John Webb

The freshwater mussel, Veksunio angasi, from the Magela Creek. Northern Territory, is a part of the traditional diet of the local aborigines. It is known to concentrate radionuclides, particularly 226Ra, within insoluble granules in its tissue. These granules also contain large amounts of calcium, magnesium, barium, phosphorus, iron and aluminium. The uptake of 226Ra into the diet can be explained in terms of calcium replacement. This study focuses on the nature and role of iron in the mussels to identify whether it is involved in biotic or abiotic processes. The mean iron concentration of whole animals was found to be 2.7 mg Fe/g wet weight with the mean content of iron in the individual organs being 2.22 mg (gills), 3.60 mg (mantle) and 8.65 mg for the visceral mass (i.e. mass of organs not dissected out) consisting of hepatopancreas and intestine. Phosphorus was also present in similar amounts (38.7 mg Pig dry wt (9.0 mg P/g wet wt.) in visceral mass and 13.4 mg P/g dry wt (2.0 mg P/g wet wt) in mantles/gills/palps. Iron in the granules s examined using 57Fe Mössbauer spectroscopy and gave a spectral doublet at a temperature of 4.2 K with spectral parameters indicating that the iron is in the form of Fe(III). The partial transformation of the doublet to a sextet at 1.5 K indicates the onset of magnetic order between 4.2 and 1.5 K. Magnetic ordering shows that the iron atoms must be close enough to each other for magnetic exchange interactions to be effective. However, the very low magnetic ordering temperature indicates that these interactions are weak, suggesting that the iron atoms are not as closely packed as would be found in a well ordered lattice of an iron oxide mineral. In addition, there may be bridging ions such as phosphate separating the iron atoms, which have the effect of reducing the strength of magnetic exchange interactions between the iron atoms. The spectral characteristics are very similar to those of nanoscale hydrated Fe(III) phosphate particles found in other biological systems such as bacterial ferritins. Preliminary indications are that the form of iron present in the mussel granules is similar to that in the water column, suggesting that it is abiotic in origin but has undergone some biotic transformation. The origin of the phosphate granules is not known, but it is known that particles of this general composition are efficient scavengers of radionuclides in the water column. The presence of radionuclides in the granules is likely to be a consequence of the mussels’ storing large quantities of iron- containing material from filter-feeding particularly the finer size fraction and colloidal particles.