Microbial hitchhikers on microplastics: The exchange of aquatic microbes across distinct aquatic habitats

Abstract Microplastics (MPs) have the potential to modify aquatic microbial communities and distribute microorganisms, including pathogens. This poses a potential risk to aquatic life and human health. Despite this, the fate of ‘hitchhiking’ microbes on MPs that traverse different aquatic habitats remains largely unknown. To address this, we conducted a 50‐day microcosm experiment, manipulating estuarine conditions to study the exchange of bacteria and microeukaryotes between river, sea and plastisphere using a long‐read metabarcoding approach. Our findings revealed a significant increase in bacteria on the plastisphere, including Pseudomonas , Sphingomonas , Hyphomonas , Brevundimonas , Aquabacterium and Thalassolituus , all of which are known for their pollutant degradation capabilities, specifically polycyclic aromatic hydrocarbons. We also observed a strong association of plastic‐degrading fungi (i.e., Cladosporium and Plectosphaerella ) and early‐diverging fungi (Cryptomycota, also known as Rozellomycota) with the plastisphere. Sea MPs were primarily colonised by fungi (70%), with a small proportion of river‐transported microbes (1%–4%). The mere presence of MPs in seawater increased the relative abundance of planktonic fungi from 2% to 25%, suggesting significant exchanges between planktonic and plastisphere communities. Using microbial source tracking, we discovered that MPs only dispersed 3.5% and 5.5% of river bacterial and microeukaryotic communities into the sea, respectively. Hence, although MPs select and facilitate the dispersal of ecologically significant microorganisms, drastic compositional changes across distinct aquatic habitats are unlikely.