Is microplastic-mediated carbon export interfering with the marine carbon cycle? A modeling approach

When microplastic ends up at the ocean surface, it can get rapidly colonized by microscopic organisms during the process of biofouling. Recent studies suggest that this biofilm that is formed has a ballasting effect on the vertical transport of microplastic in the water column. Here, we use a Lagrangian-Eulerian framework to model the vertical motion of microplastic particles caused by biofouling dynamics in response to the environmental variables provided by NEMO-MEDUSA 2.0 outputs. The sinking of initially floating particles from the surface to deeper layers may contribute to the export of carbon fixed by microorganisms attached to them, thus competing with biologically-mediated carbon cycling. The attached biofilm is able to remove and fix carbon from the surrounding seawater through algal attachment and growth and to then release it via algal respiration and other losses (such as grazing), while actively affecting particle buoyancy. Focusing on the Mediterranean Sea, an area regarded as a current and future hotspot of plastic pollution, we provide a first estimate of a threshold of microplastic concentration that could cause the onset of interference with the Mediterranean biological carbon pump. We compare this threshold with two realistic scenarios, one representative of current Mediterranean microplastic pollution and one of future projections, to estimate their respective microplastic-mediated carbon export, and any anomalies in carbon concentrations at depth. The model exploited in this study is intended as a preliminary, yet flexible and upscalable approach to help assess the impact of microplastic pollution on the global marine carbon fluxes.