ACCUMULATION AND EFFECTS OF CADMIUM ON SULFATE-REDUCING BACTERIAL BIOFILMS

Biofilms comprising a pure and a mixed culture of sulphate-reducing ba cteria (SRB) were grown in continuous culture. When exposed to 20 or 2 00 mu M Cd, both cultures accumulated Cd but the mixed culture accumul ated snore and continued to accumulate Cd during the experiment, where as accumulation by the pure cultures ceased after 4-6 d. Unlike the pu re culture, the mixed culture also accumulated both protein and carboh ydrate throughout the experiment proportionally to Cd which showed tha t accumulation required the production of biofilm material. Electron m icroscopy showed the presence of polysaccharide and particulates in bo th pure and mixed cultures, irrespective of the presence of Cd. Howeve r, energy-dispersive X-ray analysis (EDXA) showed that accumulation of Cd in the form of CdS occurred in biofilms exposed to Cd while back-s cattered electron imaging of sections indicated that the accumulation of Cd was localized in a superficial layer of the biofilm. The mechani sm of uptake, therefore, appeared to be entrapment and/or precipitatio n of CdS at the biofilm surface. The relatively low Cd uptake by the p ure culture biofilm was attributed to its less efficient growth and po lysaccharide production. These results indicate that mixed SRB culture s are more effective than pure cultures for metal removal and underlin es significant differences between the biology of pure and mixed cultu res.