The Ammonia Oxidising Archaeon <i>Nitrosopumilus maritimus</i> Does Not Alter Iodine Oxidation State in Oxic Seawater

We investigate the potential for the globally distributed marine ammonia oxidising archaeon (AOA) Nitrosopumilus maritimus to oxidise iodide (I^-), with the aim of identifying a key driver of seawater iodate (IO₃ ^-) renewal. Batch cultures of N. maritimus grew well in concentrations of 0.1 to 1 mM NH₄ ⁺ and from 0.0001 to 1 mM I^-. There was near 100% conversion of ammonium to nitrite over an 8-day growth period. No loss of I^- or production of IO₃ ^- was detected in cultures where I^- was added, indicating that N. maritimus is unable to drive I^- oxidation under the tested conditions. This contrasts with previous observations of I^- oxidation by ammonium oxidising bacteria (AOB). We explore whether differences between the metabolism of AOA and AOB could explain their differing actions on I^-. N. maritimus cultures grown with the equivalent IO₃ ^- concentrations also showed no reduction in [IO₃ ^-]. In addition, the growth of the N. maritimus culture was unaffected by inorganic iodine concentrations over 1000 times higher than in ambient seawater, suggesting a resilience to high iodine. These results suggest that AOA might have very little role in inorganic iodine turnover in the global ocean.