Occurrence, origin and potential ecological risk of dissolved polycyclic aromatic hydrocarbons and organochlorines in surface waters of the Gulf of Gabès (Tunisia, Southern Mediterranean Sea)

We investigated the occurrence, origin, and potential ecological risk of dissolved polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyl (PCBs) and organochlorine pesticides (OCPs) in 27 surface water samples collected from a highly anthropized and industrialized area in the Gulf of Gabès (Tunisia, Southern Mediterranean Sea) in October-November 2017. The results demonstrated a wide range of concentrations (ng L^-1) with the following decreasing order: Ʃ16 PAHs (17.6-71.2) > Ʃ20 PCBs (2.9-33.7) > Ʃ6 DDTs (1.1-12.1) > Ʃ4 HCHs (1.1-14.8). Selected diagnostic ratios indicated a mixture of both pyrolytic and petrogenic sources of PAHs, with a predominance of petrogenic sources. PCB compositions showed distinct contamination signatures for tetra- to hepta-chlorinated PCBs, characteristic of contamination by commercial (Aroclor) PCB mixtures. The dominant OCP congeners were γ-HCH, 2,4'-DDD and 2,4'-DDE, reflecting past use of Lindane and DDTs in the study area. Agricultural, industrial and domestic activities, as well as atmospheric transport are identified as potential sources of PAHs, PCBs and OCPs in surface waters of the Gulf of Gabès. Toxic equivalents (TEQs) suggested a low carcinogenic potential for PAHs in seawater samples (mean of 0.14 ng TEQ L^-1). Evaluation of risk coefficients revealed low risk for PAHs and PCBs, and moderate to severe risk for OCPs.

Morphological Redescription and Phylogenetical Position of Ceratomyxa truncata Thelohan (1895) and Coccomyxa morovi Léger and Hesse, 1907 (Myxozoa: Myxosporea) Infecting the Gall Bladder of Sardina pilchardus (Walbaum) from Tunisian Coast

PURPOSE

Two myxosporean species have been, so far, independently reported from the gallbladder of the European pilchard, Sardina pilchardus (Walbaum) (synonym Clupea pilchardus) in the Northern shore of the Mediterranean Sea; Ceratomyxa truncata Thélohan, and Coccomyxa morovi Léger and Hesse, 1907. The two species were described with incomplete morphological data and based only on line drawings of their mature myxospores.

METHODS

During a parasitological survey in the Southern shores of the Mediterranean coast in the gulf of Gabès off Tunisia, two coelozoic myxosporean species were found in the European pilchard and described using morphological and molecular phylogenetic tools. Morphological characterization was based on the mature myxospore study and some vegetative stages. The SSU rDNA sequences were performed for molecular and phylogenetic study.

RESULTS

The most frequently encountered species belongs to the genus Ceratomyxa Thelohan, 1892. The second species belongs to the genus Coccomyxa. Morphological examinations, allowed us to match these two recorded species with Ceratomyxa truncata and Coccomyxa morovi, respectively, as previously described in the same host species referring to the original manuscripts instead of some morphological differences. Molecular analyses based on the partial SSU rDNA sequences did not much with any of the previously reported myxozoan sequences. Phylogenetic analysis positioned C. truncate in a well-supported clade including Ceratomyxa ssp. from Mediterranean Sea, while C. morovi was positioned on the basis of the subclade grouping all Coccomyxidae species.

CONCLUSION

We provided herein a first morphological redescription of Ceratomyxa truncata and Coccomyxa morovi parasite of Sardina pilchardus from the Southern shores of the Mediterranean Sea and we successfully obtained the SSU rDNA sequences of these two species and positioned them in the phylogenetic tree.

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

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.

Effect of Acidic Industrial Effluent Release on Microbial Diversity and Trace Metal Dynamics During Resuspension of Coastal Sediment

Both industrial effluent discharge and the resuspension of contaminated marine sediments are important sources of trace metals in seawater which potentially affect marine ecosystems. The aim of this study was to evaluate the impact of the industrial wastewaters having acidic pH (2–3) and containing trace metals on microbial diversity in the coastal ecosystem of the Gulf of Gabès (Tunisia, southern Mediterranean Sea) subjected to resuspension events of marine sediments. Four trace elements (As, Cd, U, and V) were monitored during 10-day sediment resuspension experiments. The highest enrichment in the seawater dissolved phase was observed for Cd followed by U, V, and As. Cd remobilization was improved by indigenous microbial community, while U release was mainly abiotic. Acidic effluent addition impacted both trace metal distribution and microbial diversity, particularly that of the abundant phylum Bacteroidetes. Members of the order Saprospirales were enriched from sediment in natural seawater (initial pH > 8), while the family Flavobacteriaceae was favored by acidified seawater (initial pH < 8). Some Flavobacteriaceae members were identified as dominant species in both initial sediment and experiments with acidic wastewater, in which their relative abundance increased with increasing dissolved Cd levels. It could be therefore possible to consider them as bioindicators of metal pollution and/or acidification in marine ecosystems.

Natural and anthropogenic particulate-bound aliphatic and polycyclic aromatic hydrocarbons in surface waters of the Gulf of Gabès (Tunisia, southern Mediterranean Sea)

Particulate-bound aliphatic and polycyclic aromatic hydrocarbons (AHs and PAHs) were investigated in the surface waters of the Gulf of Gabès (Tunisia, southern Mediterranean Sea). Samples were collected off the Sfax and Gabès-Ghannouch coasts. Concentrations in total resolved n-alkanes ranged from 0.03 to 3.2 μg L^-1, and concentrations in total parents + alkylated PAHs ranged from bdl to 108.6 ng L^-1. The highest concentrations were recorded in the southern Sfax. AHs were mainly of biogenic origin with odd n-alkane predominance, although an anthropogenic contribution was also detected. The PAH molecular patterns revealed a mixed origin with the presence of low molecular weight and alkylated compounds, characteristic of uncombusted oil-derived products, and the presence of high molecular weight compounds, typical of combustion residues. Rainfall events induced an increase in PAH concentrations by a factor 1.5-23.5. The particle-water partition coefficients (K_oc) suggest that the partitioning of PAHs between the particulate and dissolved phases is driven by hydrophobicity and organic matter composition.

Origin and distribution of hydrocarbons and organic matter in the surficial sediments of the Sfax-Kerkennah channel (Tunisia, Southern Mediterranean Sea)

We investigated the origin and distribution of aliphatic and polycyclic aromatic hydrocarbons (AHs and PAHs) and organic matter (OM) in surficial sediments of the Sfax-Kerkennah channel in the Gulf of Gabès (Tunisia, Southern Mediterranean Sea). TOC, AH and PAH concentrations ranged 2.3-11.7%, 8-174μgg^-1sed.dw and 175-10,769ngg^-1sed.dw, respectively. The lowest concentrations were recorded in the channel (medium sand sediment) and the highest ones in the Sfax harbor (very fine sand sediment). AHs, PAHs and TOC were not correlated for most of the stations. TOC/N and δ¹³C values revealed a mixed origin of OM with both marine and terrestrial sources. Hydrocarbon molecular composition highlighted the dominance of petrogenic AHs and the presence of both petrogenic and pyrogenic PAHs, associated with petroleum products and combustion processes. This work underscores the complex distribution patterns and the multiple sources of OM and hydrocarbons in this highly anthropogenized coastal environment.

Spatial and seasonal variability of pico-, nano- and microphytoplankton at the bottom seawater in the north coast of Sfax, Eastern Mediterranean Sea

In the frame of the Taparura Project, we studied the distribution of pico-, nano- and microphytoplankton communities in relation to environmental variables at 18 stations sampled during four coastal cruises conducted between October 2009 and July 2010 at the bottom, on the north coast of Sfax (Tunisia, Eastern Mediterranean Sea). The restoration effect on coastal ultraphytoplankton (<10 μm) and microphytoplankton (<200 μm) was investigated using conventional flow cytometry and inverted microscopy. Flow cytometry analysis of ultraphytoplankton resolved six groups (Prochlorococcus, Synechococcus, nanoeukaryotes and three distinct subgroups within picoeukaryotes). In addition to these autotrophic groups, two unknown groups were characterised on the north coast. Picophytoplankton abundance shifted from a summer dominance of Synechococcus to a dominance of picoeukaryotes and Prochlorococcus during spring. Nanoeukaryotes were the most abundant in spring. Microphytoplankton was resolved into five groups, labelled Bacillariophyceae, Dinophyceae, Cyanobacteriae, Euglenophyceae and Chlorophyceae. A total of 90 microphytoplankton species were identified in all stations, with an overwhelming abundance of large diatoms, a typical trait of benthic communities (Coscinodiscus sp., Grammatophora sp., Navicula sp., Pleurosigma sp., Striatella unipunctata …). Results collected in this study are favouring a beneficial impact on the ecosystem of the Sfax north coast restoration achieved by the Taparura Project.