Effect of humic acid on water chemistry, bioavailability and toxicity of aluminium in the freshwater snail, Lymnaea stagnalis, at neutral pH
Introduction
Naturally occurring organic solutes may have significant effects on the bioavailability and toxicity of aluminium (Al) to freshwater organisms due to their ability to form strong complexes that affect Al speciation at neutral pH (Driscoll and Schecher, 1989, Martell et al., 1990). Dissolved humic material, formed from the microbial degradation of plant and animal matter (Steinberg et al., 1992), constitutes a major fraction of the total organic matter in freshwaters (Carlberg et al., 1986, Lee et al., 1993).
Humic acids are composed of aromatic and aliphatic groups and also large numbers of negatively charged carboxylic acid groups (Sauvant et al., 1999). This net negative charge forms highly stable complexes with metal cations (Schnitzer and Khan, 1972). Copper, for example, is commonly found in freshwaters bound to humic compounds with a stability constant three orders of magnitude greater than complexes with the bicarbonate ion (CO3−), another common freshwater ligand (Mantoura et al., 1978). Al also readily binds to humic acids, which thereby affect the metal's chemistry (Sauvant et al., 1999, Jekel, 1986). The nature of this association has been studied extensively (Jekel, 1986, Tipping et al., 1988, Russell et al., 1997). Previous studies have shown that humic acids form complexes with Al in the water, and that these complexes have reduced toxicity to fish (Skogheim et al., 1986, Lydersen et al., 1990). However, the influence of humic acids on tissue accumulation and endogenous toxicity of Al in freshwater organisms is largely unknown.
The snail, Lymnaea stagnalis, is a grazing omnivore found in high abundance in temperate freshwater ecosystems. It is easily maintained under laboratory conditions and its simple repertoire of behaviours is amenable to toxicological studies. Desouky (2001) recently reported the complete amelioration of Al toxicity by humic acid, on feeding behaviour in L. stagnalis, exposed to 500 μg l−1 Al for 30 days at pH 7.5. In the presence of humic acid, there was a marked reduction in loss of Al from the water column, when compared to control (Al alone), suggesting an interaction between humic acid and Al in the water column that reduced the hydroxy-polymerisation and precipitation of Al. This interaction, however, had little effect on the bioavailability of Al; levels of Al in the digestive gland (the main sink for Al; Elangovan et al., 1997) were unaffected by humic acid (Desouky et al., 2002). The major route of uptake of Al by the snail is thought to be via grazing on precipitated Al associated with mucus biofilm on the substrate (Jugdaohsingh et al., 1998). It might seem surprising therefore that accumulation of Al was unaffected by humic acid even though more Al was retained in the water column (Desouky et al., 2002).
In view of these apparently contradictory findings, the aim of this study was to further investigate the interaction between Al and humic acid in the water column and effects on Al accumulation and toxicity. We used two environmentally relevant Al concentrations: 500 μg l−1, the concentration previously used by Desouky, 2001, Desouky et al., 2002, represents more polluted waters; 250 μg l−1 provided a comparison with that found in less polluted waters.
Section snippets
Materials and methods
Lymnaea stagnalis (2–3 cm shell length), collected from Smyslov Pond (Blatna, Czech Republic), were acclimatised in standard snail water (SSW; CaCl2, 2.0 M; MgSO4, 0.08 mM; KHCO3, 0.04 mM, NaHCO3 0.70 mM; KNO3, 0.05 mM) for 14 days under standard conditions: 10 °C under a 14/10 h light/dark regime. Groups of snails were then exposed under these conditions to 250 μg l−1 or 500 μg l−1 added Al in the presence and absence of 10 mg l−1 humic acid for 30 days. Aluminium was added to the experimental tanks in the
Behavioural toxicity
Exposure of snails to added Al (250 and 500 μg l−1) resulted in behavioural suppression, seen as a fall in behavioural state score (BSS) (Fig. 1). The first evidence of Al toxicity was observed on day 2 and BSS dropped further thereafter and remained depressed throughout the (30 day) exposure period. There was no significant difference (p < 0.05) between BSS of snails exposed to 250 μg l−1 and 500 μg l−1 Al (Fig. 1A,B). Humic acid in the water column significantly reduced overall behavioural
Discussion
The aim of this study was to better understand the speciation of Al in the water column in the presence and absence of humic acid, and to relate this to its bioaccumulation and toxicity. Snails exposed to 500 μg l−1 Al in the water column showed significant accumulation of the metal in their tissues, together with behavioural depression. Surprisingly, significant tissue accumulation was not observed with the lower Al concentration (250 μg l−1), even though behaviour was still depressed. This is
Acknowledgements
We thank the Soros Foundation, UK Foreign Office and NERC, UK for financial support.
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