Cadmium sorption and toxicity in autotrophic biofilms

Autotrophic biofilms (periphyton) accumulate substantial quantities of metals from contaminated water. In this study, we measured the time course of biofilm cadmium sorption, examined the effects of current, biomass, and light on short-term cadmium sorption by biofilms, and tested the toxicity of cadmium to biofilm photosynthesis. The time course of cadmium sorption appeared to be a linear function of time over the 48-h measurement period. Biofilms in current greater than or equal to 2 cm.s(-1) sorbed three to five times more cadmium than biofilms in still water. Cadmium sorbed after 4 h was 75% greater in high-biomass biofilm (2.5 mg dry mass.cm(-2)) than in low-biomass biofilm (0.5 mg dry mass.cm(-2)), but only in moving water. Light enhanced the sorption of cadmium 40% in one biofilm type. Cadmium toxicity to photo synthesis was evident after 24 h in thin biofilms exposed to initial cadmium concentrations greater than or equal to 10 mu g.L-1; photosynthesis by thicker biofilms was not significantly impaired even at the highest concentration (100 mu g.L-1). Variations in current, biofilm biomass, and light are likely to influence the movement of metals in flowing systems.