Dynamics and controls of heterotrophic prokaryotic production in the western tropical South Pacific Ocean : links with diazotrophic and photosynthetic activity

Abstract : Heterotrophic prokaryotic production (BP) was studied in the western tropical South Pacific (WTSP) using the leucine technique, revealing spatial and temporal variability within the region. Integrated over the euphotic zone, BP ranged from 58 to 120 mg C m −2 d −1 within the Melane-sian Archipelago, and from 31 to 50 mg C m −2 d −1 within the western subtropical gyre. The collapse of a bloom was followed during 6 days in the south of Vanuatu using a La-grangian sampling strategy. During this period, rapid evolution was observed in the three main parameters influencing the metabolic state: BP, primary production (PP) and bacterial growth efficiency. With N 2 fixation being one of the most important fluxes fueling new production, we explored relationships between BP, PP and N 2 fixation rates over the WTSP. The contribution of N 2 fixation rates to bacterial nitrogen demand ranged from 3 to 81 %. BP variability was better explained by the variability of N 2 fixation rates than by that of PP in surface waters of the Melanesian Archipelago, which were characterized by N-depleted layers and low DIP turnover times (T DIP < 100 h). This is consistent with the fact that nitrogen was often one of the main factors controlling BP on short timescales, as shown using enrichment experiments , followed by dissolved inorganic phosphate (DIP) near the surface and labile organic carbon deeper in the euphotic zone. However, BP was more significantly correlated with PP, but not with N 2 fixation rates where DIP was more available (T DIP > 100 h), deeper in the Melanesian Archipelago, or within the entire euphotic zone in the subtropical gyre. The bacterial carbon demand to gross primary production ratio ranged from 0.75 to 3.1. These values are discussed in the framework of various assumptions and conversion factors used to estimate this ratio, including the methodological errors, the daily variability of BP, the bacterial growth efficiency and one bias so far not considered: the ability for Prochlorococcus to assimilate leucine in the dark.
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France van Wambeke, Audrey Gimenez, Solange Duhamel, Cécile Dupouy, Dominique Lefèvre, et al.. Dynamics and controls of heterotrophic prokaryotic production in the western tropical South Pacific Ocean : links with diazotrophic and photosynthetic activity. Biogeosciences, European Geosciences Union, 2018, 15, pp.2669 - 2689. ⟨10.5194/bg-15-2669-2018⟩. ⟨ird-02010226⟩

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