Cadmium sorption and toxicity in autotrophic biofilms

Autotrophic biofilms (periphyton) accumulate substantial quantities of meta ls from contaminated water. In this study, we measured the time course of b iofilm cadmium sorption, examined the effects of current, biomass, and ligh t on short-term cadmium sorption by biofilms, and tested the toxicity of ca dmium to biofilm photosynthesis. The time course of cadmium sorption appear ed to be a linear function of time over the 48-h measurement period. Biofil ms in current greater than or equal to 2 cm.s(-1) sorbed three to five time s more cadmium than biofilms in still water. Cadmium sorbed after 4 h was 7 5% greater in high-biomass biofilm (2.5 mg dry mass.cm(-2)) than in low-bio mass biofilm (0.5 mg dry mass.cm(-2)), but only in moving water. Light enha nced the sorption of cadmium 40% in one biofilm type. Cadmium toxicity to p hoto synthesis was evident after 24 h in thin biofilms exposed to initial c admium concentrations greater than or equal to 10 mu g.L-1; photosynthesis by thicker biofilms was not significantly impaired even at the highest conc entration (100 mu g.L-1). Variations in current, biofilm biomass, and light are likely to influence the movement of metals in flowing systems.