Seasonal comparison of bacterial communities in farmed mussels (Mytilus galloprovincialis), other benthic bivalves and surrounding coastal and offshore seawater from aquaculture zones in Greece

Bivalves are filter-feeding organisms that accumulate a wide range of microorganisms from their surrounding environment, including potentially pathogenic bacteria that can cause foodborne illnesses and can also affect mussel production and ecological stability. This study investigates microbial populations and abundance in both cultivated and wild benthic bivalves, as well as in offshore and coastal seawater samples. The samples were collected from aquaculture zones in Northern Greece and the Ambracian Gulf during two different seasons (summer and winter). Physicochemical parameters of seawater, including temperature, salinity, pH, and dissolved oxygen, were recorded during each sampling. All samples were analysed using classic microbiological methods for the enumeration of microbial groups, including aerobic plate count (APC), psychrotrophic plate count (PPC), Escherichia coli (MPN), presumptive Vibrio spp. (TCBS agar), presumptive Pseudomonas spp. (Pseudomonas agar) and presumptive Enterobacteriaceae (MacConkey agar), followed by 16S rRNA sequencing for molecular identification. Overall, microbial counts were higher in mussel samples compared to seawater, while seasonal and spatial differences were observed across the examined sites. Psychrotrophic plate counts (PPC) were higher during winter, whereas presumptive Enterobacteriaceae and presumptive Vibrio spp. increased in summer. The molecular identification revealed the presence of potential human pathogens such as Vibrio , Pseudomonas , Aeromonas , and Acinetobacter. These findings provide baseline data on the seasonal microbiological profile in farmed mussels and their surrounding marine environment and highlight the importance of continuous monitoring.