Heterotrophic and autotrophic contribution to dinitrogen fixation in the Gulf of Aqaba

We evaluated the seasonal contribution of heterotrophic and autotrophic diazotrophy to the total dinitrogen (N₂) fixation in the photic zone of a pelagic station in the northern Gulf of Aqaba, Red Sea. N₂ fixation rates were highest during a Trichodesmium bloom in winter (0.7 nmol N l−1 d−1), decreased 7-fold 1 wk later throughout the upper 200 m (~0.1 nmol N l−1 d−1), and were significantly coupled with both primary and bacterial productivity. N₂ fixation rates were generally higher in the upper 200 m (~0.4 nmol N l−1 d−1) during the thermally stratified summer and were correlated solely with bacterial productivity. Experimental enrichment of seawater by phosphorus (P) enhanced bacterial productivity and N₂ fixation rates during both seasons by 3- to 5-fold. Moreover, during the stratified season, experimental amendments to seawater applying a combination of the photosynthetic inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea and a mixture of amino acids increased both bacterial productivity and N₂ fixation rates. Our findings from the northern Gulf of Aqaba indicate that in the photic zone, a shift occurs in the diazotrophic community from phototrophic and heterotrophic populations in winter, including the cyanobacteria Trichodesmium, to predominantly heterotrophic diazotrophs in summer. These heterotrophic diazotrophs may be both carbon and P limited as illustrated by their response to additions of P and amino acids.