Summary auto-generated
This study examined cadmium accumulation by pure and mixed sulphate-reducing bacterial (SRB) biofilms grown in continuous culture. Both biofilm types accumulated cadmium when exposed to 20 or 200 μM Cd, but the mixed culture accumulated significantly more and continued accumulating throughout the experiment, whereas the pure culture ceased uptake after 4-6 days. The mixed culture showed proportional increases in protein and carbohydrate content correlated with Cd accumulation, indicating biofilm material production was essential. Electron microscopy and energy-dispersive X-ray analysis revealed that Cd accumulated as cadmium sulfide (CdS) precipitate localized in a superficial biofilm layer rather than throughout the matrix. The mechanism appeared to involve entrapment and/or precipitation of CdS particles at the biofilm surface. The pure culture's lower uptake capacity was attributed to less efficient growth and polysaccharide production. The results demonstrate that mixed SRB cultures are superior to pure cultures for metal removal applications, highlighting important biological differences between pure and mixed culture systems for bioremediation purposes.
Key findings
- Mixed culture SRB biofilms accumulated significantly more cadmium than pure culture biofilms and continued accumulating throughout the experiment, while pure cultures ceased after 4-6 days
- Cadmium accumulated primarily as insoluble cadmium sulfide (CdS) precipitate localized in a superficial biofilm layer, representing a precipitation/entrapment mechanism rather than active uptake
- Mixed culture biofilms showed proportional increases in both protein and carbohydrate content correlated with cadmium accumulation, indicating biofilm material production was required for sustained metal uptake
- Neither pure nor mixed culture biofilms showed growth inhibition at cadmium concentrations up to 200 μM, suggesting biofilm components protected cells from metal toxicity
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Abstract
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.