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Marine Ecology Progress Series

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MEPS 522:67-77 (2015)  -  DOI:

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

Eyal Rahav1,2,*, Barak Herut2, Margaret R. Mulholland3, Natalia Belkin1, Hila Elifantz1, Ilana Berman-Frank

1Mina and Everard Goodman Faculty of Life Sciences, Bar-Ilan University, Ramat Gan 52900, Israel
2Israel Oceanographic and Limnological Research, National Institute of Oceanography, Haifa 31080, Israel
3Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University, 4600 Elkhorn Avenue, Norfolk, Virginia 23529-0276, USA
*Corresponding author:

ABSTRACT: 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.

KEY WORDS: Autotrophic diazotrophs · Heterotrophic diazotrophs · Primary productivity · Bacterial productivity · N2 fixation · Gulf of Aqaba · P limitation

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Cite this article as: Rahav E, Herut B, Mulholland MR, Belkin N, Elifantz H, Berman-Frank I (2015) Heterotrophic and autotrophic contribution to dinitrogen fixation in the Gulf of Aqaba. Mar Ecol Prog Ser 522:67-77.

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