Introduction

Heavy metals in the environment influence microbial populations (Haferburg and Kothe 2007). These heavy metals mainly derive from mining activities; 240,000 km2 of the Earth's surface are influenced by mining (Furrer et al. 2002). However, there are also some environments where high metal loads have arisen due to geogenic sources, such as serpentinite soils that have developed on nickel-rich rock substrates. Plants that are able to accumulate high nickel levels have been described, especially on serpetinites, and microbial populations that are resistant to high nickel concentrations have been found in the rhizospheres of such hyperaccumulator plants and characterized (Mengoni et al. 2001; Park et al. 2004; Schlegel et al. 1991). The evolution of metal resistance can therefore be traced back to habitats that are geogenic; the subsequent spread of microorganisms or the heterologous transfer of microbial resistance mechanisms to anthropogenic metal-rich niches can thus be envisioned.

In mining areas, heavy metal loads can reach extreme levels. This is mainly due to acid mine or rock drainage (AMD, ARD). During this process, the acidification of the environment leads to high heavy metal solubility (Singer and Stumm 1970). The AMD process is accelerated by microbial processes in which the oxidation of iron leads to the release of protons and acidification (Collins and Stotzky 1992). The exposure of sulfide ores to environmental conditions and oxygen in the air allows bacteria like Acidithiobacillus ferrooxidans or Leptosprillum species to gain energy from the oxidation of iron or manganese (Schippers et al. 1996). This leads to acidification, and the oxidation power keeps the reaction running for as long as

E. Kothe (*), C. Dimkpa, G. Haferburg, A. Schmidt, A. Schmidt, and E. Schütze

Institute for Microbiology, Friedrich-Schiller- University Jena,

Neugasse 25, 07743, Jena, Germany e-mail: [email protected]

e-mail: [email protected]

e-mail: [email protected]

e-mail: [email protected]

e-mail: [email protected]

e-mail: [email protected]

I. Sherameti and A. Varma (eds.), Soil Heavy Metals, Soil Biology, Vol 19, DOI 10.1007/978-3-642-02436-8_10, © Springer-Verlag Berlin Heidelberg 2010

reduced metal (usually present as a sulfide ore) and oxygen are available. This results in the extremely acidic and metal-rich seepage waters that can be found downstream from historic and recent mines and mining heaps. An extreme example would be the Rio Tinto region in Spain, where the high iron content and subsequent precipitation of iron hydroxides lead to waterways with orange to deep-red coloration and well-adapted biota (Zettler et al. 2003).

Bacteria cope with metals in their specific environments in different ways. The mechanisms of heavy metal resistance include extracellular and intracellular sequestration, lowering the concentration of bioavailable metal in the direct surroundings of the cell, or reducing the toxicity of the metal by changing its chemical oxidation state. Another resistance mechanism involves the expression of specific exporter proteins that keep the intracellular concentration of the metal at a tolerable level (for a recent review, see Haferburg and Kothe 2007). Other mechanisms involved in enhanced metal resistance help the cell to cope with the toxic effects of heavy metals. One specific mechanism is the formation of reactive oxygen species (ROS) via the Fenton reaction. In order to cope with the ROS, superoxide dis-mutases, catalases and peroxidases that relieve the cells of the adverse effects of ROS are expressed.

In aquatic environments with high metal loads, the metal concentrations around the cell are kept constant as metal is constantly delivered to the cell surface. The cells can counteract the toxic effects best if they express highly specific metal exporter proteins for use as an optimal safety guard. In soil, in contrast, the direct surroundings of the cell can be depleted of metals by excreting components that alter the bioavailability of the metal. In this way, the environment of the cell can be altered without constant resupplying the metal. Thus, mechanisms of extracellular sequestration are more likely to be found in soil microbes, while motile bacteria are more likely to have evolved export mechanisms. This hypothesis is supported by the discovery of highly specific metal exporter systems in Gram-negative bacteria (for review: Nies 2003), while filament-forming streptomycetes have been shown to possess various mechanisms for extracellular sequestration. In both environments, intracellular sequestration and mechanisms to prevent ROS damage will further enhance heavy metal resistance. Here, we summarize the mechanisms that together lead to very high heavy metal resistance in streptomycetes, an important group of soil bacteria.

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