Regionally shared taxa and local structuring define summer bacterioplankton communities in the north-western...

Coastal zones are highly productive ecosystems exposed to fluctuating environmental conditions and multiple anthropogenic pressures. Bacterioplankton play key roles in biogeochemical cycling in these waters, owing to their rapid response to environmental perturbations, and act as indicators of ecosystem change. However, limited research has explored how land-derived inputs influences coastal bacterioplankton assemblages. Moreover, most studies have focused on the free-living (FL) fraction, with comparatively less attention given to particle-associated (PA) communities, despite their distinct genetic, metabolic, and ecological characteristics. This study examined the structure of FL and PA bacterial communities in summer across four coastal-offshore transects in the north-western Mediterranean. These transects represented different degrees of land-derived influence, associated with urbanization, agriculture, and riverine inputs. Bacterioplankton composition was analyzed together with concurrent measurements of environmental conditions. Alpha diversity showed no consistent spatial patterns, whereas community composition generally differed between coastal and offshore waters, with PA communities showing lower similarity among samples, indicating greater sensitivity to local conditions. Nevertheless, coastal amplicon sequencing variants (ASVs) accounted for up to 80% of offshore FL sequences across most transects, except in Girona, where a localised freshwater signal coincided with local shifts in bacterial diversity. A widespread pool of bacterial taxa representing more than 70% of total reads was shared among offshore FL communities across transects, whereas PA communities were more variable and site-specific. Despite extensive ASV sharing, local environmental conditions remained important drivers of community structure. Overall, our results indicate that coastal-offshore environmental gradients influence both FL and PA bacterioplankton communities, but PA communities exhibit greater spatial heterogeneity and stronger site-specific differentiation. These findings highlight lifestyle as a key factor modulating bacterioplankton responses to environmental variability and land-based pressures in coastal waters.