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Metal-tolerant plants: Latin America

BACKGROUND AND JUSTIFICATION Mining has been an important activity in Latin America since pre-Hispanic times. Gold, silver and copper were the metals most commonly used by the Indians and the presence of gold was the main reason for the Spanish settlement of the continent. Most early mining was carried out in small-scale operations, but since the early twentieth century, mining has become a largescale, high-tech operation. Arid and semi-arid areas of Latin America are important producers of several mineral commodities. Bolivia, for example, has a wide range of ores, including gold, tin and antimony. Chile is the top producer and exporter of copper. Brazil, Cuba, the Dominican Republic, Puerto Rico and Venezuela are rich in deposits of ferronickel and copper. Brazil is one of the world's top mineral producers, particularly of aluminium, gold, iron, manganese, steel and tin. Colombia is a leading producer of nickel and also has large-scale gold and ferronickel mines. Mexico has the world's highest output of silver and is among the top ten producers of arsenic, copper, lead, manganese, molybdenum and zinc. Venezuela is a significant producer of alumina, aluminium, bauxite, gold, iron ore and steel. The Peruvian Andes' rich ore deposits include copper, gold, iron, lead, silver, tin and zinc, and Argentina produces copper, gold, lead and zinc, although mining is still of only secondary importance to the national economy. 82

Metal-tolerant plants: Latin America

Although such metals as cobalt, copper, iron, manganese, molybdenum, nickel and zinc are essential for the healthy growth of plants, at high concentrations they can be toxic. Some plants, however, are tolerant to metals and can survive with ease in mineral-rich areas that occur either naturally (e.g., in serpentine soils) or as a result of human interventions (e.g., in the hinterlands of metal smelting or mining activities). Metal-tolerant plants (MTPs) have evolved biological mechanisms that allow them to resist metals in concentrations that would be toxic to many other plants, and in many cases, they are indifferent to the presence of these metals. The metal resistance of these plants has been laboratory tested and found to be particularly effective with cadmium, copper, lead, manganese, nickel, selenium and zinc. Some MTPs—representing about 0.2 per cent of angiosperms (flowering plants)—have the ability to accumulate very high concentrations of certain trace elements that are far in excess of normal physiological requirements and to far greater concentrations than the levels found in most other plant species, including those that are tolerant to soils containing high levels of metal salts. Known as hyper-accumulating plants, they have evolved efficient metal-uptake mechanisms and effective methods for sequestering metals in their tissues. In some cases, the adaptation is so specialized that they cannot complete their life cycles when they are grown on normal soils.

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MTPs typically have very restricted geographical distributions and they are often endemic to a few small areas of metal-rich soils. Therefore, many species are rare and some are known from only a single site, including some Becium (Lamiaceae) species that are found only on natural copper outcrops in the Democratic Republic of the Congo. Others, however, are adapted variants (ecotypes) of common species that grow on metal-rich soils, e.g., those polluted by the metal mining industry, including the grass species Holcus lanatus and the common monkey flower, Mimulus gutattus (Scrophulariaceae). As these examples demonstrate, MTPs occur in many different plant taxa and families. Their metal-specific adaptations and the large range of ecotypes, species and families that they represent give MTPs a special place in biodiversity and genetic resource conservation. The relationship between MTPs and the mining industry is a long one. Metal prospectors looking for areas to mine used the presence of MTPs as an indication that there were valuable metals in the area. Since then, the roles have changed somewhat and MTPs are often used in the restoration and revegetation of former mine sites, where soils have been polluted and degraded. They can also be used in phytoremediation, i.e., the removal of toxic metals from soils (a process known as phytoextraction) or in the bio-mining of low-grade ore that cannot be processed economically by other methods. Furthermore, deposits of metal-rich waste from

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VOLUME 9: EXAMPLES OF THE SUCCESSFUL CONSERVATION AND SUSTAINABLE USE OF DRYLAND BIODIVERSITY

mining provide new habitats that can be used for the microevolution of metaladapted variants of common species or for colonization by MTPs. In this way, rather than being a liability, abandoned mining sites can be seen as a resource base for unique genetic materials.

my of many Latin American countries makes the identification and characterization of such species and varieties a priority for both economic and environmental reasons. The region's scientists, therefore, have recently started to develop such a project.

This is especially so given the wealth of genetic diversity already present in Latin America. Although the five Mediterranean-climate regions of the world cover less than five per cent of the Earth's land mass, they are home to around 20 per cent of the known species of vascular plants. Arid and semi-arid areas of Latin America, in particular, are major centres of plant diversity, and eight of the 25 recognized biodiversity hotspots are located in Latin America, including one in central Chile where over 45 per cent of the region's 3,400 plant species are endemic. Arid and semi-arid areas of Latin America, therefore, are areas where metal-tolerant and hyperaccumulating plants can potentially be found, not only because of the presence of a high number of ore deposits and metal-enriched areas but also thanks to the area's high plant diversity.

For example, three hyper-accumulating plants (Bidens cynapiifolia (Asteraceae) and the grass species Paspalum racemosum and P. tuberosum) have been identified growing near a copper mine in the northern Peruvian Andes. B. cynapiifolia is also thought to be a copper-tolerant plant. In addition, one copper hyper-accumulating plant and 106 nickel-tolerant plants have been found growing in serpentine soils in Cuba; a selenium hyper-accumulating tree grows in Venezuela; three zinc-tolerant plants and other plants that may be tolerant to lead and arsenic have been found in Ecuador; and copper-tolerant plants were found growing near copper smelting and mining works and on tailing sands in Chile. Despite the restricted areas in which these metal-tolerant ecotypes have been found, many are from species that have wide geographical distributions. This implies that they could also have evolved such features in other mineral-rich areas of Latin America.

DESCRIPTION Despite the wealth of biodiversity among the plants of Latin America and the wide variety of mineral deposits, no systematic study has yet been undertaken to identify those plants with metal-tolerant and hyper-accumulating properties. The importance of mining to the econo-

Traditional knowledge is another potential source of useful information about MTPs in Latin America that has not yet been extensively tapped. For example, small-scale miners in Chile are known to have used the presence of the native soapbark tree (Quillaja saponaria, Rosaceae) as an indication of copper ore

Metal-tolerant plants: Latin America

in the subsoil and it seems likely that similar traditional knowledge could be gathered from other places.

FUTURE

PLANS

Regeneration of mine-damaged areas is a particularly serious issue in Latin America. For many decades, a complete lack of environmental legislation left mining companies free to exploit natural resources without any thought for the future of those resources or of the areas in which they were found. Scientists, governments and mining companies throughout the region should therefore make a strong effort to detect and study MTPs in arid and semi-arid areas. Otherwise, the plants will be eliminated by mining activities long before their value has been discovered and they are protected by legislation and appropriate management practices. Recently, progress has been made at the policy, regulatory and technical levels in the governments and mining industries of such Latin American countries as Chile and Peru. Other countries, however, prefer to encourage investment, trade liberalization, technology transfer and crossborder mergers and acquisitions, which can lead to increased influence from large corporations and investor pressure, often at the expense of environmental protection and conservation. International action to protect biodiversity, such as the Convention on Biological Diversity, has increased the attention given to this issue. However, Latin American mining

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continues to attract vast amounts of international investment, and new laws and regulations are not going to be enough to prevent present and future threats to MTPs, mainly because of habitat loss. Since the initial presentation of this report, the author has taken up a new position that is allowing a more systematic search for MTPs, focusing particularly on north-central Chile. Mining companies are now being contacted for both their knowledge on mineral anomalies in wild areas and for their economic support. It is hoped that they can be interested in the potential use of MTPs in rehabilitation (or bioremediation) programmes for their own solid wastes. The use of MTPs to both stabilize and clean up mine wastes should not only bring about local environmental benefits but will also allow the ex situ conservation of these valuable plant species and ecotypes. Mining companies, therefore, can contribute to the identification and conservation of important MTP germplasm collections—the current priority of the research programme. In the long term, bio-mining with MTPs is a possibility. At present, using bacteria to extract metals from ores is proving to be an efficient process on an industrial scale. In the future, using MTPs, especially hyper-accumulating varieties, may be more realistic as a cheaper and more ecologically friendly way to remove metals from polluted soils than traditional physicochemical technologies, but further research is needed to develop this technique.

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P R E PA R E D

BY

Rosanna Ginocchio Departamento de Ecología P. Universidad Católica de Chile Alameda 340, Santiago, Chile New address Centro de Investigación Minera y Metalúrgica (CIMM) Av. Parque Antonio Rabat 6500 Vitacura-Santiago, Chile Tel: (+56) 2 364 3538 Fax: (+56) 2 364 3570 E-mail: [email protected] Web site: www.cimm.cl