Prokaryotic Diversity and Distribution Along Physical and Nutrient Gradients in the Tunisian Coastal Waters (South Mediterranean Sea).

Marianne Quéméneur,Malika Bel Hassen,Fabrice Armougom,Yosra Khammeri,Rim Lajnef,Amel Bellaaj-Zouari

doi:10.3389/fmicb.2020.593540

Abstract

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 PO43–) 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.

Highlights

Prokaryotes are abundant in marine ecosystems, with number estimates of 1 × 1029 in oceans and concentration estimates of 103 bacteria per microliter in surface seawater (Azam and Malfatti, 2007; Flemming and Wuertz, 2019)
Surface (S) and bottom (B) waters were collected using 12L Niskin bottles fitted on a Rosette sampler equipped with conductivity, temperature, and depth (CTD) sensors (SBE 9, SeaBird Electronics) that led to the salinity and temperature data reported in the Figures 1C,D
These areas are subjected to different physical water properties: the Modified Atlantic Water mass was reported along the northeastern coasts whereas the saltier and warmer Modified Mediterranean Water (MMW) was detected in the Gulf of Gabès (Khammeri et al, 2020)

Summary

Introduction

Prokaryotes are abundant in marine ecosystems, with number estimates of 1 × 1029 in oceans and concentration estimates of 103 bacteria per microliter in surface seawater (Azam and Malfatti, 2007; Flemming and Wuertz, 2019). Both PCoA and heatmap clustering analysis revealed some differences in the prokaryotic relative abundance at ASV and genus levels between the northern or the southern bays, as observed at higher taxonomic ranks, e.g., class, order (Figure 3B).

Results

Conclusion