The crucial and versatile roles of bacteria in global biogeochemical cycling of iodine

Abstract

Iodine (I) is a trace element with health and environmental significance. Iodate (IO 3 - ), iodide (I - ) and organic iodine (org-I) are the major species of iodine that exist in the environment. Dissimilatory IO 3 - -reducing bacteria reduce IO 3 - to I - directly under anoxic conditions via their IO 3 - reductases that include periplasmic iodate reductase IdrABP 1 P 2 , extracellular DMSO reductase DmsEFAB and metal reductase MtrCAB. IdrAB and DmsEFAB reduce IO 3 - to hypoiodous acid (HIO) and H 2 O 2 . The reaction intermediate HIO is proposed to be disproportionated abiotically into I - and IO 3 - at a ratio of 2:1. The H 2 O 2 is reduced to H 2 O by IdrP 1 P 2 and MtrCAB as a detoxification mechanism. Additionally, dissimilatory Fe(III)- and sulfate-reducing bacteria reduce IO 3 - to I - directly via their IO 3 - reductases and indirectly via the reduction products Fe(II) and sulfide in the presence of Fe(III) and sulfate, respectively. I - -oxidizing bacteria oxidize I - to molecular iodine (I 2 ) directly under oxic conditions via their extracellular multicopper iodide oxidases IoxAC. In addition to I 2 , a variety of org-I compounds are also produced by the I - -oxidizing bacteria during I - oxidation. Furthermore, ammonia-oxidizing bacteria oxidize I - to IO 3 - directly under oxic conditions, probably via their intracellular ammonia-oxidizing enzymes. Many bacteria produce extracellular reactive oxygen species that can oxidize I - to triiodide (I 3 - ). Bacteria also accumulate I - during which I - is oxidized to HIO by their extracellular vanadium iodoperoxidases. The HIO is then transported into the bacterial cells. Finally, bacteria methylate I - to org-I CH 3 I, probably via their methyltransferases. Thus, bacteria play crucial and versatile roles in the global biogeochemical cycling of iodine via IO 3 - reduction, I - oxidation and accumulation and org-I formation.