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

다국어 초록 (Multilingual Abstract)

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.
Nitrite is a key intermediate between reduced and oxidized forms of nitrogen that is involved in numerous biological processes in the marine nitrogen cycle. Throughout the world's oceans, concentrations of nitrite vary spatially, with concentrations in the subtropics often demonstrating a subsurface maximum, while concentrations are often elevated in the euphotic zone in the tropics and subpolar regions. To date, the mechanisms and processes governing the vertical distributions of nitrite remain controversial. By investigating a suite of processes catalyzing nitrite cycling across the Northwestern Pacific, we show that spatially variable interactions between phytoplankton and planktonic nitrifiers regulate pathways and rates of nitrite cycling. These processes together shape nitrite distribution patterns at the basin scale. Additionally, we demonstrate the significance of urea oxidation in marine nitrogen recycling in terms of contributing to the primary nitrite maximum formation and maintaining the homeostasis of the two steps of nitrification.



At the basin scale, the PNM exhibits a unique meridional pattern

Nitrate‐dependent phytoplankton‐nitrifier interactions result in distinctive nitrite accumulation patterns in different ocean regions

Following ammonia oxidation, urea oxidation represents the second largest nitrite source playing an essential role to form the PNM


At the basin scale, the PNM exhibits a unique meridional pattern
Nitrate‐dependent phytoplankton‐nitrifier interactions result in distinctive nitrite accumulation patterns in different ocean regions
Following ammonia oxidation, urea oxidation represents the second largest nitrite source playing an essential role to form the PNM