IMMOBILIZATION OF CADMIUM BY MICROBIAL EXTRACELLULAR PRODUCTS

An axenic bacterial culture was isolated from Morrison loam soil (pH 6.4) and was capable of growth in the presence of CdO (175-mu-g/mL of Cd). During the first two days of incubation, growth of the culture was accompanied by a decrease in the concentration of soluble Cd. This effect was due to two phenomena: (1) a decrease in the solubility of CdO in medium conditioned by the microorganisms; and (2) to sorption of Cd to bacterial cells. The solubility of CdO in filter-sterilized spend medium was very low (4-6-mu-g/mL), yet the solubility was high (110-160-mu-g/mL) in autoclaved, filtered spent medium. Changes in the pH value of the medium were not responsible for this effect. Incubation of spent medium in the presence of increasing concentrations of Cd as Cd(NO3)2 resulted in a decrease in the metal concentration of the liquid, due to the formation of a precipitate in quantities proportional to the amount of added Cd. Similar precipitate formation was observed when 100-mu-g/mL of Hg, Cu, or Zn were added but not when 100-mu-g/mL of Mg was introduced. The compound(s) responsible for the precipitate formation was not dialyzable from tubing with a pore size of 12,000-14,000 and was not retained by a C18 Sep-Pak cartridge. Dialysis of a suspension of the precipitate against water (pH 3.0) resulted in partial dissolution of the complex. Approximately 50% of the bound Cd was released and this was accompanied by the release of a protein of molecular weight of 42,600 daltons. The results suggest that this microbial protein forms a water insoluble precipitate with Cd and therefore plays a major role in decreasing the concentration of soluble Cd during microbial growth.