Biomethylation of thallium by bacteria and first determination of biogenicdimethylthallium in the ocean

To investigate a possible biomethylation of thallium, incubation experiment s 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 an aerobic conditions with a fresh-water lake sediment, by adding Tl(I) nitrat e to these systems. Only in the case of the anaerobic sediment was a signif icant production of dimethylthallium observed (after three-weeks). Analysin g different surface seawater samples and those from a single depth profile down to 4000 m, dimethylthallium was determined above the detection limit o f 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 tot al thallium concentration, varied from 3 to 48%, The determination of such low dimethylthallium content was possible by applying a new sensitive analy tical method of positive thermal-ionization isotope-dilution mass spectrome try connected with a species-unspecific thallium enrichment by anion-exchan ge 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-l ead, monomethylcadmium and dimethyl sulphide were also found with peak conc entrations 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 oce an. However, even at depths of 1000 and 4000 m significant amounts of dimet hylthallium could be analysed, which suggest bacteria as the biogenic sourc e in the deep sea. Copyright (C) 2000 John Wiley & Sons, Ltd.