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Cu Zn

Zinc

Cobalt

In the ionized form, at higher concentrations, these metals can directly inhibit microorganisms. They are often required at lower concentrations as trace elements.

At higher levels, clearance from higher organisms occurs by reaction with plasma proteins and other mechanisms. Many of these metals serve as trace elements at lower concentrations.

commonly shared oxidants and reductants. For example, some sulfate-reducing microorganisms can reduce Fe3+ using H2 or organic matter as a reductant and also can oxidize elemental sulfur to sulfate when Mn(IV) is present as an electron acceptor. The Mn(IV)-dependent production of sulfate under anaerobic conditions, carried out by Desulfobulbus propionicus, provides a way to link sulfur and manganese cycling anaerobically.

Recently, a unique energetic couple has been described by B. Schinck and M. Friedrich. They isolated a lithoautotrophic bacterium from anaerobic sediments that can link the oxidation of phosphite (PO33~) to phosphate (PO43 ) with the reduction of sulfate to hydrogen sulfide. Schinck and Friedrich have suggested that this energetic cycle could have operated on the early Earth.

1. What major forms of iron, manganese, and phosphorus are important in biogeochemical cycling?

2. Why is Aquaspirillum considered to be a magneto-aerotactic bacterium?

3. What are some important microbial genera that contribute to manganese cycling?

4. What is phosphine? Under which conditions will it be produced?

5. Describe some links between oxidants and reductants that have been discovered recently.

Microorganisms and Metal Toxicity

In addition to metals such as iron and manganese, which are largely nontoxic to microorganisms and animals, there are a series of metals that have varied toxic effects on microorganisms and homeothermic animals. Microorganisms play important roles in modifying the toxicity of these metals (table 28.6).

The "metals" can be considered in broad categories. The "noble metals" tend not to cross the vertebrate blood-brain barrier but can have distinct effects on microorganisms. Microorganisms also can reduce ionic forms of noble metals to their elemental forms.

The second group includes metals or metalloids that microorganisms can methylate to form more mobile products called organometals. Some organometals can cross the blood-brain barrier and affect the central nervous system of vertebrates. Organometals contain carbon-metal bonds. These bonds are their unique identifying characteristics.

The mercury cycle is of particular interest and illustrates many characteristics of those metals that can be methylated. Mercury compounds were widely used in industrial processes over the centuries. One has only to think of Lewis Carroll's allusion to this problem when he wrote of the Mad Hatter in Alice in Wonderland. At that time mercury was used in the shaping of felt hats. Microorganisms methylated some of the mercury, thus rendering it more toxic to the hatmakers.

A devastating situation developed in southwestern Japan when large-scale mercury poisoning occurred in the Minamata Bay region because of industrial mercury released into the marine environment. Inorganic mercury that accumulated in bottom muds of the bay was methylated by anaerobic bacteria of the genus Desulfovibrio (figure 28.25). Such methylated mercury forms are volatile and lipid soluble, and the mercury concentrations increased in the food chain (by the process of biomagni-fication). The mercury was ultimately ingested by the human population, the "top consumers," through their primary food source—fish—leading to severe neurological disorders.

A similar situation has occurred in many of the freshwater lakes in the north-central United States and in Canada, where mercury compounds were used to control microbial growth in pulp mills. Even decades later the fish in lakes downstream from these pulp mills cannot yet be used for food, and fishing is only for recreation.

The third group of metals occurs in ionic forms directly toxic to microorganisms. The metals in this group also can affect more complex organisms. However, plasma proteins react with the ionic forms of these metals and aid in their excretion unless excessive long-term contact and ingestion occur. Relatively high doses of these metals are required to cause lethal effects. At lower concentrations many of these metals serve as required trace elements.

Figure 28.25 The Mercury Cycle. Interactions between the atmosphere, aerobic water, and anaerobic sediment are critical. Microorganisms in anaerobic sediments, primarily Desulfovibrio, can transform mercury to methylated forms that can be transported to water and the atmosphere. These methylated forms also undergo biomagnification. The production of volatile elemental mercury (Hg0) releases this metal to waters and the atmosphere. Sulfide, if present in the anaerobic sediment, can react with ionic mercury to produce less soluble HgS.

Figure 28.25 The Mercury Cycle. Interactions between the atmosphere, aerobic water, and anaerobic sediment are critical. Microorganisms in anaerobic sediments, primarily Desulfovibrio, can transform mercury to methylated forms that can be transported to water and the atmosphere. These methylated forms also undergo biomagnification. The production of volatile elemental mercury (Hg0) releases this metal to waters and the atmosphere. Sulfide, if present in the anaerobic sediment, can react with ionic mercury to produce less soluble HgS.

The differing sensitivity of more complex organisms and microorganisms to metals forms the basis of many antiseptic procedures developed over the last 150 years (see section 7.5). The noble metals, although microorganisms tend to develop resistance to them, continue to be used in preference to antibiotics in some medical applications. Examples include the treatment of burns with silver-containing antimicrobial compounds and the use of silver-plated catheters.

1. What are examples of the three groups of metals in terms of their toxicity to microorganisms and homeothermic animals?

2. How can microbial activity render some metals more or less toxic to warm-blooded animals?

3. Why do metals such as mercury have such major effects on higher organisms?

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