| Summary: | In the coastal upwelling system of the eastern tropical North Atlantic (ETNA), dissolved organic phosphorus (DOP) production and its release by phytoplankton is increasingly recognized as an important supply mechanism for phosphorus (P) to the oligotrophic open ocean. Photoautotrophs and dinitrogen (N2) fixing organisms (diazotrophs) are thought to be able to consume DOP, thus alleviating P stress in areas with extremely low dissolved inorganic phosphate (DIP) concentrations. In this study, nutrient addition bioassay experiments were conducted to investigate the bioavailability of different organic and inorganic phosphorus components to the phytoplankton community in the ETNA. We specifically examined how DIP and DOP amendments affected N2 fixation in that area. Our observations showed that phytoplankton growth was primarily limited by nitrogen. Moreover, DIP addition resulted in a significant increase in N2 fixation rates in almost all experiments relative to control treatments, suggesting that diazotrophs were limited by P availability in our study. At very oligotrophic sampling stations, all P compounds stimulated N2 fixation rates compared to the control. This demonstrates the capability of the diazotrophic community to utilize various DOP compounds, especially under P limiting conditions. Our findings provide a mechanism explaining how high N2 fixation rates can be sustained under extremely low DIP concentrations in the oligotrophic North Atlantic.
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