Oxidation of thallium by freshwater plankton communities

Thallium is a toxic metal that is of emerging concern in the Great Lakes. It can exist as either TI(I) or TI(III), and its oxidation state affects its complexation and subsequent bioavailability and toxicity. We conducted lab and field incubations with ²⁰⁴TI(I) and natural plankton assemblages to study the occurrence and mechanism of TI oxidation. We observed that TI(III) comprised 74% of total dissolved TI after a 60 h incubation in surface waters from Lake Ontario, revealing a maximum specific oxidation rate of 0.014 h^-1. No TI(I) oxidation was observed in sterile-filtered control treatments, indicating that solar radiation alone does not oxidize TI(I) to TI(III). Additional incubations with pond water revealed that TI(I) oxidation is mediated by microbial activity and is not related to the presence of abiotic particles or phytoplankton or protozoa. We also identified a minor fraction (5-13%) of nonion-exchangeable (Chelex-100 resin; pH 1.5) TI that may represent dimethylthallium or complexed thallium. This study demonstrates that planktonic bacteria are responsible for oxidizing the thermodynamically stable TI(I) to the more abundant TI(III).