Bioavailability and toxicity of aluminium to the freshwater pond snail, Lymnaea stagnalis at neutral p

This study examines the role of silicon (Si) in the amelioration of aluminium (Al) toxicity in the freshwater pond snail, Lymnaea stagnalis at neutral pH. Aluminium is markedly accumulated by the snail and other grazing invertebrates, its bioavailability dependent on pH and the presence of complexing ligands. Previous work suggested that exogenous Si ameliorates Al toxicity as a result of the formation of non-toxic hydroxyaluminosilicates (HAS). However, Si also acts endogenously. Experiments were carried out to investigate the effect of endogenous Si on Al toxicity in L. stagnalis. Snails were exposed to Al or Si for 25 - 30 days and then to Si or Al respectively for 15 - 20 days. Behavioural measurements showed that Al toxicity was completely ameliorated in snails pre- or post-loaded with Si in the presence of Al. The amelioration of toxicity observed may be as a result of increased turnover/loss of Al, whereby the metal combines with endogenous Si to form non-toxic HAS which is then excreted. Lymnaea stagnalis accumulates Al possibly via drinking or grazing on biofilm in the water column. In order to understand the role food plays in the bioavailability and toxicity of Al, snails were exposed to artificial food containing Al nitrate or oxalate for 20 days. Possible amelioration of Al toxicity by endogenous Si was also investigated. Behaviour was significantly depressed in snails exposed to food containing Al nitrate. Snails exposed to food containing Al oxalate showed less toxicity, probably due to the strong complex formed between Al and oxalic acid, making Al less bioavailable. Amelioration of Al toxicity by Si occurred in snails fed on food containing Al oxalate alternating with Si, possibly due to the interaction between Al oxalate and Si in the tissues, resulting in the formation of non-toxic HAS. Aluminium and Si may be taken up together as HAS in the water column or independently within the tissues of L. stagnalis. X-ray microprobe analysis of digestive gland detoxificatory granules shows a Si:Al ratio of 1:2.3, suggesting the presence of non-toxic HAS, proto-imogolite. Experiments were conducted to examine the influence of Al accumulation on granule number and snail behaviour over 16 days. Characterisation of the Al-Si entity was also investigated using nuclear magnetic resonance (NMR) spectroscopy in snails exposed to Al or Si over 20 days. The number of granules increased in response to Al loading while depression in behaviour was associated with reduction in granule number. The NMR spectra of the Al-Si entity observed suggest a variety of HAS formed within the digestive gland, from proto- imogolite allophane to Si- and stream deposit - allophane. Concentration of Al/Si, pH and the presence of other ligands were suggested as key factors influencing the type of HAS formed within the digestive gland of L. stagnalis. Possible mechanisms of Al accumulation/toxicity and the role of endogenous Si in amelioration of Al toxicity are discussed.