Aquatic aluminium all-clear

Inductively coupled plasma atomic emission spectroscopy was used to track the path of an aluminium contaminant in water as it travels through the food chain. The study indicates that the toxic metal accumulates only in the inedible parts of shellfish.

The results indicate that in comparison to vertebrates, aquatic invertebrates ingest a higher proportion of Al but apparently it does not accumulate in tissues that may pose a threat to people eating the shellfish.

Rachel Walton, Francis Livens, Cathy McCrohan, and Keith White of The University of Manchester, UK, have focused on the grazing detritivore, the freshwater snail Lymnaea stagnalis, and a predator, the signal crayfish Pacifastacus leniusculus, in order to understand the potential for "trophic" transfer of aluminium through the aquatic food chain.

"The extent and impact on organisms of the uptake and trophic (food chain) transfer of aluminium through the food web is not clearly understood," the team explains. "Most studies of Al toxicity to aquatic organisms examine fish and invertebrates such as crayfish, in which extracellular interaction with aluminium in the water column means that the gill is the main site of accumulation and damage."

They first exposed the snails to either aqueous aluminium (500 micrograms per litre; this value is well within the range of pollution found in the environment) in the presence or absence of inorganic phosphate (equivalent concentration) for one month. A control group were exposed to neither metal nor ligand. "The bioavailability, accumulation and toxic effects of metals are affected by a wide range of factors, including their speciation and associations with other ligands," the team explains. "In general, soluble and organic forms are considered more toxic to the target organism and more bioavailable to biota higher up the food chain."

They then partitioned off any aluminium that may have accumulated in snail tissues using ultracentrifugation and quantified the content in soft tissues and subcellular fractions using inductively coupled plasma atomic emission spectroscopy.

They found that the snails exposed to aluminium and phosphate accumulated more metal per snail than those exposed to aluminium alone. (291 micrograms as opposed to 206 micrograms). Moreover, the team also found that those snails exposed to phosphate had more detoxified aluminium (39 as opposed to 26 percent). The creatures produced detoxified inorganic granules and heat stable proteins to protect themselves from the toxic effects of the metal.

In the next step, the team fed a number of exposed and control snails to individually housed crayfish over a 40 day period. They collected water samples, uneaten snail tissue and faeces throughout the experiment in order to assess the fate of the aluminium contaminant. They watched for changes in the behaviour of the crayfish at four points during the experiment and saw none. Intriguingly, the team found that crayfish fed the non-phosphate snails accumulated significant amounts of aluminium whereas those fed the snails given aluminium plus phosphate did not, despite the concentration of the metal being so much higher in that snail group.

They add that about 16 to 17 percent, much higher than accumulation rates in vertebrates, of the available aluminium was accumulated by the crayfish into the creatures' green gland (kidney), gut and hepatopancreas but not the gills or flexor muscle in any of the groups. People usually only consume the flexor muscle of crayfish.

"This study indicates that in comparison to vertebrates, aquatic invertebrates accumulate a higher proportion of Al via oral ingestion but it does not accumulate in tissues that may pose a threat to human consumers," the team concludes.