Iodine is one of essential trace elements for humans and animals, and is a constituent of thyroid hormones thyroxine and triiodothyronine. Insufficient iodine in the diet can cause iodine deficiency disorders such as endemic goiter and cretinism. Iodine has one stable isotope, ¹²⁷I, and several radioisotopes, including ¹²⁹I and ¹³¹I. From a radioecological viewpoint, long-lived ¹²⁹I is of great concern because it is one of the most persistent radionuclides released into the environment from nuclear facilities and nuclear weapon testing. Given its long half-life (15.7 million years), ¹²⁹I is expected to behave like a stable isotope over long time periods and it may accumulate in the human thyroid gland. Therefore, it is important to understand geochemistry of iodine for accurate safety assessments of ¹²⁹I. The predominant chemical forms of iodine in the environment are iodate (IO₃^-; oxidation state, +5), iodide (I^-; oxidation state, -1), and organically bound iodine. Recent studies have demonstrated that chemical forms of iodine are influenced or regulated by environmental organisms, especially bacteria. In this review, bacterially catalyzed iodine chemistries, including volatilization, accumulation, oxidation, reduction, sorption, and reductive dehalogenation of iodine, are summarized.