From plastic pollution to carbon cycling: the role of aquatic fungi in coastal sedimentary systems

Microorganisms play a central role in the degradation of organic matter and the balance between carbon storage and CO₂ release in aquatic systems. Plastics add a novel, anthropogenic carbon pool to the biogeochemical cycles that govern carbon drawdown, respiration, transport, and storage across atmosphere, water and sediment domains. Plastics are quickly colonized by microorganisms in the environment, forming a biofilm named the plastisphere. The plastisphere is complex, consisting of, i.e., diverse bacteria, fungi, diatoms, as well as salts, minerals and metals reflective of the host environment. Through their metabolic activity, some microorganisms in the plastisphere mediate plastic transformation into CO₂ or biomass. Fungi are universal decomposers, with specialized enzyme systems for recalcitrant polymers such as lignin and chitin. These capabilities position them as key players in plastic polymer transformation, influencing the fate of plastics in the natural environment. Yet, the role of aquatic fungi on plastics remains largely understudied, particularly regarding their diversity, ecological functions, and contribution to carbon cycling, transport and storage. To address this gap, we deployed two plastics, the conventional plastic polyethylene (PE) and the bioplastic polylactic acid (PLA), for two months along the freshwater, brackish, and marine transect of a Danish estuary. The plastics were placed in water columns, sediments, and rhizospheres to test how habitat and root-associated processes influence fungal colonization and plastic degradation. In current and future work, NanoSIMS will track carbon assimilation in individual fungi isolated from the system, revealing fungi as overlooked agents in the environmental fate of plastics. By coupling plastic degradation to sedimentary carbon fluxes, aquatic fungi may influence whether plastic-derived carbon is respired, recycled, or sequestered in sediments. Understanding these processes is essential to refine our models of the coastal carbon cycle.