Prokaryotes play an important role in biogeochemical cycling in marine ecosystems, but little is known about their diversity and composition, and how they may contribute to the ecological functioning of coastal areas in the South Mediterranean Sea. This study investigated bacterial and archaeal community diversity in seawater samples along the Tunisian coast subject to important physicochemical disturbances. The 16S amplicon sequencing survey revealed higher prokaryotic diversity in the northern Tunisian bays than in southeastern waters (Gulf of Gabès). The major taxonomic groups identified in all samples were Alphaproteobacteria (40.9%), Gammaproteobacteria (18.7%), Marine Group II Euryarchaeota (11.3%), and Cyanobacteria (10.9%). Among them, the relative abundance of Alteromonadales, Prochlorococcus, and some clades of Pelagibacterales (SAR11) significantly differed between the northern and the southern bays, whereas no difference was observed across coastal waters in the archaeal Candidatus Poseidoniales (MGII), Synechococcus, and Pelagibacteraceae (SAR11 clade Ia), for which no relationship was observed with the environmental variables. Both Pseudoalteromonas and Alteromonas levels increased with the increasing salinity, density and nutrients (NH₄⁺ and/or PO₄^3–) gradients detected toward the southern waters, while the SAR11 clades Ib and IV and Prochlorococcus, decreased in the shallow, salty and nutrient-rich coastal waters of the Gulf of Gabès. Rhodobacteraceae was positively correlated with Synechococcus and chlorophyll levels, suggesting a relationship with phytoplankton biomass. The present study provides the first insights into planktonic prokaryotic community composition in the South Mediterranean Sea through the analysis of Tunisian seawaters, which may support further investigations on the role of bacterioplankton in the biogeochemistry of these ecosystems.

原核生物在海洋生态系统的生物地球化学循环中起着重要作用，但对其多样性和组成以及它们如何促进南地中海沿海地区的生态功能了解甚少。这项研究调查了突尼斯沿海海水样本中细菌和古细菌群落的多样性，这些细菌和古细菌群落受到重要的理化干扰。16S扩增子测序调查显示，突尼斯湾北部的原核生物多样性高于东南部水域（加贝斯海湾）。在所有样品中鉴定出的主要分类组为丙酸杆菌 （40.9％）， 丙型杆菌 （18.7％），海军陆战队II Euryarchaeota （11.3％），和 蓝细菌（10.9％）。其中，相对丰度甲轮虫， 原球菌和一些进化枝 上层杆菌 （SAR11）在北部和南部海湾之间存在显着差异，而古细菌在沿海水域中未观察到差异 念珠菌属 波塞冬胺（MGII）， 球菌和 杆菌科（SAR11进化枝Ia），其与环境变量没有相关性。都假性铝单胞菌 和 链霉菌随着盐度，密度和养分（NH ₄ ⁺和/或PO ₄ ^3–）向南部水域的梯度增加，水平面升高，而SAR11进化枝Ib和IV和原球菌，在加贝斯湾的浅水，咸水和营养丰富的沿海水域减少。 红细菌科 与...成正相关 球菌和叶绿素水平，暗示与浮游植物生物量的关系。本研究通过对突尼斯海水的分析，提供了对地中海南部浮游原核生物群落组成的初步见解，这可能支持进一步研究浮游植物在这些生态系统的生物地球化学中的作用。