Extracellular Precipitation of Metals

Exceeding a solubility product leads to precipitation of an insoluble salt of the reacting species. Sulfide anion (S22) and oxalate anion (C2O4_) are produced by some species of micro-organisms and these anions can form very insoluble salts with heavy metal ions which exhibit very small solubility products as noted by Veglio and Beolchini (1997). Copper phosphate precipitation occurring within the matrix of mycelia of the fungus Penicillium ochro-chloron after 4 days of incubation in shake flask cultures at pH 4, was demonstrated using scanning electron microscopy and energy dispersive x-ray microanalysis (Crusberg et al. 1994). Wrinkled 40-50 mm dia. spheres of insoluble copper phosphate, inferred from EDX analysis, are trapped within the mycelia of the fungus grown for 4 days in aerated cultures in the presence of 100mg/l Cu2+. Penicilllium and Aspergillus have been shown to produce extracellular acid phosphatases which correlate with copper removal from solution (Haas et al. 1991; Tsekova et al. 2002). It is likely that the extracellular phosphatases are localized in the periplasmic space, sandwished between the membrane and cell wall materials. There they can act on extracellular organophosphates, hydrolyzing the phosphate and organic moieties for uptake via specialized transport systems. As a means of detoxification, the free phosphate anions can then react with and precipitate heavy metal ions as insoluble heavy metal phosphates before those ions enter the cell. A. niger has also been recently shown to immobilize metals as insoluble oxalate salts (Cunningham and Kuiack 1992; Sayer and Gadd 1997). In fact, oxalate is a common secondary metabolite of Penicillium spp. (Sayer and Gadd 1997) and is known to precipitate calcium oxalate under natural conditions in litter (Tait et al. 1999). Lead oxalate precipitation between pH 4 and 5 has also been demonstrated as a means of detoxifiction using living A. niger biomass (Sayer et al. 1999). Fungi therefore possess two mechanisms to protect themselves against extracellular heavy metals involving the precipitation of heavy metals as insoluble salts. Recently, Mukherjee et al. (2001) reported the bioreduction of AuClJ ions by Verticillum sp. In which gold nanoparticles 20 nm dia., characterized by a vivid purple color over the surface of the fungus growing on culture plates were observed.

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