An ocean iodine cycling model is presented, which predicts upper ocean iodine speciation. The model comprises a three-layer advective and diffusive ocean circulation model of the upper ocean, and an iodine cycling model embedded within this circulation. The two primary reservoirs of iodine are represented, iodide and iodate. Iodate is reduced to iodide in the mixed layer in association with primary production, linked by an iodine to carbon (I:C) ratio. A satisfactory model fit with observations cannot be obtained with a globally constant I:C ratio, and the best fit is obtained when the I:C ratio is dependent on sea surface temperature, increasing at low temperatures. Comparisons with observed iodide distributions show that the best model fit is obtained when oxidation of iodide back to iodate is associated with mixed layer nitrification. Sensitivity tests, where model parameters and processes are perturbed, reveal that primary productivity, mixed layer depth, oxidation, advection, surface fresh water flux and the I:C ratio all have a role in determining surface iodide concentrations, and the timescale of iodide in the mixed layer is sufficiently long for non-local processes to be important. Comparisons of the modelled iodide surface field with parameterisations by other authors shows good agreement in regions where observations exist, but significant differences in regions without observations. This raises the question of whether the existing parameterisations are capturing the full range of processes involved in determining surface iodide, and shows the urgent need for observations in regions where there are currently none.

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.