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Microbiological tolerance of metals

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Briefly describe the methods used by bacteria and fungi to tolerate environmental heavy metals. Include references and your views on the potential commercial applications of bacteria and fungi.

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Solution Summary

This is an 1800+ word essay describing the methods by which fungi and bacteria can tolerate heavy metals in their environment. The essay is fully referenced with peer-reviewed articles. The question is from a fourth year course in environmental biology, but can be adapted to other levels and related courses.

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Mechanisms used by Bacteria and Fungi to Tolerate Heavy Metals.

A good indication of the health of an ecosystem is the diversity of its microflora. An ecosystem that displays a rich array of bacteria, fungi, and other microorganisms, can be described as a healthy environment. Stresses applied to an organism, can cause a change in that microbe's growth rate. When a microorganism, such as a bacterium or fungus, is exposed to various stresses, a typical response is a decrease in the rate of growth of the organism. An important stress like low temperature results in a reduction of the molecular motion in biological systems (Taylor, 1987). With a decrease in molecular motion comes a subsequent reduction in growth rate. In the natural environment the microflora must overcome countless obstacles. Such stresses as temperature, water activity, and nutrient availability are impediments that fungi and bacteria face constantly. On top of the naturally occurring stresses, artificial stresses influence the activity of fungi and bacteria. These include changes in soil pH, man-made drought and flooding and the dumping of heavy metals.
Heavy metal stress is caused by excessive amounts of the class B/borderline heavy metals that includes copper, nickel, zinc, lead, silver, cadmium and mercury (Nieboer and Richardson, 1990). To tolerate heavy metals, organisms have two basic mechanisms at their disposal: avoidance and sequestration (Baker and Walker, 1990). This paper is concerned with the techniques used by bacteria and fungi to tolerate excessive heavy metals present in the ecosystem. Table 1 describes the general methods bacteria and fungi use to tolerate metal stresses discussed in this paper. There are many different methods used apart from the ones below. Bacteria and fungi can employ a range of mechanisms and are not just limited to a particular mechanism.

Table 1. Mechanisms of heavy metal tolerance discussed.
Cellular Process Tolerance Mechanism
Cell Wall Avoidance
Chemical transformation Avoidance
Metal-binding proteins Sequestration
Chelating agents Sequestration

The tolerance displayed by bacteria and fungi may be thought of as the result of man-made pollution. However, lots of these mechanisms of tolerance are genetic and probably arose from the pre-human environments plagued by volcanic pollution. Since the earth contains these heavy metals in varying amounts, we can assume that all organisms on the planet have adapted ways of coping with these metal ions. Some of the metal ions mentioned above are required in trace amounts for growth of some organisms. Cooper, zinc and lead are essential micronutrients, but are toxic when present in excess (Tomsett, 1993). This poses a difficulty in maintaining homeostasis. Fungi and bacteria have to evolve to accept certain metal ions, but reject excessive amounts.

The mechanisms of avoidance in fungi and bacteria are similar. The principal component that helps these organisms and that sets it apart from other cells is the cell wall that ...

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