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 (IO3-) 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 IO3-. A significant increase in IO3- concentrations
compared to media-only controls was observed in cultures of the
ammonia-oxidising bacteria Nitrosomonas sp. (Nm51) and Nitrosoccocus
oceani (Nc10) supplied with 9-10 mM I-, indicating I- oxidation to IO3-.
Cell-normalised production rates were 15.69 (±4.71) fmol IO3- cell-1 d-1
for Nitrosomonas sp., and 11.96 (±6.96) fmol IO3- cell-1 d-1 for
Nitrosococcus oceani, 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 I- to IO3- oxidation.
If the link between ammonia and I- oxidation observed here is
representative, our ocean iodine cycling model predicts that future
changes in marine nitrification could alter global sea surface I- fields
with potential implications for atmospheric chemistry and air quality.