Phytoplankton-Nitrifier Interactions Control the Geographic Distribution of Nitrite in the Upper Ocean

As a key intermediate in the nitrogen cycle, nitrite is involved in multiple biological pathways that regulate the distribution and availability of nitrogen in the ocean. In the oligotrophic gyres, nitrite accumulates near the base of the euphotic zone, demonstrating a subsurface maximum, termed the primary nitrite maximum; while in subpolar regions, nitrite concentrations are elevated in the near-surface ocean. As yet, the mechanisms controlling this meridional pattern remain unclear. Here, we present vertically resolved profiles of rates of nitrite production and consumption extending from the Subtropical Gyre to the Subarctic Front in the North Pacific Ocean. Our results indicate that the latitudinal distributions of nitrite across this basin are influenced by variations in phytoplankton-nitrifier interactions. In the well-lit oligotrophic surface, phytoplankton dominates rapid nitrite cycling via coupled release and re-assimilation; below the euphotic zone, diminished light stress on nitrite oxidizers results in rapid turnover and limits nitrite. By contrast, in subpolar regions where nitrate concentrations are elevated in the euphotic zone, nitrite is released during assimilative nitrate reduction and competition between phytoplankton and nitrifiers for ammonium is relaxed, facilitating ammonia oxidation. These processes, together with differential light sensitivities of ammonia and nitrite oxidizers, allow net accumulation of nitrite. Furthermore, we demonstrate a substantial contribution of urea oxidation in forming the primary nitrite maximum and balancing the two steps of marine nitrification. Our findings reveal physical-biological interactive controls on nitrite cycling and distributions in the ocean and help disentangle the complex effect of phytoplankton-microbe interactions on marine nitrogen biogeochemistry.