Contribution of Urea to Nitrite Production in Southern Ocean Waters with Contrasting Nitrifying Communities

We compared the contribution of ammonia and urea to nitrite production in >100 samples of Southern Ocean waters with abundant and diverse ammonia-oxidizing archaeal (AOA) communities. Ammonia (AO) and urea (UO) oxidation rates were distributed uniformly within a water mass across coastal and slope waters west of the Antarctic Peninsula; however, rates and AOA community composition displayed strong vertical gradients. Rates in most samples from Antarctic surface and slope water were at or below the limit of detection. Highest mean rates of both processes were in the Winter Water (WW, epipelagic, 21.2 and 1.6 nmol N L-1 d-1), and the Circumpolar Deep Water (CDW, mesopelagic, 7.9 and 2.5 nmol N L-1 d-1), for AO and UO, respectively. However, we also found that the response of AO and UO to substrate amendments varied by water mass. AO rates in WW samples increased by [~]200% with 44 vs 6 nM amendments, but decreased (down to 7%) in CDW samples. UO rates responded similarly, but to a lesser degree. This response suggests that even low NH4+ amendments may inhibit AO by mesopelagic Thaumarchaeota populations. AO and UO rates were not correlated, nor were they correlated with the abundance or ratios of abundance of marker genes, or with the concentrations of ammonium or urea. Our data suggest that while ammonium is the primary substrate, urea-N is responsible for a significant fraction ([~]25% of that from AO alone) of nitrite production in the Southern Ocean, comparable to its contribution at lower latitudes. IMPORTANCESouthern Ocean nitrification fuels denitrification in oxygen depleted zones at higher latitudes, one of the controls of N:P ratios in the global ocean. N2O, a powerful greenhouse gas, is by-product of nitrification. We contrast the contributions of ammonium and urea-N to nitrification in the Southern Ocean. Our work constrains rates and demonstrates that the contribution of urea-N to nitrite production in polar waters is comparable to that in temperate oceans. Correlations between activity and the abundance or ratios of Thaumarchaeota marker genes were weak, questioning their use as indicators of activity. We document differential responses of activity to substrate amendments by water mass: enhanced in epipelagic but inhibited in mesopelagic samples. We interpret this difference in the context of community composition and the production of reactive oxygen species. Our insights into environmental controls of nitrification are relevant to microbial ecologists studying Thaumarchaeota and to modeling the global nitrogen cycle.