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 Al in neutral freshwater is its ability to form complexes with organic material such as humic acids and extracellular polymeric substances (EPS). This paper examines the influence of EPS, secreted by a natural bacterial biofilm associated with a pebble substrate ("sediment biofilm") and by the snail Lymnaea stagnalis, on the behaviour of Al in the water column and its bioavailability to the snail. Both sediment biofilm and snails were a significant source of aqueous EPS. Added Al stimulated the production of EPS by the snail 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 resulted in a progressive rise of Al in the water column in the absence but not in the presence of sediment biofilm. Up to 150 mug Al.cm(-2) was associated with the sediment biofilm, and we suggest that sediment is a significant "sink" for aqueous Al. EPS avidly binds colloidal Al, and we propose that the sediment biofilm is an important influence on the behaviour and bioavailability of Al in running waters when amounts of humic substances are low.