Diversity and abundance of diazotrophic cyanobacteria in the central coastal area of the Gulf of Gabès (South-eastern Tunisia)

Gabès 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 Gabès 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 μm and 0.2–3​ μm) during July in the central coastal area of the Gulf of Gabès. 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: 0.8–1.5μm and large cells: 1.6–3.2μm) 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 Gabès 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. •Diazotrophic cyanobacteria investigation in polluted central coasts in Gabès gulf.•Observation of blooming Richelia intracellularis free living and hosted in diatoms.•Diazotrophic UCYN are predominant as small cells ranging from 0.8 to 1.5μm