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

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 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.

Keywords: Gulf of Gabès; Tunisia; archaea; bacteria; bacterioplankton; diversity; seawater.

FIGURE 1

Sampling sites along the Tunisian coast during INCOMMET cruise (A) in the South Mediterranean Sea (B) with vertical profiles of salinity (C), temperature (D), ammonium (E), orthophosphate (F), and chlorophyll a (Chla) (G) concentrations along the section defined by the station latitude from the Gulf of Tunis to the Gulf of Gabès (modified from Khammeri et al., 2020). Stations are indicated by blue or red circles (for North or South bays) on map. Stations and sample depth are indicated by line in vertical profiles.

FIGURE 2

Comparison of alpha diversity indices (Observed, Shannon, Simpson) between sampling sites and location groups (from North or South bays) along Tunisian coast.

FIGURE 3

Composition of prokaryotic communities in the seawater samples along the Tunisian coast (South Mediterranean Sea). Relative abundance of all prokaryotic phyla (A) and dominant orders (>10% in average, B).

FIGURE 4

Principal Coordinate Analysis (PCoA) ordination based on Bray-Curtis distance matrix from the prokaryotic community (ASV level) across all seawater samples collected along Tunisian coast. Sample locations are indicated by blue (North) or red (South) symbols (squares or triangles for bottom or surface waters, respectively). The distribution of the 25 most abundant ASVs is indicated by circles of different colors and their taxonomic affiliations are given in the legend at genus-level rank.

FIGURE 5

Heatmap visualizing the Z-score distribution from the relative abundance of the dominant genera (>1% in average) in the seawater samples along the Tunisian coast (South Mediterranean Sea). The dendogram clusters according to the Bray-Curtis similarity index.

FIGURE 6

Panel figure showing the relative abundance of dominant ASVs retrieved from the seawater samples collected along Tunisian coast. Representative ASVs of each dominant genus were selected for visualization among top 40 ASVs (mean relative abundance >.5%).

FIGURE 7

Canonical correspondence analysis visualizing the relationships between physicochemical environmental variables (arrows) and sampling sites (black circles) derived from ASV abundance data. Significant environmental variables are displayed (p < 0.05). S1 to S4 sample originated from North Tunisian coast; S5 to S11 samples originate from South Tunisian coast. B and S mean Bottom and Surface waters, respectively.