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RESEARCH PRODUCT

Biomethylation of thallium by bacteria and first determination of biogenic dimethylthallium in the ocean

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To investigate a possible biomethylation of thallium, incubation experiments were carried out under aerobic conditions with a sewage sludge and with a mixed bacterial culture isolated from a sewage sludge, as well as under anaerobic conditions with a fresh-water lake sediment, by adding Tl(I) nitrate to these systems. Only in the case of the anaerobic sediment was a significant production of dimethylthallium observed (after three-weeks). Analysing different surface seawater samples and those from a single depth profile down to 4000 m, dimethylthallium was determined above the detection limit of 0.4 ng L−1 in about 20% of all samples, ranging from 0.5 to 3.2 ng l−1. The proportion of dimethylthallium in these samples, compared with the total thallium concentration, varied from 3 to 48%. The determination of such low dimethylthallium content was possible by applying a new sensitive analytical method of positive thermal-ionization isotope-dilution mass spectrometry connected with a species-unspecific thallium enrichment by anion-exchange chromatography and a species-specific extraction step. This is the first time that dimethylthallium could be detected in environmental samples. In surface seawater samples the occurrence of dimethylthallium correlated well with relatively high concentrations of chlorophyll-a, used as an indicator for bioactivity. Mostly, other biomethylated compounds such as trimethyl-lead, monomethylcadmium and dimethyl sulphide were also found with peak concentrations at these locations. The depth profile of dimethylthallium shows a maximum in concentration between 40 and 200 m, which corresponds with the highest level of bioactivity normally found in the water column of the ocean. However, even at depths of 1000 and 4000 m significant amounts of dimethylthallium could be analysed, which suggest bacteria as the biogenic source in the deep sea. Copyright © 2000 John Wiley & Sons, Ltd.