Abstract
Abstract. Nitric oxide (NO) is a short-lived intermediate of the oceanic nitrogen cycle. However, our knowledge about its production and consumption pathways in oceanic environments is rudimentary. In order to decipher the major factors affecting NO photochemical production, we irradiated several artificial seawater samples as well as 31 natural surface seawater samples in laboratory experiments. The seawater samples were collected during a cruise to the western tropical North Pacific Ocean (WTNP, a N–S section from 36 to 2∘ N along 146 to 143∘ E with 6 and 12 stations, respectively, and a W–E section from 137 to 161∘ E along the Equator with 13 stations) from November 2015 to January 2016. NO photoproduction rates from dissolved nitrite in artificial seawater showed increasing trends with decreasing pH, increasing temperature, and increasing salinity. In contrast, NO photoproduction rates (average: 0.5±0.2×10-12 mol L−1 s−1) in the natural seawater samples from the WTNP did not show any correlations with pH, water temperature, salinity, or dissolved inorganic nitrite concentrations. The flux induced by NO photoproduction in the WTNP (average: 13×10-12 mol m−2 s−1) was significantly larger than the NO air–sea flux density (average: 1.8×10-12 mol m−2 s−1), indicating a further NO loss process in the surface layer.
Highlights
Nitric oxide (NO) is a short-lived intermediate of the oceanic nitrogen cycle; see Bange (2008) and Kuypers et al (2018)
NO concentrations generated from photolysis of artificial seawater samples with an initial NO−2 concentration of 0.5 μmol L−1 increased with increasing DAF-2 concentrations, and the reached a maximum at a DAF-2 concentration of 1.4 μmol L−1 (Fig. 2a)
This demonstrated that photolysis samples with NO, which were allowed to react with DAF2, could be stored for at least 1 d at 4 ◦C in the dark
Summary
Nitric oxide (NO) is a short-lived intermediate of the oceanic nitrogen cycle; see Bange (2008) and Kuypers et al (2018). NO is produced and consumed during various microbial processes such as nitrification, denitrification, and anammox (Schreiber et al, 2012; Kuypers et al, 2018). Apart from (micro)biological processes, NO can be produced photochemically from dissolved nitrite (NO−2 ) in the sunlit surface ocean (Zafiriou and True, 1979; Zafiriou and McFarland, 1981): NO−2 + H2 O hν −→ NO OH q OH−. Mack and Bolton (1999) reviewed the possible subsequent reactions hydroxyl orafdRiceaalct(iOonH(q1));offoRr eeaxcatmiopnle(,1t)hecopurolddurecaecdtNtoOparnodduce HNO2 reversely (Reaction 2) and some reactions that consumed NO or its oxides like Reaction (3) to Reaction (8) It is known that both phytoplankton and zooplankton can metabolize NO, and they are influenced by ambient (extracellular) NO concentrations (Singh and Lal, 2017; Wang et al, 2017; Astier et al, 2018).
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