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Validation of the Biotic Ligand Model (BLM) to Metal Mixtures

Author: ChenZhongZhi
Tutor: ZhuLin
School: Nankai University
Course: Environmental Science
Keywords: biotic ligand model (BLM) bioavailability metal toxicity bioaccumulation gene expression
CLC: X171.5
Type: PhD thesis
Year: 2010
Downloads: 111
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Abstract

Contamination rarely occurs for single metals and is invariably due to complex mixtures in the environment. However, risk assessments of toxic compounds are routinely based upon effects evaluations of single substances. During recent years, the biotic ligand model (BLM) approach has gained widespread interest amongst the scientific, regulated and regulatory communities because of its potential for use in developing water quality criteria (WQC) and in performing risk assessments for metals. The BLM approach has significant implications for advances in the area of metal mixture risk assessment. Unfortunately, the BLM approach cannot yet be easily used to assess the effects in metal mixtures, which is more common. It is also unclear whether simple modifications of the BLM will be sufficient to account for such complex interactions. Therefore, the BLM should be used with caution when attempting to model metal interactions. Based on the development and theoretical basis of the biotic ligand model (BLM) overview, we examined the advantages and limitations in application of the BLM. Some important future research directions were identified. Based on the review we assessed the predictive capacities of two biotic ligand models (BLMs) for acute zinc toxicity to Daphnia magna as applied to a number of landscape waters from Tianjin and synthetic laboratory-prepared waters. To examine the possibility of extending the BLM approach to other organisms (or life stage), we tested the hypothesis that increased Ca2+ content increases the sensitivity of the developing embryos and larvae of zebrafish (Danio rerio) to Pb. And the aim of the study was to investigate the extent to which calcium can individually mitigate lead ion toxicity based on the concept of biotic ligand model (BLM). Embryos of the zebrafish were exposed to various Pb concentrations. Different chemical characteristics of water and some representative toxicological endpoints of zebrafish embryo were recorded. And general growth retardation as a major toxicological endpoint was used for analysis at 72 h due to its sensitivity and facility. After that, copper (Cu), an essential micronutrient and lead (Pb), a nonessential toxic metal, uptake by the unicellular green alga, Chlamydomonas reinhardtii, have been quantified in single metal exposures and in metal mixtures in order to test some of the key assumptions of the BLM. Furthermore, we tested the potential of the BLM to predict other biological effects, gene expression. Their responses to various combinations of Cu and Pb in the green algae C. reinhardtii were studied by following bioaccumulation and expression levels of genes known to be involved in the metal responses. The study concentrated on the expression levels of genes coding for natural resistance associated macrophage protein (nrampl), copper transporter (ctr2), oxidative stress responding gene-glutathione peroxidase (gpx5) and glutathione S-transferase (gsts2) after an exposure to Pb and Cu. The software of Visual MINTEQ (version 2.5.2) was employed to calculate the biotic ligand models and the chemical speciation in the solution. Results of this study can be used to help extend current predictive toxicity models (e.g. the BLM) to metal mixtures.The results indicated that:(1) Within these ranges of water chemistry parameters, the model based on toxicological data of acute zinc to Daphnia magna may be considered as a useful tool for taking into account bioavailability in regulatory assessments of zinc. This model generally predicted 80% effect concentrations by an error of less than a factor of 1.5. However, the predicted values of the acute Zn-BLM, which was developed in fish and subsequently adopted for Daphnia magna through downward adjustment of LA50 were normally lower than measured values. Acute toxicity to Daphnia magna (LC50) was underestimated by a factor 2 to 4. The method of adjusting the LA50 to extent the model applicability should be considered only the simplest means of calibrating the BLM, and might be suspected.(2) The results showed that when Ca2+ concentration increased from 0.25 mmol/L to 2.00 mmol/L, the toxicity of lead on embryos of zebrafish (Danio rerio) decreased markedly after 72 h. And a large part of these decrease can be explained by the positive linear relations between EC50 {Pb2+}/EC50 [Pb]T (express as lead ion activity/dissolved total concentration) and activity/total concentration of Ca2+ through which the influence of Ca2+ on toxicity could be predicted. The results support the assumptions of the BLM and have often been associated with competition between lead and calcium for binding on transport and toxic action sites on biological surfaces. However, when Ca2+ concentration increased from 2.00 mmol/L to 4.00 mmol/L, the toxicity of lead on embryos of zebrafish (Danio rerio) seems to be constant at 72 h.(3) Stability constants for the interaction of the metals with biological uptake sites by Chlamydomonas reinhardtii were determined from measured short term internalization fluxes. In the absence of competition, a value of 105.8M-1 was obtained for Cu while 105.9M-1 was obtained for Pb. Competition experiments did not show a straightforward antagonistic competition, as would be predicted by the BLM. Only at high Cu2+ concentrations (>1μM) did Cu behave as a competitive inhibitor of Pb transport. Surprisingly, low concentrations of Cu2+ had a synergistic effect on Pb uptake. Furthermore, Cu uptake was independent of Pb when Cu concentrations were below 10-7 M. In order to explain the observed discrepancies with the BLM, membrane permeability and Cu transporter expression levels were probed. The expression of ctr2, a gene coding for a Cu transporter, increased significantly in the presence of Pb, indicating that bioaccumulation is much more dynamic than assumed in the equilibrium models.(4) The variability of interactive effects across genes suggests they are the product of intracellular feedback or regulatory processes specific to each gene and often binary metal combination. That is not consisting with one assumption of steady-state model:no significant biological regulation is induced by binding to sensitive sites, even in the short-term exposure experiments. Variable effect of different metal transporter (ctr2 and nrampl) suggested that the carrier molecule might possess regulatory and modification. The assumption of BLM considers biological effect as a function of metal uptake following bound amount of metal to the organism. Although the metal must first be accumulated before an effect is observed, it is oversimplification to link metal accumulation to proportional toxic effect. Besides that, protection effect of Cu suggesting that accumulation of heavy metals is not always related to their toxicity, which is not consistent with the assumption of BLM.

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