Study of bioaccumulation and biotransformation by microanalytical X-ray techniques: investigation of distribution and speciation of Cu and Cr in the body of the plant-feeding nematode, Xiphinema vuittenezi

Free-living nematodes, due to their microscopic size and quite complex life processes can be widely applied as biological model organisms to study the effect of soil pollutants. The modern X-ray spectrometric methods enable the individual analysis of microscopic biological samples, hereby expanding the scope of environmental and toxicological sciences. In order to study the uptake and bioaccumulation of Cu and Cr by nematodes (Xiphinema vuittenezi), the worms were starved in tap water for five days, and then treated with solutions of CuSO4, Cu(NO3)2, KCr(SO4)2 or K2CrO4 for 1-120 h. The effect of two soil-polluting elements, Cu, which is often applied in herbicides, and Cr, which is essential in the form of Cr(III), but toxic if it occurs as Cr(VI), on soil-inhabiting nematodes was studied. The uptake of these toxicants and their bioaccumulation was investigated by TXRF, following a two-step sample digestion method (digestion with HNO3 and using cold O plasma). Their effect on the essential element content of nematodes was also studied. Differences were found between the uptake of Cr(III) and Cr(VI). The Cr(III) causes decreased Zn content comparing with untreated nematodes, for Cr(VI) it was not observed. Elemental maps in the cross-sections of the worms prepared by focused gallium ion beam, following a special sample preparation method (quick-freezing in liquid nitrogen for 2 min and lyophilization for 72 h) were collected by scanning electron microscope equipped with energy dispersive X-ray detector. The distribution of Cr-P and Cu-P showed a similar pattern, however, the distribution of S deviated from these ones. According to these findings, the role of P-containing ligands in the binding of Cu and Cr is more characteristic than of S-containing ones. With the purpose of studying the speciation of the Cu and Cr in the worm, X-ray absorption near edge structure spectrometry was applied. The sample preparation method was the same as in the case of the elemental mapping. For Cu, the sample spectrum was fitted by the spectra of reference compounds, and Cu3(PO4)2, was found to be the main component, which harmonized with the results of the elemental distribution measurements. However, the role of S-containing ligands in Cu detoxification cannot be excluded, since the spectrum of Cu-cysteine complex was a minor component of the fit. It was also established that in the chromate treated nematodes a considerable part of Cr(VI) was reduced to Cr(III).