Plastic pollution is a topical environmental threat, yet many aspects of its impact on ecosystem functioning remain poorly explored. In particular, the effects of microplastics (MP) on sedimentary organic matter (OM) degradation are still largely unknown. To provide insights on this, we investigated in mesocosm the short-term (after 10 and 20 days of exposure) effects of microplastics of different particle size ranges (210-500µm and 70-210µm) and polymer type (polyethylene, polyurethane and tire wear particles) on quantity, biochemical composition and enzyme-mediated degradation of OM in coastal marine sediments.

Our results reveal that smaller microplastic particles, regardless of polymer composition, can trigger enhanced aminopeptidase activity, especially in the very short term (i.e., by 10 days from incubation). Furthermore, increased β-glucosidase activity was recorded after 10 and 20 days of exposure to each microplastic polymer and size.

Notably, OM quantity showed polymer-specific changes. Tire wear-treated sediments showed significantly increased organic C quantity for the whole duration of the experiment, mainly because of the rise of (constitutive) lipid contents.

These results resulted in a size-driven positive influence of microplastics on the OM degradation and turnover rates, pointing to the likely relevance of both chemical and physical MP properties in influencing sedimentary biogeochemistry.

Our results, though based on a manipulation experiment in mesocosm, suggest that the effects of microplastics on sedimentary biogeochemistry are tightly dependent on the plastic particle size and polymeric composition. In particular, the observed positive effects on C degradation, if contextualized in warmer seas, where C degradation will be accelerated as well, pose concern about the exacerbated oligotrophic nature of the sea bottom, with potentially negative consequences on benthic trophic webs.