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Abstract. Across the Southern Ocean in winter, nitrification is the dominantmixed-layer nitrogen cycle process, with some of the nitrate producedtherefrom persisting to fuel productivity during the subsequent growingseason. Because this nitrate constitutes a regenerated rather than a newnutrient source to phytoplankton, it will not support the net removal ofatmospheric CO2. To better understand the controls on Southern Oceannitrification, we conducted nitrite oxidation kinetics experiments insurface waters across the western Indian sector in winter. While allexperiments (seven in total) yielded a Michaelis–Menten relationship withsubstrate concentration, the nitrite oxidation rates only increasedsubstantially once the nitrite concentration exceeded 115±2.3 to245±18 nM, suggesting that nitrite-oxidizing bacteria (NOB) require aminimum (i.e., “threshold”) nitrite concentration to produce nitrate. Thehalf-saturation constant for nitrite oxidation ranged from 134±8 to403±24 nM, indicating a relatively high affinity of Southern OceanNOB for nitrite, in contrast to results from culture experiments. Despitethe high affinity of NOB for nitrite, its concentration rarely declinesbelow 150 nM in the Southern Ocean's mixed layer, regardless of season. Inthe upper mixed layer, we measured ammonium oxidation rates that were two-to seven-fold higher than the coincident rates of nitrite oxidation,indicating that nitrite oxidation is the rate-limiting step fornitrification in the winter Southern Ocean. The decoupling of ammonium andnitrite oxidation, combined with a possible nitrite concentration thresholdfor NOB, may explain the non-zero nitrite that persists throughout theSouthern Ocean's mixed layer year-round. Additionally, nitrite oxidation maybe limited by dissolved iron, the availability of which is low across theupper Southern Ocean. Our findings have implications for understanding thecontrols on nitrification and ammonium and nitrite distributions, both inthe Southern Ocean and elsewhere. 

Abstract. As a key biogeochemical pathway in the marine nitrogen cycle, nitrification (ammonia oxidation and nitrite oxidation) converts the most reduced form of nitrogen – ammonium–ammonia (NH4+–NH3) – into the oxidized species nitrite (NO2-) and nitrate (NO3-). In the ocean, these processes are mainly performed by ammonia-oxidizing archaea (AOA) and bacteria (AOB) and nitrite-oxidizing bacteria (NOB). By transforming nitrogen speciation and providing substrates for nitrogen removal, nitrification affects microbial community structure; marine productivity (including chemoautotrophic carbon fixation); and the production of a powerful greenhouse gas, nitrous oxide (N2O). Nitrification is hypothesized to be regulated by temperature, oxygen, light, substrate concentration, substrate flux, pH and other environmental factors. Although the number of field observations from various oceanic regions has increased considerably over the last few decades, a global synthesis is lacking, and understanding how environmental factors control nitrification remains elusive. Therefore, we have compiled a database of nitrification rates and nitrifier abundance in the global ocean from published literature and unpublished datasets. This database includes 2393 and 1006 measurements of ammonia oxidation and nitrite oxidation rates and 2242 and 631 quantifications of ammonia oxidizers and nitrite oxidizers, respectively. This community effort confirms and enhances our understanding of the spatial distribution of nitrification and nitrifiers and their corresponding drivers such as the important role of substrate concentration in controlling nitrification rates and nitrifier abundance. Some conundrums are also revealed, including the inconsistent observations of light limitation and high rates of nitrite oxidation reported from anoxic waters. This database can be used to constrain the distribution of marine nitrification, to evaluate and improve biogeochemical models of nitrification, and to quantify the impact of nitrification on ecosystem functions like marine productivity and N2O production. This database additionally sets a baseline for comparison with future observations and guides future exploration (e.g., measurements in the poorly sampled regions such as the Indian Ocean and method comparison and/or standardization). The database is publicly available at the Zenodo repository: https://doi.org/10.5281/zenodo.8355912 (Tang et al., 2023).