Colonization of Microplastics by Different Strains of <i>Pseudomonas Syringae</i> Increases Ice-Nucleation Activity

Microplastics (MPs) are increasingly detected throughout the atmosphere, raising questions about their persistence and influence on cloud-relevant ice nucleation processes. Recent studies suggest that MPs may act as ice-nucleating particles (INPs), potentially enhanced by biological colonization. Here, we quantify the ice nucleation activity (INA) of polystyrene (PS) and polyethylene (PE) MPs before and after surface aging and microbial colonization. Strains of <i>Pseudomonas syringae</i>, spanning a range of INA, were cultured onto pristine and aged 100 μm PS and PE MPs. Uncolonized MPs (0.5 to 100 μm PS; 100 μm PE) exhibited INA, with median freezing temperatures ranging from -21.0 °C to -23.8 °C. Hydrothermal and photooxidation exposure, produced small, statistically insignificant increases in freezing temperature. PE MPs nucleated ice at higher temperatures than PS MPs, while the size of MPs did not appear to impact mean freezing temperatures. However, biofilm colonization increased median freezing temperatures by ∼6.5 °C (<i>p</i> < 0.0001) and enhanced INA relative to noncolonized MPs and cells alone. These results indicate that atmospherically relevant MPs modified by aging and microbial growth exhibit elevated INA, highlighting an under represented pathway by which MPs may influence cloud microphysics.