Contribution of dinitrogen fixation to bacterial and primary productivity in the Gulf of Aqaba (Red Sea)

Abstract. We evaluated the seasonal contribution of heterotrophic and autotrophic diazotrophy to the total dinitrogen (N₂) fixation in a representative pelagic station in the northern Gulf of Aqaba in early spring when the water column was mixed and during summer under full thermal stratification. N₂ fixation rates were low during the mixed period (∼ 0.1 nmol N L⁻¹ d⁻¹) and were significantly coupled with both primary and bacterial productivity. During the stratified period N₂ fixation rates were four-fold higher (∼ 0.4 nmol N L⁻¹ d⁻¹) and were significantly correlated solely with bacterial productivity. Furthermore, while experimental enrichment of seawater by phosphorus (P) enhanced bacterial productivity and N₂ fixation rates during both seasons primary productivity was stimulated by P only in the early spring. Metatranscriptomic analyses from the stratified period identified the major diazotrophic contributors as related to heterotrophic prokaryotes from the Euryarchaeota and Desulfobacterales (Deltaproteobacteria) or Chlorobiales (Chlorobia). Moreover, during this season, experimental amendments to seawater applying a combination of the photosynthetic inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and a mixture of amino acids increased both bacterial productivity and N₂ fixation rates. Our findings from the northern Gulf of Aqaba indicate a~shift in the diazotrophic community from phototrophic and heterotrophic populations, including small blooms of the cyanobacterium Trichodesmium, in winter/early spring, to predominantly heterotrophic diazotrophs in summer that may be both P and carbon limited as the additions of P and amino acids illustrated.