Weathered microplastics alter deep sea benthic biogeochemistry and organic matter cycling: insights from a microcosm experiment

The deep seafloor is the largest reservoir for plastic contamination; yet, it remains unclear whether Carbon introduced through synthetic polymers might alter key ecosystem functions. We assessed the potential alterations in organic matter (OM) cycling triggered by a pulse-injection of microplastics (MPs) into deep sea sediments collected from the upper bathyal zone at 530 m depth. Sediments were exposed for 15, 30 and 45 days to environmentally relevant concentrations (∼0.25 % in weight) of naturally weathered MPs (size range 70-210 μm) of polyethylene, tyre wear particles, and a mixture of polymers. Weathered MPs significantly affected the semi-labile fractions of sedimentary OM and extracellular enzymatic activities, ultimately impairing carbon degradation and turnover rates. Polymer-specific effects were observed in carbohydrate and lipid contents, as well as in the activities of alkaline phosphatase and β-glucosidase, with tyre wear particles exerting the most intense impact. Notably, the polymer mixture triggered a less intense effect compared to the same concentration of single polymers. This study highlights how MP contamination can disrupt biogeochemical cycles in deep-sea soft bottoms. Given that these ecosystems constitute the largest portion of the oceans' seafloor, impacts here documented may signal possible cascading effects throughout the ecological hierarchy. Further research is needed to fully understand the dynamics and effects triggered by substantial accumulation of plastic-derived Carbon in these ecosystems.