The natural processes of bioconversion that are so important in the global recycling of elements may have unwanted consequences for humans. Prominent among these is acid mine drainage, a frequently encountered problem in mining regions. Bacterial oxidation of mineral sulphides, particularly the ubiquitous iron pyrite, leads to the release of a highly acidic leachate into streams and rivers. This also contains dissolved metals, including ferric iron. When it mixes with stream water, the pH is raised sufficiently for the iron to precipitate as unsightly orange ferric hydroxides, blanketing the stream bed and wiping out plant and animal life. The main culprits in the formation of acid mine drainage are sulphur-oxidising bacteria, notably Acidithiobacillus ferrooxidans; as we shall see in the final chapter, under controlled conditions this same organism can also provide economic benefits to the mining industry by extracting valuable metals from low grade ores.
Another area in which environmental microorganisms can have detrimental effects is that of biodeterio-ration, whereby economically important materials such as wood, paper, textiles, petroleum and even metals and concrete may be subject to damage by a range of microorganisms, mainly fungi and bacteria.
The most important microorganisms in the biodete-rioration of wood are members of the Basidiomycota. Wood is only susceptible to fungal attack when its moisture level reaches around 30 per cent. The major component of wood that is subject to microbial attack is cellulose, although some forms can also degrade lignin. There are two main forms of rot; white rot, which involves the degradation of lignin as well as cellulose, and brown rot, in which the lignin is unaltered. The dry rot fungus Serpula lacrymans produces thick strands of hyphae called rhizomorphs, which it uses to conduct water and nutrients from damper areas. These are very strong, and able to travel over brickwork and masonry barriers. S. lacrymans is able to generate water as a metabolic end product and thus, once established, is able to grow even on dry wood. Dry rot flourishes in areas of static dampness such as badly ventilated, uninhabited properties.
Because cellulose is also an important component of paper and textiles, its breakdown is clearly of great economic importance. Degradation by fungi, and, to a lesser extent, bacteria, results in a loss of strength of the material in question. The paper-making process provides warm, wet conditions rich in nutrients, ideal for microbial growth, which can clog up machinery and discolour the finished product. A variety of biocides are used in an effort to minimise microbial contamination.
The discoloration referred to above raises the point that biodeterioration of a material need not necessarily affect its physical or chemical make-up; aesthetic damage can lessen the economic value of a material by altering its appearance. The blackening of shower curtains by moulds growing on surface detritus, familiar to generations of students, is another example of this!
Biodeterioration is the damage caused to materials of economic importance due to biological (mainly microbial) processes.
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