Metal Resistance and Accumulation in Cyanobacteria
The indiscriminate discharge of metals into the environment generated from various industrial processes, modern agricultural practices, acid mine drainage and human wastes, has long been recognized as an important source of these pollutant. Metals constit
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Metal Resistance and Accumulation in Cyanobacteria Marli F. Fiore, David H. Moon and Jack T. Trevors
Introduction
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he indiscriminate discharge of metals into the environment generated from various industrial processes, modern agricultural practices, acid mine drainage and human wastes, has long been recognized as an important source of these pollutant. Metals constitute more than 75% of all known elements and occupy groups lA to 6A (representative metals), groups IB to 8B (transition metals) and the lanthanide and actinide metals. l The term "heavy metals" has been redefined over the years and although not completely satisfactory from a chemical point of view, the most widely used is those elements with a density greater than 5 glcm3. The definition problem has been thoroughly discussed by Gadd~ and to avoid further confusion the term heavy metal will not be used here. Microbial cells can accumulate metals essential for growth and metabolic processes3 as well as other metals with no known biological functions.4.5 Due to their well documented ability to bind metals and to their abundance in natural environments, a significant contribution to metal sorption has been attributed to microbial cells.6 -lo For about 150 years cyanobacteria were considered a special group of algae, the blue-green algae. l1 However, the comparison of 16S and 5S rRNA sequences suggest that the cyanobacteria are a phylogenetically coherent group within the Gramnegative eubacteria.12 According to Gibbons and Murraf3 the cyanobacteria are members of the Kingdom Procaryotae and are included within the division Gracilicutes (bacteria with a Gram-negative cell wall), class Photobacteria, subclass Oxyphotobacteriae, order Cyanobacteriales. A provisional classification was proposed by Rippka et aF4 where cyanobacteria can be subdivided into five major groups (sections) and this classification is included in the eighth edition of the Bergey's Manual of Determinative Bacteriology.15 Cyanobacteria are the largest, most diverse, and most widely distributed group of oxygenic photosynthetic microorganisms in freshwater, marine and terrestrial environments. l6 They also have the distinction of being the oldest known fossils, Wastewater Treatment with Algae, edited by Yuk-Shan Wong and Nora F.Y. Tam. Springer - Verlag and Landes Bioscience 1998.
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Wastewater Treatment with Algae
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more than 3.5 billion years old, and morphologies within the group have remained much the same for all this time. 17 These microorganisms occur as unicellular, colonial, filamentous and branched filamentous forms and unlike most of the other prokaryotes, are usually seen and often recognizable (even at the species levels) in the environment and present in many cases as plankton blooms or as dense turfs or mats containing few other microbial species. They can exist as macrophytes forming branched thalloid plants or colonies several centimeters in length or diameterl8 and have ecologiCally important roles as initial colonizers of arid land, primary producers of organic
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