Plastisphere assemblages differ from the surrounding bacterial communities in transitional coastal environments

Marine plastic contamination is currently considered ubiquitous in aquatic environments. These particles present a re-sistant and hydrophobic substrate known to promote microbial colonisation and biofilm formation in aquatic ecosys-tems, the so-called Plastisphere, raising concerns about its potential ecological risks. The novelty of this topic translates into a relatively low number of studies, including for transitional coastal ecosystems, such as sandy beaches or estuarine habitats. Therefore, a sampling campaign was conducted in two transitional coastal ecosystems -the Mondego estuary (Portugal) -and adjacent sandy beaches (winter 2020). After visual sorting and filtering of suspected particles under sterile conditions DNA extraction and 16S rRNA amplicon high throughput sequencing was used to profile the bacterial communities on the surface of plastic particles and from those found on the water and sediments from the sampled transitional coastal ecosystems. All particles were characterised according to type, colour and size, and the chemical nature of the particles was determined by FTIR-ATR or mu-FTIR spectroscopy after DNA extraction. All samples contained plastics in several sizes (micro and mesoplastics), shapes (higher abundances of fragments on beaches and fibres in the estuarine waters), colours and polymers. Although no significant differences were detected in the alpha-diversity indexes of the bacterial communities between plastics and their surrounding environments, data showed the occurrence of unique key bacterial groups on plastics from both environments, such as pathogens (e.g., Lactococcus, Staphylococcus and Streptococcus) and groups commonly associated with wastewater treatment plants (e.g., members of the phylum Firmicutes). This highlights the concerns for plastics to act as vectors of transmis-sion and spread of these bacterial groups in transitional coastal ecosystems. Furthermore, it raises the possibility that (micro)plastics entering the estuary from the sea play a substantial contribution to overall dynamics of (micro)plastics and their microbial assemblages in the estuarine system.

Automated summary

Marine plastic contamination is widespread in aquatic environments, promoting microbial colonization and biofilm formation. There is a lack of studies on transitional coastal ecosystems, such as sandy beaches and estuaries. A sampling campaign was conducted in the Mondego estuary and adjacent sandy beaches in Portugal, revealing the presence of diverse plastic particles with unique bacterial communities, including pathogens and those commonly found in wastewater treatment plants. This highlights concerns about the transmission and spread of these bacteria by plastics in transitional coastal ecosystems.