Tellurium (Te) is a group 16 metalloid element related to sulphur and oxygen. It possesses stable oxidation states of +VI (tellurate), +IV (tellurite), 0 (elemental Te), and −II (telluride). The majority is found in the hydrosphere as tellurate and in the lithosphere as tellurides of gold and silver . The most harmful forms of Te to microorganisms are the oxyanions, especially tellurite, with concentrations as low as 1 µg/mL being highly toxic . However, the ability to reduce it to the elemental form allows certain bacterial species to resist concentrations up to 4000 µg/mL . The means by which this compound exerts its toxicity is still debated, however, the strong oxidative properties , confirmed by an E° of 0.827 V for the TeO3−2/Te redox couple , are likely among the reasons.