Impacts of anthropogenic pollutants on benthic prokaryotic communities in Mediterranean touristic ports

Ports and marinas are part of the logistics chain and the transport networks with a decisive role in the development of coastal areas. They are hot spots of several pollutants, such as polycyclic aromatic hydrocarbons (PAHs) and metals. In particular, port sediments act as a long-term sink of anthropogenic contaminants, which usually exhibit higher concentrations than the adjacent coastal areas. Moreover, the origin and composition of contaminant mixtures in sediments vary substantially among different ports as a result of the heterogenous impacts exerted by port infrastructures as well as tourist, industrial and urban activities surrounding the port area. Several port activities (e.g. dredging, navigation) imply sediment disturbance such as resuspension of particle bound pollutants and their mobilization to the water column, thereby changing the environmental conditions in benthos and potentially altering the diversity and functionality of the whole aquatic ecosystem. Within the EU legislation, the peculiar features of ports justify their designation as Heavily Modified Water Bodies and the development of monitoring programs specific and adequate to their characteristics. Existing sediment quality assessment tools include the chemical analyses combined with the ecotoxicological approach and macrobenthic community descriptors, as ecological indicators of sediment quality. Currently, an ecological approach based on the analysis of bacterial communities is not yet established for regulatory purposes.With these premises, the general objective of this work was to evaluate the impacts of anthropogenic pollutants on the bacterial communities in port sediments. A coordinated sampling campaign was performed in three Mediterranean ports: Cagliari, (Sardinia, Italy), El Kantaoui (Tunisia), and Heraklion (Crete, Greece). The pollution status of the three sites has been previously determined by chemical characterization of hydrocarbons (aliphatic hydrocarbons and PAHs), metals and metalloids (Al, As, Cd, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sb, V and Zn) in sediments collected from different port sectors. Moreover, the environmental properties of the three harbors have been defined based on the combined assessment of physical parameters, chemical variables (i.e. nutrients, pigments), and macrobenthic diversity. In this study, the structure and composition of the benthic bacterial communities were assessed by targeted metagenomic analysis of the V4 hypervariable region of the 16S rRNA gene and the link between the abiotic variables and community structure and composition was explored. For the assessment of metal impacts on bacterial communities, the normalization of metal contents to Al as reference element was applied to compensate the natural variability in the grain-size and mineralogical composition of the studied sediments, collected from geographical distant sites. The analysis of benthic bacterial communities by NGS demonstrated that the port site was the main factor structuring communities in superficial sediments. Moreover, the sector use was another prevailing driver of bacterial community as sediments collected near the shipyard in the Heraklion port displayed a peculiar community composition which could be ascribed to the low redox potential, the high percentual of silt and clay, and the high levels of aliphatic hydrocarbons. When this peculiar sector was excluded, the structure of the benthic bacterial community was demonstrated to be strongly associated with the content of organic carbon, a parameter directly linked to the bioavailability of pollutants in sediments, as well as the emission source of PAHs and the level of copper. In ports, copper is a toxic metal introduced into the marine ecosystems by shipping activities due to the use of Cu-based antifouling paints on ship hulls. It is notable that the analysis of sediments collected during the coordinated sampling campaigns of the three investigated ports has established that the shipyard sector in Heraklion is significantly degraded in terms of macrobenthic diversity. Similarity to bacterial communities, the macrobenthonic patterns have also been demonstrated to be strongly affected by copper pollution.Overall works performed under the umbrella of the ENPI CBCMED project MAPMED demonstrated that the benthic bacterial community is sensitive to the pollution status of port sediments. We were able to link changes in its structure and composition to the observed differences among ports in the copper levels and in the origin of PAHs entering sediments. In conclusion, our results designate the benthic bacterial community as a suitable biological indicator of sediment quality in ports, with the added value of tracking bioremediation potential and processes.