A European study has revealed that water salinity acts as a natural barrier, preventing harmful bacteria from hitching rides on microplastics from rivers into the sea. Researchers sampled microbial communities on plastics in nine major rivers, including the Seine and the Rhine, and found that the bacterial species present in freshwater differed markedly from those in marine environments.

Pathogens such as Aeromonas and Prevotella were found only in river samples, while marine plastics hosted different species like Vibrio, with no overlap observed. The findings suggest that the transition from freshwater to seawater creates strong selective pressure, hindering the long-distance travel of harmful bacteria via microplastics.

The study offers new insights into the so-called 'plastisphere' – the microbial biofilms that form on plastic debris – and underscores the limited risk of certain pathogens reaching marine ecosystems from inland waterways. These findings support ongoing EU efforts to address microplastic pollution through legislation such as REACH, the Marine Strategy Framework Directive, and the Water Framework Directive.

By closing key knowledge gaps, this research helps inform future environmental and health policy on microplastics and water quality – while pointing to the need for broader studies on other microbes and tidal influences.