Abstract

Nitrification, the stepwise oxidization 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 extending from the South China Sea to the western North Pacific Subtropical Gyre. Our results show active urea oxidation in the presence of experimental ammonium amendment, indicating direct urea oxidation. Urea oxidation rate covaries with ammonia oxidation rate, and the depthintegrated rates of urea oxidation and ammonia oxidation are comparable, demonstrating urea oxidation is a significant source of nitrite that helps to balance the two steps of nitrification in the oligotrophic ocean. Nitrifiers exhibit high affinity for their substrates, and the apparent halfsaturation constants for ammonia and nitrite oxidation decreased with depth. The apparent halfsaturation constant for urea oxidation is 1.2 to 11-fold (median 2.2) higher than that for ammonia oxidation at the corresponding depths, but with no clear vertical trend. Such kinetic traits may account for the relatively higher urea concentration compared to ammonium and nitrite concentrations in the ocean's interior. Moreover, combining our results with a review of the previous literature shows a trend of increased urea oxidation relative to ammonia oxidation, from the more eutrophic coastal zone to the oligotrophic open ocean, revealing a substrate-dependent biogeographic distribution of urea oxidation across marine environments.