Hamza I. S., Biegala Isabelle, Zouari A. B., Akrout F., Keskes F. A., Hamza A., Hassen M. B. (2021). Diversity and abundance of diazotrophic cyanobacteria in the central coastal area of the Gulf of Gabes (South-eastern Tunisia). Regional Studies in Marine Science, 42, 101653 [10 p.]. ISSN 2352-4855.
Titre du document
Diversity and abundance of diazotrophic cyanobacteria in the central coastal area of the Gulf of Gabes (South-eastern Tunisia)
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.
