ABSTRACT
Graduate School of Biotechnology and Bioengineering, Yuan Ze University, Chung-Li, Taiwan.
Email: ccchien@saturn.yzu.edu.tw
Heavy metals comprise a group of loosely-defi ned subset of metallic elements including the transition metals and some metalloids (Duffus 2002). There is no authoritative defi nition of heavy metals and here we will only focus on metals/metalloids associated with contamination and potential toxicity or ecotoxicity. Heavy metals are ubiquitous and abundant on Earth since the formation of the planet. The evolutions of life on Earth, including microorganisms, have been exposed to these toxic elements since the beginning of history. There is no doubt that the features for toxic metal resistance in microorganisms in this diverse environment have not evolved in response to anthropogenic activities but rather, are an adaptation to the natural environment over billions of years (Silver and Phung 2005). In the natural environment, the amount of biologically available heavy metals can vary greatly in different locations. For example, elevated concentrations of toxic heavy metals can be found in volcanic soils, hot springs and sediments, but are generally low in the average ecosystem (Gadd 2009a). Therefore, the occurrence of microbial resistance systems to toxic metals, albeit widespread in the microbial world, can vary greatly with frequencies ranging from a few percent in pristine environments to nearly all microbial isolates in heavily polluted environments (Silver and Phung 2005, 2009, Gadd 2010). However, the disturbances of environmental niches by anthropogenic activities has led to the redistribution of toxic heavy metal ions in aquatic and terrestrial ecosystems and signifi cantly affected the populations and diversity in different microbial habitats (Haferburg and Kothe 2007).
