Hamza I. S. auteur aut IRD Biegala Isabelle auteur aut IRD Zouari A. B. auteur aut IRD Akrout F. auteur aut IRD Keskes F. A. auteur aut IRD Hamza A. auteur aut IRD Hassen M. B. auteur aut IRD text journalArticle eng text/pdf born digital access Gabes Gulf is limited by ultra-oligotrophic waters which are regularly fertilized with both phosphate and iron enriched atmospheric dust. Despite the lack of river run nitrogen source, an essential nutrient for the ecosystem productivity. Diazotrophic cyanobacteria are an important source of new nitrogen in oligotrophic oceans and seas. So far only the filamentous diazotrophic cyanobacteria Trichodesmium spp. has been investigated in Gabes gulf, while it is known that other diazotrophic cyanobacteria, such as the heterocysted Richelia intracellularis and unicellular cyanobacteria (UCYN), are major source of new nitrogen in oligotrophic ecosystems. In this study, we investigated the diversity and abundance of marine diazotrophic cyanobacteria within two size fractions (> 10 mu m and 0.2-3 mu m) during July in the central coastal area of the Gulf of Gabes. Richelia intracellularis were present under free living and hosted in the diatoms Hemiaulus hauckii. Unicellular diazotrophic cyanobacteria were classified under tow size classes (small cells: 0.8-1.5 mu m and large cells: 1.6-3.2 mu m) with a predominance of the small cells (98 %). Richelia intracellularis reached concentrations that can be considered as a bloom phenomenon (up to 227.5 cells mL(-1)), while the concentration of UCYN (from 8.8 to 23.9 cells mL(-1)) were slightly higher than mean concentrations observed across the Mediterranean Sea. High temperature and dissolved nitrogen and orthophosphate concentrations in the central coastal area of the Gulf of Gabes seem to the main factors leading to such abundances. Contribution of each diazotrophic cyanobacteria to the N/P balance in the central coastal area of Gulf was also discussed. specialized Diazotrophic cyanobacteria Richelia intracellularis UCYN Central coastal area of the Gulf of Gabes TUNISIE MEDITERRANEE 034 036 42 101653 [10 ] 2021 2352-4855 https://www.documentation.ird.fr/hor/fdi:010081499 10.1016/j.rsma.2021.101653 2352-4855 [F B010081499] https://www.documentation.ird.fr/hor/fdi:010081499 https://www.documentation.ird.fr/intranet/publi/2021-07/010081499.pdf Accès réservé (Intranet de l'IRD) IRD - Base Horizon / Pleins textes 2021-07-02 2025-02-24 fdi:010081499 fre