric acid) decreased Sm influx rates. However, the decreases were much less thanthat predicted by the BLM, possibly due to the direct internalization of Sm complexed by simple organic acids. The competition effects of two major cations (Ca2+and Mg2+) and three lanthanide cations (La3+, Ce3+and Eu3+)were successfully modeled by the BLM, with the stability constants of binding estimated to beKCa=103.96M-1, KMg=102.72M-1, KLa=106.82M-1, KCe=106.94M-1and KEu=107.08M-1. The stability constants and Jmax were very similar among the four investigated lanthanides and varied progressively with atomic number; therefore, the results obtained in the present study probably can be extrapolated to other lanthanides.Then short-term (60min) Eu biouptake fluxes by the freshwater green alga, Chlamydomonas reinhardtii, were investigated in presence and absence of ligands (e.g. malic acid and citric acid) and a second rare earth metal (Sm). Data were interpreted in the context of the biotic ligand model (BLM), which uses experimentally determined stability constants in order to take into account both the competition and complexation of the metal of interest. In the absence of ligands or competitors, Eu biouptake was well described by a Michaelis-Menten equation with the constants:Jmax=1.59×10-14mol cm-2s-1and Km=10-7.3M (corresponding to an affinity constant of1073M-1). Biouptake of Eu (or Sm) decreased as the concentration of a competing rare earth element (REE, i.e. Sm (or Eu)) was increased, quantitatively consistent with equilibrium modeling predictions. On the other hand, when hydrophilic complexes were formed with citric and malic acid, Eu biouptake was much greater than predicted on the basis of free ion concentrations alone. Overall, the results showed that the REE were likely to share a common biouptake pathway, resulting in an overall reduced biouptake of the mixtures but that complexation could increase bioaccumulation by Chlamydomonas reinhardtii.Finally, gene expression exposed by mixtures of Sm and Eu of the freshwater green alga, Chlamydomonas reinhardtii, were investigated. Meanwhile, bioaccumulation was also tested for the two metals. During the2-hour exposure, bioaccumulation of Sm and Eu competes with each other obviously, which means that they shared the same tranporter during the biouptake. The results are consistent with that we got in the biouptake experiments. In the gene expression experiments, two mRNAs (divalent metal transporter and GSTS2) were selected. During the2-hour exposure, DMT and GSTS2did not change significantly, which could be explained by the following3reasons:(1) DMT and GSTS2could not response to Sm and Eu;(2) DMT and GSTS2could response to Sm and Eu, but not in the range of our experimental concentrations (3×10-8-3×10-7M);(3) under the pressure of rare earth metals, Chlamydomonas reinhardtii did not show any biological response to that. In these three assumptions, only the third one is consistent with that of BLM, which assumed that the alga would not show any biological response during the short-time exposure.Samarium and europium are non-essential metals. The mixture experiments of concentration and competition can be explained by BLM’s assumptions. However, BLM can not explain complexation experiments. In addition, as non-essential metals, in the test range we selected, the relevant gene expressions were not affected by metals, which is consistent with BLM’s assumption that short-term exposure could not cause biological regulation. So BLM can explain the process of concentration, competition and gene expression caused by two non-essential metals (samarium and europium), except for complexation results. |
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