Microplastic Removal and Biodegradation by Native Mediterranean Fungus Alternaria alternata

The threat and predominance of microplastics (MPs) in marine environments has prompted a growing interest in their interactions with microorganisms that naturally colonize them (i.e., the plastisphere). This study investigates the interaction of Alternaria alternata─a fungus native to the Mediterranean Sea─with polystyrene (PS) MPs, focusing on the potential of the fungus to remove and degrade MPs in seawater. We first designed and constructed a custom laboratory setup in which an immobilized benthic fungal mat, contained in temperature-controlled glass vials, was exposed to ∼1.7 μm weathered MPs ranging in size from 0.45 to 30 μm. This scenario emulates environmental conditions occurring in the benthic layer of seagrass habitats. We observed a 96% reduction in PS MP particle concentration within 24 h in the presence of a live fungus, which was significantly higher than the removal of the MP from sedimentation or exposure to inactivated fungus. Micro- and nanoscale visualizations illustrate the capture and entrapment of MPs within the fungal biomass. The fungus displayed selectivity, favoring PS over polyethylene terephthalate (PET) and polypropylene (PP). Further analyses indicated the formation of a transformation product following interaction between the fungus and PS MPs, indicating that active fungal interaction, rather than gravitational settling, was the dominant driver of MP removal. In addition, thermogravimetric analysis revealed structural alterations within the fungal cell wall upon exposure to PS-based MPs, further supporting the hypothesis of PS utilization by the fungus. Overall, this study offers new insights into the use of the fungus A. alternata for the biological decomposition of PS MPs and serves as an effective natural method for removing MP from seawater without disrupting the ecological balance.