Influence of sediment biofilm on the behaviour of aluminum and its bioavailability to the snail Lymnaea stagnalis in neutral freshwater

An important influence on the behaviour, bioavailability, and toxicity of A l in neutral freshwater is its ability to form complexes with organic mater ial such as humic acids and extracellular polymeric substances (EPS). This paper examines the influence of EPS, secreted by a natural bacterial biofil m associated with a pebble substrate ("sediment biofilm") and by the snail Lymnaea stagnalis, on the behaviour of Al in the water column and its bioav ailability to the snail. Both sediment biofilm and snails were a significan t source of aqueous EPS. Added Al stimulated the production of EPS by the s nail but not by bacterial biofilm. Repeated elevation of the concentration of Al in the water by 500 mug Al.L-1 but not 100 mug Al.L-1 over 10 days re sulted in a progressive rise of Al in the water column in the absence but n ot in the presence of sediment biofilm. Up to 150 mug Al.cm(-2) was associa ted with the sediment biofilm, and we suggest that sediment is a significan t "sink" for aqueous Al. EPS avidly binds colloidal Al, and we propose that the sediment biofilm is an important influence on the behaviour and bioava ilability of Al in running waters when amounts of humic substances are low.