Reaction with iodide (I) at the sea surface is an important sink for atmospheric ozone, and causes sea-air emission of reactive iodine which in turn drives further ozone destruction. To incorporate this process into chemical transport models, improved understanding of the factors controlling marine iodine speciation, and especially sea-surface iodide concentrations, is needed. The oxidation of I to iodate (IO) is the main sink for oceanic I, but the mechanism for this remains unknown. We demonstrate for the first time that marine nitrifying bacteria mediate I oxidation to IO. A significant increase in IO concentrations compared to media-only controls was observed in cultures of the ammonia-oxidising bacteria sp(Nm51) and (Nc10) supplied with 9-10 mM I, indicating I oxidation to IO. Cell-normalised production rates were 15.69 (±4.71) fmol IO cell d for sp., and 11.96 (±6.96) fmol IO cell d for , and molar ratios of iodate-to-nitrite production were 9.2±4.1 and 1.88±0.91 respectively Preliminary experiments on nitrite-oxidising bacteria showed no evidence of ItoIO oxidation. If the link between ammonia and I oxidation observed here is representative, our ocean iodine cycling model predicts that decreases in marine nitrification under ocean acidification could lead to significantly higher sea surface I. A global sensitivity analysis suggests a 0.13 nM increase in sea surface I concentrations per percentage decrease in nitrification rate. In turn, this could result in increased O deposition to the sea surface and sea-air iodine emissions, with implications for atmospheric chemistry and air quality.