MUCUS SECRETION BY THE FRESH-WATER SNAIL LYMNAEA-STAGNALIS LIMITS ALUMINUM CONCENTRATIONS OF THE AQUEOUS ENVIRONMENT

Extracellular mucopolysaccharide (EPS) is a significant component in m any waters, its role in the cycling and mobilization of metals is uncl ear. In vitro studies were conducted to examine the influence of EPS, secreted by the freshwater pond snail, Lymnaea stagnalis, on soluble w ater Al concentrations at near-neutral pH. Snails maintained in aerate d water of known ion content (pH 7.3, 10 degrees C) and added aluminum (500 mu g/L Al), significantly (p < 0.05; at 24 and 48 h) reduced Al (measured by inductively coupled plasma optical emission spectroscopy) in solution as compared to controls (absence of snails). Although sna ils accumulated Al into soft tissue, this only accounted for a small p ercentage of the total reduction. The remaining Al was recovered follo wing acidification of the water. This observation was attributed to pe dal EPS secreted by L. stagnalis (quantified by periodic acid Schiff s taining), which is chiefly insoluble and substrate bound. The Al that remained in solution was more labile, possibly due to the influence of soluble EPS. Further experiments with isolated EPS, confirmed that th is poorly soluble film binds and reduces Al in solution. The influence of EPS on the solution chemistry and bioavailability of Al and possib ly other metals may be important in natural waters.