Journal
MARINE ECOLOGY PROGRESS SERIES
Volume 522, Issue -, Pages 67-77Publisher
INTER-RESEARCH
DOI: 10.3354/meps11143
Keywords
Autotrophic diazotrophs; Heterotrophic diazotrophs; Primary productivity; Bacterial productivity; N-2 fixation; Gulf of Aqaba; P limitation
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Funding
- Israel Science Foundation grants [996/08]
- German Ministry of Education and Research (BMBF) [03F0640A]
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We evaluated the seasonal contribution of heterotrophic and autotrophic diazotrophy to the total dinitrogen (N-2) fixation in the photic zone of a pelagic station in the northern Gulf of Aqaba, Red Sea. N-2 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 (similar to 0.1 nmol N l(-1) d(-1)), and were significantly coupled with both primary and bacterial productivity. N-2 fixation rates were generally higher in the upper 200 m (similar to 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 N-2 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 N-2 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 cyano bacteria 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.
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