In the oceanic Oxygen Minimum Zones, the widespread picocyanobacterium Prochlorococcus forms a secondary chlorophyll maximum (SCM) below the oxycline, in waters without detectable O₂. As a result of oxygenic photosynthesis, traces of oxygen released in this layer can support activity of aerobic microbial processes in apparent anoxia. Beside organotrophic aerobic respiration, other aerobic processes such as nitrite oxidation might use the released O₂, altering the otherwise anaerobic element cycling in this layer below the oxycline. During two cruises in the Pacific OMZs, water collected in the SCM was incubated simulating the dim bluish light and trace O₂ levels found in situ. Net O₂ metabolism was measured using high-resolution O₂ optodes and rates of incorporation of C isotopes. Isotopically labelled ¹⁵N-nitrite was also added to the incubations to measure its oxidation and reduction pathways. Our results indicate that a substantial fraction of the O₂ produced by photosynthesis in the SCM was used for the oxidation of nitrite to nitrate, and could even account for the total net O₂ consumption during some incubations. Nitrite oxidation in the SCM had a high affinity for O₂, revealing half-saturation constants of only a few tens of nanomolar for O₂, consistent with the trace levels of O₂ found in situ. Nitrite oxidation was detected even under apparently anoxic conditions, suggesting an anaerobic pathway of nitrite oxidation. Regardless of O₂ concentration, oxidation was the dominant nitrite transformation pathway, highlighting the role of the SCM in N retention in the OMZs

Primary Presenter: Emilio Garcia Robledo, Universidad de Cadiz (emilio.garcia@uca.es)

Authors:

Laura Bristow, University of Goteborg  ()

Bo Thamdrup, University of Southern Denmark  ()

Sam Fortin, Princeton University  ()

Bess Ward, Princeton University  ()

NIels Peter Revsbech, Aarhus University  ()