Accumulation and effects of cadmium on sulphate-reducing bacterial biofilms

Biofilms comprising a pure and a mixed culture of sulphate-reducing bacteria (SRB) were grown in continuous culture. When exposed to 20 or 200 μM Cd, both cultures accumulated Cd but the mixed culture accumulated more and continued to accumulate Cd during the experiment, whereas accumulation by the pure cultures ceased after 4-6 d. Unlike the pure culture, the mixed culture also accumulated both protein and carbohydrate throughout the experiment proportionally to Cd which showed that accumulation required the production of biofilm material. Electron microscopy showed the presence of polysaccharide and particulates in both pure and mixed cultures, irrespective of the presence of Cd. However, energy-dispersive X-ray analysis (EDXA) showed that accumulation of Cd in the form of CdS occurred in biofilms exposed to Cd while back-scattered electron imaging of sections indicated that the accumulation of Cd was localized in a superficial layer of the biofilm. The mechanism of uptake, therefore, appeared to be entrapment and/or precipitation of CdS at the biofilm surface. The relatively low Cd uptake by the pure culture biofilm was attributed to its less efficient growth and polysaccharide production. These results indicate that mixed SRB cultures are more effective than pure cultures for metal removal and underlines significant differences between the biology of pure and mixed cultures.