UPTAKE AND TRANSFORMATION OF METALS AND METALLOIDS BY MICROBIAL MATS AND THEIR USE IN BIOREMEDIATION

Constructed microbial mats, used for studies on the removal and transf ormation of metals and metalloids, are made by combining cyanobacteria inoculum with a sediment inoculum from a metal-contaminated site. The se mars are a heterotrophic and autotrophic community dominated by cya nobacteria and held together by slimy secretions produced by various m icrobial groups. When contaminated water containing high concentration s of metals is passed over microbial mars immobilized on glass wool, t here is rapid removal of the metals from the water. The mats are toler ant of high concentrations of toxic metals and metalloids, such as cad mium, lead chromium, selenium and arsenic (up to 350 mg L(-1)). This t olerance may be due to a number of mechanisms at the molecular, cellul ar and community levels. Management of toxic metals by the mats is rel ated to deposition of metal compounds outside the cell surfaces as wel l as chemical modification of the aqueous environment surrounding the mats. The location of metal deposition is determined by factors such a s redox gradients, cell surface micro-environments and secretion of ex tra-cellular bioflocculents. Metal-binding flocculents (polyanionic po lysaccharides) are produced in large quantities by the cyanobacterial component of the mat. Steep gradients of redox and oxygen exist from t he surface through the laminated strata of microbes. These are produce d by photosynthetic oxygen production at the surface and heterotrophic consumption in the deeper regions. Additionally, sulfur-reducing bact eria colonize the lower strata, removing and utilizing the reducing H2 S, rather than water, for photosynthesis. Thus, depending on the chemi cal character of the microzone of the mat, the sequestered metals or m etalloids can be oxidized, reduced and precipitated as sulfides or oxi des. For example precipitates of red amorphous elemental selenium were identified in mars exposed to selenate (Se-VI) and insoluble precipit ates of manganese, chromium, cadmium, cobalt, and lead were found in m ats exposed to soluble salts of these metals. Constructed microbial ma ts offer several advantages for use in the bioremediation of metal-con taminated sites. These include low cost, durability, ability to functi on in both fresh and salt water, tolerance to high concentrations of m etals and metalloids and the unique capacity of mats to form associati ons with new microbial species. Thus one or several desired microbial species might be integrated into mars in order to design the community for specific bioremediation applications.