Significance of Urea in Sustaining Nitrite Production by Ammonia Oxidizers in the Oligotrophic Ocean

Nitrification, the stepwise oxidation of ammonia to nitrate via nitrite, is a key process in the marine nitrogen cycle. Reported nitrite oxidation rates frequently exceed ammonia oxidation rates below the euphotic zone, raising the fundamental question of whether the two steps are balanced and if alternative sources contribute to nitrite production in the dark ocean. Here we present vertically resolved profiles of ammonia, urea, and nitrite oxidation rates and their kinetic traits in the oligotrophic Subtropical North Pacific. Our results show active urea-derived nitrogen oxidation (urea-N oxidation) in the presence of experimental ammonium amendment, suggesting direct urea utilization. The depth-integrated rates of urea-N oxidation and ammonia oxidation are comparable, demonstrating that urea-N oxidation is a significant source of nitrite. The additional nitrite from urea-N oxidation helps to balance the two steps of nitrification in our study region. Nitrifiers exhibit high affinity for their substrates, and the apparent half-saturation constants for ammonia and nitrite oxidation decrease with depth. The apparent half-saturation constant for urea-N oxidation is higher than that for ammonia oxidation and shows no clear vertical trend. Such kinetic traits may account for the relatively higher urea concentration than ammonium concentration in the ocean's interior. Moreover, a compilation of our results and reported data shows a trend of increased urea-N oxidation relative to ammonia oxidation from the eutrophic coastal zone to the oligotrophic open ocean. This trend reveals a substrate-dependent biogeographic distribution of urea-N oxidation across marine environments and provides new information on the balance and flux of the marine nitrification process.