EXECUTIVE SUMMARY

 Introduction

The Aquatic Effects Technology Evaluation Program, AETE, has been established to assist the Canadian mining industry in meeting its environmental effects monitoring and related requirements, in as cost-effective a manner as possible. The program is coordinated by the Canadian Center for Mineral and Energy Technology (CANMET). The present report is a technical evaluation of metallothionein as a biomonitoring tool (biomarker) for the mining industry.

Metallothioneins (MT) are low molecular weight, cysteine-rich metal-binding proteins that show high affinity for Group IB and IIB metal ions. Studies involving aquatic animals have suggested a central role for these molecules in the regulation of the essential metals Zn and Cu; in the detoxification of these metals, when present in excess, and of nonessential metals such as Cd; and in the acquisition of metal tolerance for populations living in metal-contaminated environments.

Evaluation

The present evaluation of metallothionein is based principally on published field studies performed in mining regions. Peer-reviewed literature and reports of studies carried out by individual mining companies and by the AETE program were consulted. Criteria defined for biomarkers and in the terms of reference of the contract were used as guides for the evaluation process. Conclusions of this evaluation are provided point by point in the following.

Criteria: the indicator should respond in a dose-dependent manner to changes in ambient levels of the contaminant; the indicator should be specific to a particular contaminant or a class of contaminants.

Conclusion: strong field evidence (15 studies) supports the fact that metallothionein responds specifically in a dose-dependent manner to changes in ambient levels of a trace metal or of a group of trace metal (e.g. Cd, Cu, Zn, Ag).

Criterion: levels of the indicator should be related to the health or «fitness» status of the organism.

Conclusion: only four field studies examined this issue; results were in agreement with the criterion. In these studies, high metallothionein levels were associated with detrimental effects at the organism and population levels of biological organization. Hypothetical causes were an overwhelming of the detoxification mechanism including MT, or a metabolic cost, associated with MT synthesis, affecting directly the growth and/or reproduction of the host organism.

Criterion: the indicator should have an early warning capacity, i.e., the biochemical response should be predictive of effects at higher levels of biological organization and should precede them.

Conclusion: one single study provided results consistent with an early warning capacity of the MT tool. MT decreases in a fish species and improvements in water quality preceded recoveries of phytoplanktonic, zooplanctonic, and benthic communities. This link appeared empirical.

Criterion: the basic biology/physiology of the biomonitor organism should be known so that sources of uncontrolled variation (growth and development, reproduction, food sources) can be minimized.

Conclusion: Peer-reviewed literature on the subject is scarce. The author concluded that nontoxicological factors influencing MT levels have not been adequately evaluated.

Criterion: applicability.

Conclusion: reliable analytical protocols for MT detection and quantification have been defined. MT is easy to quantify by metal-saturation methods, and metal-saturation methods would be easy to standardize on a countrywide basis.

Criterion: commercial availability.

Conclusion: metallothionein analyses are not available in the private sector, and specific cost estimates could not be made. MT certified reference materials are commercially available.

Criterion: practical limitations for carrying field work.

Conclusion: fresh samples have to be frozen quickly (<6 h after collection) and protected against long-term oxidation.

Recommendations

• Metallothionein can already be considered to be a useful biomarker of exposure to certain metals (e.g. Cd, Cu, Zn, and Ag).
• Metallothionein is not a «stand on its own» tool. As for any monitoring tool, the MT level in an organism has to be used in conjunction with other biotic and abiotic measurements to be interpreted unambiguously (e.g. section 6.9).
• Use of metallothionein as a means of evaluating metal effects on cells, organisms and populations is less well established. There is a need for fundamental, mechanistic research on understanding the role of metallothionein in metal toxicology.
• The early warning capacity of metallothionein is not established. Research efforts are to be directed notably to an increased understanding of fundamental ecological mechanisms, and the characteristics of tolerant organisms to metal exposure in nature.
• Some of the research needed on the use of metallothionein as a biomonitoring tool could be conducted under the auspices of the AETE program. This includes an investigation of the use of different species as sentinel organisms at selected sites, and their calibration with respect to the use of MT as a biomarker of exposure.
• Standardization of protocols of sample preparation, metallothionein extraction and quantification, and QA/QC checks would be required on a countrywide basis. The private sector could relatively rapidly develop the infrastructure required to offer MT services.
• Protocols of organism capture/collection and handling need to be standardized to minimize undesired variations in metallothionein concentrations.

Other recommendations are indicated in Chapter 7.