Thallium is highly toxic to humans and other mammals (ToxFAQs: Thallium) and several radioactive thallium isotopes are known. In microorganisms, thallium is known to inhibit growth and to be accumulated via potassium transport systems (Norris et al, 1976; Damper et al, 1979). Methylated thallium species have been detected in natural waters and production of dimethylthallium by anaerobic microorganisms in lake sediment has been reported (Schedlbauer and Heumann, 2000.)

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Schedlbauer OF, Heumann KG. Biomethylation of thallium by bacteria and first determination of biogenic dimethylthallium in the ocean. Appl Organometal Chem. 2000;14:330-40.