Extracellular Polymer Metal Interactions

Many bacteria produce large amounts of extracellular polysaccharides that have ionic properties and thus function as efficient biosorbents for metal cations. Interactions with metal ions are generally considered to be a direct consequence of the presence of negatively charged functional groups on these exopolymers. These groups include pyruvate, phosphate, hydroxyl, succinyl, and uronic acid. pH-dependent binding the positively charged cation to these groups can occur rapidly, with stability constants in excess of those measured for humic substances and other naturally occurring ligands. Extracellular polymers such as those produced by Zoogloea sp. are strongly involved in metal removal from sewage treatment processes, and the extraction and removal of polymers from these and other bacterial cultures can greatly reduce biosorption capacities and also increase metal sensitivity. The extracellular matrices act as an efficient barrier and prevent significant entry of metal ions into the cells. Similar interactions are also likely for cyanobacteria, algae, and fungi that produce extracellular polymers. Many organic metabolites are important in metal detoxification because of their chelating or complexion properties. Organic or inorganic acids produced by microorganisms, including Thiobacillus, Serratia, Pseudomonas, Penicillium, and Aspergillus are able to extract metals from solid substrate. Citric acid is an efficient metal chelator, whereas oxalic acid can precipitate metals as insoluble oxalates around cell walls and in the external medium. For example, citric acid production by Penicillium has been used to extract Zn selectively from industrial waste. Oxalic acid producing fungi often exhibit marked metal tolerance, and this detoxification mechanism is often found in wood-rotting fungi, particularly those exposed to chromated copper arsenate wood preservatives, in Poria, as well as in other species such as Aspergillus niger, Penicillium spinulosum, and Verticillium psalliotae.

Certain organisms, particularly the sulfate-reducing bacteria of the genus Desulfovibrio, are involved in the formation of sulfide deposits that contain large amounts of metal. Sulfide formation thus leads to metal removal from solution, and this is associated with resistance in a variety of microbes. Metal-resistant strains of Klebsiella aerogenes precipitate lead, mercury, or cadmium as insoluble sulfide granules on the outer surfaces of cells, and particles of Ag2S are deposited on Thiobacilli were grown in silver containing sulfide leaching system. Strains of the green algae Gyanidium caldarium can grow in acidic water at 45°C containing high concentrations of metal ions. Fe, Cu, Ni, Al and Cr can be removed from solution by precipitation at cell surfaces as metal sulfides. Cells can contain up to 20% metals on a dry weight basis. Yeast can also precipitate metals as sulfides in and around cell walls, and colonies may appear dark brown in the presence of Cu (Gadd 1990). Many other examples of crystallization and precipitation on microbial surfaces are known, with some of these representing resistance mechanisms. Microbes and several bacteria are implicated in the formation of ferromanganese nodules on ocean floors; for example, Hyphomicrobium, algae, and fungi promote Mn2+ oxidation in a variety of habitats and can become encrusted with manganese oxides. Other bacteria can become encrusted with oxidized iron compounds by metabolism-dependent and independent processes. In metal-resistant strains of Citrobacter, one major mechanism of metal uptake is the activity of cell-bound phosphatase, induced by growth in glycerol-2-phosphate, which can precipitate Cd, Pb, Cu, and U as insoluble metal phosphates on the cell surface.

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